BOT 732: PHYSIOLOGICAL PLANT ECOLOGY LECTURE NOTES

What is Physiological Plant Ecology?

Physiological Plant Ecology is a combination of Plant Physiology with Plant Ecology; and

therefore is concerned with nature or functioning of plants in relation to their natural

environment. Physiological Plant Ecology, as Autecology as it is, is concerned with the response

of the individual plant to its environment and the ways in which changes in the environment are

accommodated by the reactions of the plant.

Plants Responses to the Environment

Plants respond to both physical (abiotic) environment and to each other (biotic environment).

The mutual interactions may be influenced by the form and function of a particular plant species.

These interactions may be modified by the environment and may in turn alter the environment

responses. Plants may also vary in their responses on a seasonal basis and show different

reactions at various stages in their life history.

Responses to environmental factors

All plants have certain basic requirements without which they cannot exist. Each factor usually

has a minimum and maximum level beyond which a plant cannot survive. However, the extreme

values tolerated by one plant are not necessarily identical to those tolerated by another.

Consequently, the study of the tolerance of plants to the environmental extremes may provide a

means of discriminating between plants with different ecologies. The optimum response is

somewhere between the two extremes and may also differentiate between species. There is a

common dissimilarity between laboratory and field responses as the later is modified by

competition. The overall relationship of a species to its natural environment is its ecological

amplitude. Species usually have separate but overlapping amplitudes; those that appear to have

identical amplitudes may have different demands upon the environment as in the case of a forest

tree and woodland herb.

Unit 2: Microclimate

Plants exist in a physical environment near the surface of the ground; their roots penetrate a little

distance into the earth’s crust and their shoots a little way into the atmosphere, but on a global

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scale, they can be considered as only superficial. However, within this narrow layer, the

interaction between plant and environment is complex: the physical environment determines the

plants which grow, whilst the plants influence the physical environment.

Energy relations of surfaces

The sun as a source of energy: The radiation balance:

The earth receives a broad spectral range of radiation from the sun. the distribution of the solar

flux is skewed so that only 25% of the total lies in wavelengths shorter than the peak whilst there

is an extended tail into the longer wavelengths. The radiation maximum lies in the visible range

360-760 nm which constitutes light and is also the range over which photosynthesis occurs.

Wavelengths shorter that the visible are termed UV whilst those longer than 760 nm are termed

infrared.

The integrated flux density of solar radiation incident upon a surface perpendicular to the sun’s

rays at the outer edge of the earth’s atmosphere is at 1,400 Wm-2 (2.0 cal cm-2 min-1); this is the

solar constant.

The shortwave input during the day consists of both direct solar radiation (I) and non-directional

sky radiation that has been scattered and diffused by the constituents of the atmosphere. In the

visible spectrum, these components are recognized as direct sunlight and diffused daylight (D)

The total (I+D) is termed global radiation. At the surface, some of the radiations are lost by

reflection (r) and yet more (b) is reradiated at a longer wavelength in accord with the Stefan-

Boltzmann Law which relates the total intensity of radiation emitted from a “black body” (the

body which completely absorbs all radiation incident upon it) to the fourth power of the absolute

temperature (T). Thus:

Where σ=5.67×10-8 Wm-20K-4 i.e. a constant of proportionality

T= temperature

b= radiation

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1. Light and Plant life

Absorption of light:

Three different properties of light may separately affect the metabolism and development of a

plant:

1) Its spectral quality

2) Its intensity and

3) Its duration.

When plant pigments absorb light either photoreaction takes place and this leads to energy

conversion, or the energy is dissipated in different ways. We must here refer to Planck’s

quantum theory of radiation transfer, which states that the transfer of radiation takes place in

discrete packets called quanta.

E=hυ

Where E is the energy transferred, h= Planck’s constant and υ the frequency of the radiation

For energy of photon (i.e. energy of a single quantum of light)

E=

Where E=energy, h=Planck’s constant, c= speed of light and λ = wavelength

One mole of a compound absorbs N photons, having and energy E=Nh; Energy, N=6.02×1023

(Avogadro’s number), h= Planck’s constant. This total energy absorbed per mole is called an

Einstein (E).

NB: An Einstein of red light has less energy than an Einstein of blue light, though both have the

same number of quanta, N

There are three main groups of pigments associated with photo-responses in plant and these are:

1) Chlorophyll concerned with photosynthesis,

2) Phytochrome concerned with morphogenetic changes and

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3) β-carotene or flavin concerned with phototropism.

PHYSIOLOGY OF PHOTOSYNTHESIS

The amount of photosynthesis carried out by a plant is influenced by three properties of light –

spectral quality, intensity and duration.

Spectral quality: This is of great importance. An intense source of monochromatic green light at

530 nm would be not be absorbed significantly by pure chlorophyll a in solution; this is not

expected to be effective in photosynthesis. However, the action spectrum shows that green light

is nevertheless effective, though the main peaks are in the red and blue light.

Intensity of light: this markedly affects the rate of oxygen evolution or carbon dioxide uptake

in photosynthesis. At the lowest light intensities, the real rate of photosynthesis becomes less

than the rate of respiration and the net rate of photosynthesis becomes negative. The intensity at

which both reach a balance is called compensation point. It is important to note that a plant

cannot survive at less its compensation point.

Duration of light: the greater the duration of illumination the more photosynthesis will be

accomplished and if a plant is exposed to a longer day it can be expected to fix more CO 2. It is

important to note that if plants are provided with a prolonged period of light they may

discontinue photosynthesis because of a temporary inability of the chloroplasts to store all the

extra starch. While higher temperature may cause wilting and stomatal closure, thus limiting CO2

uptake. Continuous supply of light makes plants to become bleached and chlorotic.

2. LIGHT AND PLANT PIGMENTS: OTHER PIGMENTS.

Phytochrome: Associated with different types of responses by plants; mode of action not clear;

is a large molecule of molecular weight f about 120,000; its protein contains a high proportion of

acidic and basic amino acids; the sulphur containing amino acid- cysteine; is highly charged and

reactive molecule; is potentially capable of many rearrangements leading to differences in shape.

Location of phytochrome in the plant: relatively easy to detect in non-green and etiolated parts

of plants, but difficult to detect in green plants; no evidence about intracellular location;

phytochrome + cytoplasm corresponds to inactive fraction but phytochrome + membrane

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corresponds to active pool; experiments showed nuclear membrane to be an important site for

phytochromes; and also found in plasmalema.

Daily rhythms: phytochrome is involved in a number of plant responses associated with daily

rhythm of light and darkness.

Plant movements: the daily movement of leaves is a phytochrome-controlled phenomenon.

Stomatal movements: another daily rhythm in plants in the opening and closing of the stomata;

to get rid of CO2 stomata open, although water is also lost via transpiration. The phasing of this

stomatal sequence is phytochrome-controlled.

3. GERMINATION AND SEEDLING ESTABLISHEMENT.

Many aspects of plant development depend on responses to the presence or absence of light.

Some responses depend on daily rhythm of light and darkness e.g. photosynthesis and

translocation. Light is important throughout the whole of the life cycle of a plant. The successful

establishment of an angiosperm seedling may require light to break dormancy, to promote

extension above the soil surface, to produce leaves of a size, shape, orientation and chlorophyll

content adequate for efficient photosynthesis.

Dormancy break: light provides the stimulus necessary for breaking one form of seed

dormancy, it is effective only after water has been imbibed and it acts by removing a blockage

in the metabolic pathway of the embryo. Light requirement for germination can be considered

in view of a) trigger effect, b) light inhibition and c) the effect of continuous or periodic

irradiation.

-Trigger effect: some seeds require very small amounts of light for germination e.g. Lactuca

sativa variety. Both the percentage and rate of germination depend on the quantity of light

applied. If the quantity of light required is small and can be supplied in a single dose, it is called

light trigger reaction.

-Light inhibition: germination in some seeds is inhibited by light. This effect is controlled by

phytochrome. In some species, only a brief flash of low intensity far-red illumination is required

to inhibit germination, in others, treatment with far-red must be prolonged.

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-Photoperiodism: some seeds require alternation of light and darkness to promote germination

and light has to be given over a very long period. Illumination of the cotyledons promotes

germination in Lactuca sativa but illumination of the radicle is ineffective.

Extension growth: activation of the embryo results in the synthesis of nucleic acids, proteins

and differences in hormones levels etc.

Establishment of photosynthesis: unlike the angiosperms, some lower vascular plants can

produce chlorophyll in the dark and so, as soon as the sporeling emerges and is exposed to the

light, photosynthesis can begin. Light is required for the completion of chlorophyll synthesis as

well as the organization of the chloroplast lamellar system and for the synthesis of some

chloroplast enzymes.

Lamellar structure: in meristematic tissues of an angiosperm seedling, plastids are present as

eoplasts –colourless, spherical or ovoid sacs bounded by a double membrane and containing

stroma.

Chlorophyll synthesis: the greening of photosynthetic tissues is one of the obvious results that

follow the emergence of a young angiosperm plant into the light. If a young plant on

development finds itself in darkness, it becomes more or less etiolated. Thus, etioplasts are

formed instead of chloroplasts.

4. FURTHER DEVELOPMENT OF THE PLANT

Flowering: the most thoroughly investigated aspect of the influence of light o changing patterns of

growth during the angiosperm life cycle is the effect of photoperiod on flowering. It was realized

during the latter part of the last century that day length might influence flowering and the

importance of a critical period of darkness was recognized soon after. It was the famous report of

W. W. Garner and H. A. Allard in 1920 that drew worldwide attention to the close relationship

between day length and several plant processes, including flowering.

5. LIGHT AS FACTOR WITHIN THE ECOSYSTEM.

Introduction: light acting through a variety of responses plays a major part in the functioning,

structure and survival of any ecosystem. The conversion of light energy (E) to chemical energy by

means of photosynthesis is the primary process required for the establishment of a food chain. In

plants, the close relationship between day-length perception and those processes directly concerned

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with population continuity and survival is particularly well marked e.g. reproduction and dormancy.

However, it is important to note that the overall effect of light on complex ecosystems remains

poorly understood.

The species present within a plant community depend on numerous factors e.g. topography, soil,

climate (including light) etc. the structure of a community both affects and is affected by light e.g.

in a forest, the light which reaches the uppermost leaves in the canopy differs both in intensity and

spectral composition from that which reaches leaves of plants close to the ground.

Productivity: the total amount of light falling annually on the earth is quite large but very little is

directly used by plants. In the tropics, the intensity of radiation is almost constant all the year round

but in higher latitudes, there are profound seasonal differences. The amount of cloud cover may also

greatly affect the intensity of the radiation reaching the vegetation. Of the radiation falling on a

well-managed pasture, about half is absorbed and converted to heat energy, which supplies the

energy for the conversion of liquid H2O into vapour H2O. Some of the H2O evaporated comes

directly from the surface of the leaf and the soil, and by affecting cloud cover and humidity,

contributes to the effect of climate on the plant.

Light and plant distribution: plant distribution is primarily influenced by temperature, with water

supply as an important secondary factor. Temperature is broadly related to latitude while day-length

is precisely determined by latitude. It is therefore not surprising to find a close relationship between

plant distribution and the precision of day length. The effects of light on pollination biology are

therefore highly diverse and the considerable importance in the reproduction and hence the

selection and distribution of many plant species.

PHYSIOLOGICAL PLANT ECOLOGY BY LARCHER

Radiation and Temperature: Energy, Information, Stress.

Radiation: all life on earth is supported by the stream of energy radiated by the sun and flowing

into the biosphere. Even the relatively small amount of radiant energy in the form of latent chemical

energy by the photosynthesis of plants suffices to maintain the biomass and vital processes of all

members of the food chain. By far, the larger number of radiation absorbed is transformed

immediately into heat; part of this fraction is used in the evaporation of water and the rest produces

an increase in the temperature on the earth’s surface.

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Radiation within the atmosphere:

Attenuation of the radiation by the atmosphere: at the outer limits of the earth’s atmosphere, the

intensity of radiation is 1.39 kWm-2. Of this, only an average of 47% reaches the earth’s surface. Of

the radiation that reaches the ground or plant cover, about half has passed directly through the

atmosphere while the remainder is diffused by the atmosphere and clouds.

-Distribution of radiation in plant cover: in stands of plants, photosynthesis occurs within a stack

arrangement of leaves which partially overlap or shadow one another. The incident light is absorbed

progressively in its passage through these many layers so that most of it is utilized. J. Wiesner

(1907) introduced the notion of “relative irradiance”, expressed as the average percentage of the

external light. In deciduous forests, an open stand of conifers of the temperate zone, an average of

10-20% of the incident radiation reaches the herbaceous stratum during the growing season; when

the trees are bare, this figure increases to 50-70%. In dense coniferous forest and tropical forests

with their abundance of species, the relative irradiance at the ground falls to a few percentages, or

even less that 1%. As a rule, the limits of the existence of vascular plants lie in this range.

The attenuation of radiation in a stand of plants depends on the density of the foliage and the

arrangement of leaves. The foliage density can be expressed as Leaf Area Index (LAI).

LAI =

Ordinarily, the units used for leaf area and ground area are identical (m2) so that LAI has no units.

With a LAI of 4, a given area of ground would be covered by 4 times that of area of leaves –

arranged in several layers. On its way through the plant canopy, the radiation must pass these

successive layers of leaves. In the process, its intensity decreases almost exponentially with

increasing amount of cover in accordance with Lambert-Beer Extinction Law.

The extinction coefficient indicates the degree of attenuation of light within the canopy for a given

area index. The interception of radiation in a stand can also be characterized in terms of the space

taken up by the foliage. Here, the LAI is referred not to ground area but to the volume occupied by

the stand. This takes into account differences in arrangement and position of leaves.

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-Uptake of radiation by plants: the ecosphere receives solar radiation at wavelength ranging from

290-300 nm. Radiation at shorter wavelength is absorbed in the upper atmosphere. By the O3 and

O2 in the air; the long wavelength limit is determined by the H2O vapour and CO2 content of the

atmosphere. About 40-50% of the solar energy received falls in the spectral region of 380-780 nm,

which we perceive as visible light. This region is bounded on the short-wavelength end by UV

radiation.

Radiation and plan life:

The direct effects of radiation:

For a plant, radiation is a source of energy and a stimulus regulating development, but it can also

cause injury. All these effects of radiation result from the capture of quanta. Because only those

quanta that are absorbed can be photochemically active, every radiation-dependent process is

mediated by particular photoreceptors. Each of these is characterized by an absorption spectrum

corresponding to the action spectrum of the associated photobiological events.

In photoenergetic processes, the energy provided by absorption of radiation serves to drive

metabolic reactions or cause chemical transformations, in a manner directly dependent on the

amount of quanta absorbed. Energy-rich compounds can be constructed, molecular structures can

be altered, reactions can be accelerated or the structure of a molecule can be destroyed. The primary

processes of photosynthesis in green plants are driven by radiation in the range of wavelength

between 380-710 nm. This PAR is an important quantity in plant ecology. The photoreceptors

involved in photosynthesis are chlorophylls with absorption maxima in the red and blue, along with

accessory plastid pigments that absorb in the blue and UV region.

Photodestructive effects occur c extremely high-intensity visible radiation or are caused by UV. In

both cases, photoenergetic processes are involved. The damage brought about by intense light

consists primarily of photo oxidation of chloroplast pigments. An important ability that helps a

plant to tolerate strong light is the rapid re-synthesis of decomposed plastid pigments. UV below

300 nm causes not only photo oxidation but also photo destruction of nucleic acids (NA) and

protein bodies, and acute damage to protoplasm. UV damage to plants is evident in various

symptoms: reduced capacity for photosynthesis, difference in the activity of enzymes, disturbance

in the process of growth, cell death etc.

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PRODUCTIVITY AND THE MORPHOLOGY OF CROP STANDS.

Patterns with leaves:

The primary productivity of communities made up of autotrophic green plants is initially dependent

of photosynthesis.

1. Relationship between productivity and morphology.

Community organization:

A. Density of the vegetation cover

The most obvious features of foliage canopies as related to production is the density of foliage

canopy. Less than full cover permits solar radiation to escape interception by the photosynthetic

apparatus. This problem is of considerable importance with cultivated crops during early stages of

growth. According to Shibles and Webar (1965, 1966) and Williams et al. (1965a), when cover is

scant, production is directly related to the fraction of light intercepted. With annual crops, it usually

takes a very long time for even a densely sown crop to achieve as much as 75% interception

(Santhirasegaram and Black, 1968)

Chlorophyll and leaf area have both been used to characterize the amount of photosynthetic

material in the cover. Depth distribution of chlorophyll, light and production rates, are roughly

related (Steeman Nielson, 1957, Talling, 1961). For both aquatic and terrestrial systems,

photosynthetic capability of the elements increases with chlorophyll concentration up to a saturation

level (Gabrielson, 1948). This level for leaves is about 3 mg chlorophyll (a + b) dm -2 surface. At

this level, differences in chlorophyll concentration strongly affect the extinction coefficient of the

leaves, but the absolute amount of light absorbed is little affected (kasanaga and Monsi, 1954).

Most higher plant’s leaves contain at least the level of chlorophyll required for saturation of the CO2

assimilating capacity and the “excess” chlorophyll is not correlated with production. Further, the

response of a leaf in assimilating CO2 becomes a diminishing returns response with increasing light

flux. Thus, chlorophyll indices require, for quantitative purposes of two curvilinear reactions.

NB: Because the distribution of chlorophyll of higher plant leaves is essentially in sheets whose

surfaces are restrictive to CO2 exchange, and whose lateral dimensions largely determine light

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interception. Leaf Area Indices (LAI) is a more functional basis for describing canopy morphology.

Crop growth rate equal to net assimilation rate of leaves times Leaf Area Index.

C = EL

C= net dry matter production.

E= mean rate of net photosynthesis

L= area of leaf per unit area of ground.

Where C is related to total leaf density, L, two kinds of relationships have been found. In one, C

increased as L increased up to some optimum value of L and then declined (Watson 1958, Black,

1963). In the other cases, a plateau response has been found with C remaining constant as L

increased (Brougham 1956, Shible and Weber 1965).

B. Horizontal patters among leaves

Full cover could be provided by one continuous sheet of leaves. However, horizontal distribution

are such that L = 3 or more is needed for complete interception of light. Leaf distribution may range

from uniform to random, and to contagious distribution (clumped or aggregated patterns).

Contagious or regular patterns in foliage may be of several size scales. The individual plants,

branches, leaves and leaflets each serve as aggregation centers in contagious. According to Sacki,

Iwaki and Monsi (1968) “clusters” foliage model are efficient in light interception. Permit L. the

argument is that widely dispersed clusters of leaves would have a smaller extinction coefficient than

would disperse foliage and hence, more leaves could be illuminated at large. L. As L increases,

additional leaves are added to existing clusters thus the extinction coefficient decreases with

increasing L as proposed by Verhagen et al (1963) for an ideal foliage.

C. Vertical separation of leaves.

The influence of variation in the vertical density of leaves is also relatively unexpected. According

to Nichiporovich (1961) leaf size in relation to vertical separation strongly influences the solid

angle occlusion and hence the sky light pattern within a canopy. Large but widely separated leaves

like those of sunflower may actually create a diffuse light pattern quite similar to a shorter

community with small leaves like alfalfa (Anderson 1966 b)

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NB: “better” canopy structure would result in leaf width was reduced or if the leaves were whorled

to reduce the contagious distribution resulting from the opposite and alternate arrangement.

D. Vertical distribution of leaves and light interception

It has been found that attenuation at any depths can usually be related to interposed L by a simple

analytical expression, the Bouquer-Lambert Law.

Where I and I0 are light fluxes to horizontal receivers at points within and above the canopy, L is

Leaf Area Index from the top of the canopy and K is an extension coefficient.

Variations in K have been related to variations in canopy structure, especially to angle of leaf

display (Monsi and Saeki, 1953). This relationship is well illustrated through the comparative

morphology of ryegrass and subterranean clover stands. According to Stern and Donald (1962),

sunlight diminished much more strongly per unit LAI in clover stands than in grassy stands.

Fig

Foliage Angle:

According to Boysen-Jensen (1932) foliage angle affects not only the relative illumination of a fully

exposed leaf but also the projected shadow area of the leaf and thus the flux of light available to

lower leaves.

Light distribution models

The actual flux of light received by each individual leaf must be known in order to estimate its

photosynthesis, a consequence of the curvilinear response of photosynthetic light response curve is

that higher production and hence more efficient light utilization is achieved by illuminating many

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leaves at a modest level of light than by exposing a few leaves to full sun. Mathematical models

which would predict high distribution within canopies have been developed.

(Monsi and Saeli 1963)

Preliminary results indicate that that productivity levels and optimum canopy structure differ

appreciably for various sky conditions.

Monteits (1965) included inclined leaves in his own approach by introducing a parameter S equal to

the fraction of light which passes a unit leaf layer without interception. Thus, S = 0 for a continuous

sheet of foliage normal to a distant point-light source and 1.0 for leaves parallel to the light rays.

The resulting equation for illumination penetrating the Nth layer is

NB: variations in S have a larger effect on calculated productivity.

Azimutal Orientation

According to Ross and Nielson (1967b) in an adaptation whereby crops orientate their leaves to

specific directions for maximum light interception. This could be east to west or north to south e.g.

maize have a tendency to orientate its leaves in the east west direction.

Non-Leaf Structure

Light interception by non-reproductive tissue is an additional feature of canopy morphology.

According to Duncan et al. (1967) 9% of the daily insulation may be intercepted by tassels of a

maize crop at commercial densities (50,000 plants/ha). With herbaceous plants, stem, petiole and

inflorescence parts may contain appreciable chlorophyll and thus represent productive as well as

light intercepting structures.

II. RELATION OF CANOPY MORPHOLOGY TO PRODUCTION

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Establishing relationships between canopy morphology and yields presents a number of difficulties.

It is important to note that the shorter the season, the more dependent crop yield will be upon the

rate at which full cover is reached; and on the efficiency of the canopy at small values of L. thus, a

short season crop such as Cucumis melo develops only a small leaf area but one containing highly

dispersed horizontal leaves. Beyond these factors, canopy morphology affects more than just

visible light distribution among leaves and photosynthesis. The pattern of leaf distribution influence

air circulation, canopy roughness and hence the efficiency of eddy turbulence. These factors in turn

affect CO2 and H2O vapour and heat transfer. Since leaf distribution also determines the receipt and

loss of short and long wave radiation, canopy architecture in effect determines microclimate.

A. Simulation of Crop Productivity

According to DeWit (1965), Monteith (1965) and Duncan et al. (1967) when L is small, horizontal

leaves are advantageous; at large values of L more erect leaves give greater production.

NB: Duncan used his own model to compute the production. Photosynthesis rate was computed for

each hour of the day and then summed and corrected for respiration to give an estimate of daily

production with βmax for the day at 740 (elevation angle of light) inclined leaves show a marked

advantage only when L exceeds 2 to 4, and erect leaves only when L approaches high values of 8 or

more. This was true for the photosynthetic functions of both maize and clover

FIG

The influence of varying sky conditions, latitude, physiological functions and leaf optimal

properties have been explored briefly with Duncan’s model.

B. Some Experimental Results

Establishing relationship between canopy morphology and yield presents a number of difficulties.

The following experiments illustrate some of the problems in establishing cause and effect

relationships between canopy morphology and agronomic yield. Pendleton et al. (1968) working

with two isolines of maize with “normal” and “upright” leaves compares for grain production with

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a moderately high density of plants with L reaching 4.0. Unfortunately, the normal variety was

intolerant of high densities. Thus, when the “upright” lines yielded 41% more grain, large part of

this difference was related to differences in numbers of barren plants. A second phase of the

experiment demonstrated a striking influence of leaf angle. Leaves of a planophile variety were

position upright by mechanical means with L=4.1, the normal display intercepted 99% of the

incident light near noon as compared to 90% intercepted when leaves above the ear were upright,

and 84% with all leaves upright. Grain yields were 10,700; 12,200 and 11,400 kgha-1 respectively.

Ref: I. R. Cowan (1968). J. applied ecology 5, 367-379 in the interception and absorption of

radiation in plant stands.

The interception and absorption of radiation in plant stands cannot be treated in isolation with the

geometry and orientation of plant foliage parts. Thus the spatial distribution, the orientations, the

size and optical properties of the leaves as well as the angular and spectral distribution of radiation

incident on the plant community are of primary importance.

Interception of radiation incident on a stand:

When solar energy is radiated into a plant stand, part of the radiation will penetrate the stand

without suffering interception by the leaves. The unintercepted intensity I is averaged over a

horizontal plane at any level within the stand. The amount of radiation intercepted by the unit leaf

area may be equated to the decrease of I per unit length of optical path expressed in terms of

cumulative leaf area.

Thus,

p = mean area projected per unit leaf area of leaves on a plane normal to the radiation.

By integration:

IL = intensity of a beam incident on the top of the stand

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L = leaf Area Index; A = cumulative leaf area

In a stand of plants, the flux of unintercepted radiation varies exponentially with height. In a stand

of horizontal leaves, the penetration and interception are independent of the angular distribution. In

stands of other leaf habits, the radiation incident at large angles is more efficiently intercepted than

that incident at small angles. Thus, the angular distribution of radiation is modified by the stand so

that the more nearly normal components of diffuse radiation can penetrate to a relatively greater

depth.

The condition of the sky leading to isotropic diffuse radiation is described by Anderson (1966) as

uniformly overcast (UOC). The diffuse radiation from the sky is never truly isotropic – even at the

extremes of complete absence of clouds or a complete cloud cover. Anderson therefore prefers to

deal with the penetration of light from a stand overcast sky which makes allowance for greater

brightness near the zenith than near the horizon.

Exchange of radiation within the leaf canopy

For a longer wave radiation, the transmission and reflection coefficient are negligibly small and

therefore, the amount of radiation reflected, transmitted and emitted downwards by a leaf layer is

equal to the emissive power of a black surface at leaf temperature. Since the variation of leaf

temperature in a crop is usually small, the emissive power of a black surface at leaf temperature is

taken to be constant. If the irradiative temperature of the sky were to be the same as that of the

leaves, then the flux density will be equal to the emissive power of black surface at the leaf.

Radiation in a stand of horizontal leaves

The attenuation of radiation fluxes is not dependent on the angular distribution of radiation, hence

the optical properties of plant leaves vary with the wavelength of the radiation. Gate (1965)

examined the absorption of cottonwood and found out that a split of solar spectrum into three parts

may suffice for crude radiation fluxes. The reflection coefficient of the ground (RG) , has been taken

as 0.10 and 0.20 for the radiation fluxes of wavelengths 0.31-0.73 and 0.73-1.2μ in a stand of

horizontal leaves respectively (Bowers and Hanks 1965). The similarity between the absorption

profile of radiation of all wavelengths and that of visible radiation is interesting. It occurs because,

first, because little absorption of radiation in the band 0.73-1.2 μ occurs, and second, the absorption

of radiation in the band 1.2-5.5 μ is largely compensated by the net loss of thermal radiation. It

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seems likely that an approximate similarity of this kind may occur quite frequently in real

communities, with clear skies and large income of solar radiation.

A. DIFFERENCES IN NITROGEN USE EFFICIENCY IN C3 AND C4 PLANTS AND ITS IMPLICATIONS IN ADAPTATION AND EVOLUTION

The discovery of the C4 cycle of photosynthetic CO2 fixation stimulated intensive study in

differences in photosynthetic efficiency among plant species. The higher CO2 uptake rates in leaves

of species with C4 photosynthesis results, at least in part from high concentration of CO2 in the cells

surrounding the vascular bundles (bundle sheath cell, BSC), in which the pentose phosphate CO2

fixation cycle (C3) is operative. This high concentration of CO2 in the BSC is apparently due to the

high activity of PEP carboxylase and its affinity for CO2 in mesophyll cell (MC) and the rapid

movement of resulting carboxylation products, malate and aspartate, to the BSC where they are

decarboxylated.

The high concentration of CO2 in BSC has the advantage of suppressing photorespiration by

reducing the competition of O2 for CO2 reaction sites on ribulose diphosphate carboxylase

(RuDPC). The cleavage of RuDPC in the presence of O2 results in formation of phosphoglycolate –

the substrate for photorespiration. So, the higher photosynthetic rate in leaves of C4 species results

from concentrating CO2 around RuDPC and eliminating CO2 loss in photorespiration.

The higher photosynthetic capacity of C4 leaves gives these plants advantages in certain ecological

situations; though the advantages of the C4 cycle in crop production has been questioned. One

advantage of C4 plants is their greater water use efficiency i.e. the ability to produce more dry

matter per unit of water transpired (Shantz, H. L., and R. L. Piemeisel 1927). This greater water use

efficiency by C4 plants results from higher CO2 fixation and lower transpiration rates in C4 species

compared to C3 species. This advantage has lead to the speculation that C4 plants evolved under dry

climates.

The lack of inhibition of photosynthesis by O2 in C4 species and their ability to extract CO2 from

atmospheres low in CO2 suggests that C4 photosynthesis may have evolved during geologic periods

of elevated atmospheric O2 content and/or lower CO2 content. Finally, the adaptation of C4 species

17

to conditions of high irradiance and high temperature indicates that C4 plants may have evolved in

response to such environment.

Nitrogen Use Efficiency

Another characteristic difference between C3 and C4 species, at least among Gramineae, is their use

of Nitrogen. Grasses depend primarily on inorganic forms of Nitrogen and growth responds

strongly to increased nitrogen supply (SS). In fact, productivity of cultivated grass crops depends

heavily on Nitrogen supplied from commercial inorganic sources.

Nitrogen use efficiency is defined as biomass produced or CO2 fixed per unit of nitrogen in plant.

The main difference in Nitrogen use efficiency of C3 and C4 species appears to be based on

partitioning of N among leaf proteins and the related CO2 fixation pathways.

Cultivated tropical (C4) grasses respond to applied Nitrogen to a much greater degree than

temperate (C3) grasses. In addition, the nitrogen concentration in leaves of tropical grasses is

considerably lower than in temperate species. Wilson (1975) showed C4 species have a higher

relative growth rate over a wide range of leaf concentration than the C3 species. It therefore means

that in areas where soil Nitrogen is very low, the higher Nitrogen use efficiency by C 4 species

would be of a particular advantage.

The greater Nitrogen use efficiency of C4 species appears to be associated with the

compartmentalization of PEPC and RuDPC in mesophyll and bundle sheath tissues respectively.

Implications of Nitrogen Use Efficiency in Adaptation and Evoloution of C4 Species

The greater Nitrogen use efficiency of C4 plants may give them an adaptive advantage, particularly

on sites low in Nitrogen. The advantage may also me be extended to sites high in Nitrogen, since it

is commonly observed that C4 species become aggressive weeds in cultivated fertile fields. It is

entirely possible that this advantage in Nitrogen use efficiency was a major factor in the evolution

of C4 plants and C4 Photosynthetic pathways or photosynthesis. The percentage of C4 grasses tends

to increase and soil Nitrogen tends to decrease as the climate becomes warmer and drier. It is

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therefore possible that C4 grasses evolved in the dry tropics in response to a combination of factors,

including high temperatures, water stress and Nitrogen availability.

NB: plants which exhibit the C4 dicarboxylic acid pathway possess certain anatomical and

biochemical characteristics which may be used to distinguish them from C3 plants. The

characteristics are:

Low CO2 compensation concentration (Γ)

Lack of apparent photosynthesis

Requirement of high light intensity for saturation of photosynthesis

High temperature optimum for net photosynthesis

Presence of a specialized layer of BSC around the leaf vascular tissues

The formation of C4 dicarboxylic acid as the initial product of photosynthesis in contrast to

plants possessing the C3 pathway of CO2 fixation (Hofstra and Hesketh, 1969)

Reference: Monteith, J. L. (1972): Solar Radiation in Tropical Ecosystems. J. applied Ecology

9 (3) 741-766

The efficiency with which plants store solar energy can be expressed as the product of f factors

which describe the dependence of dry matter production on latitude and season (εg), on

cloudiness and aerosol content of the atmosphere (εa), on the spectral composition of the

radiation (εs) and the quantum needed of the photochemical process (εq), on the leaf index and

leaf arrangement (εi), on the concentration of CO2 in the canopy and diffusion resistance of

individual leaves (εd) , and the fraction of assimilate used for photosynthesis (εr). The analysis

can be extended to estimate animal and human populations from the relation between

production and rates of metabolism.

LIGHT ENERGY

The general global distribution

All life on earth is supported by the stream of energy radiated by the sun into the biosphere.

Even the relatively small amount of the sun’s energy bound in the form of latent chemical

energy by the photosynthesis of plants suffices to maintain the biomass and the vital processes

of all members of the food chain.

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By far the larger fraction of radiation absorbed is transformed immediately into heat; part is

used in the evaporation of water and the rest produces an increase in the temperature of the

earth’s surface.

*Radiation within the Atmosphere

The biosphere receives solar radiation in the range of wavelengths (λ) between 290 nm and

about 3,000 nm. Radiation at shorter λ is absorbed in the upper atmosphere by O3 and

atmospheric O2, and the long λ cut-off is caused by the air’s content of water vapour and CO2.

About 40-45 % of the energy incident from the sun consists of λs between 380 and 720 nm.

This is the region of the spectrum we perceive as visible light. The chloroplasts pigments absorb

radiation between 380 and 740 nm. Below this range is UV radiation and above the range is the

infra-red radiation. Between 400 and 700 nm is utilized in photosynthesis – PhAR – long wave

radiation.

Attenuation of Radiation of Radiation by the Atmosphere (see* above)

At the outer limits of the earth’s atmosphere, the intensity of radiation is 1360 Wm -2 (1 W = 1

Js-1) or 1.36 KJm-2s-1. This figure is known as the solar constant. Of this only, an average of 47%

reaches the earth’s surface. Of the incident sunlight, 25% is reflected by clouds and 9% is

scattered by atmospheric particles and returned to outer space. A further 10% is absorbed by

clouds and 9% by water vapour. The radiation eventually reaching the ground or the plant cover

is composed of direct sunlight (averaging 24% of the incident radiation), diffuse radiation from

the sky (sky light, 6%) [24+ 17+ 6 = 47%].

Depending on the latitude of a site, its attitude above sea level, the nature of the terrain and the

frequency of clouds, there are large regional and local differences in the supply of radiation.

Thus, the high pressure regions in the tropics, where clouds are few that receive greater than

average quantity of solar radiation. At greater altitudes, owing to the shorter optical path of the

rays and the lesser degree of air turbidity, more radiation reaches the ground than in lower-lying

places.

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Solar Radiation and Productivity in Tropical Ecosystems (Monteith 1970) The incident energy on the earth’s atmosphere is 136 KJm -2s-1 (solar constant). As the energy

stored by plants is about 17 KJ per gram of dry matter, the solar constant is equivalent to the

production of dry matter at the rate of about 1gm-2 every 12 s, 7.2 kgday-2 or 2.6 tm-2year-1. The

annual yield of agricultural crops ranges from a maximum of 30-60 tha-1 in field experiments to

less than 1 tha-1 in some forms of subsistence farming.

The efficiency, E with which crops or natural communities produce dry matter is defined as the

net amount of solar energy stored by photosynthesis in any period, divided by the solar constant

integrated over the same period. This efficiency depends on many factors and Monteith (1070)

put forward a model that explicitly defines seven factors related to radiation and plant

characteristics that determine efficiency. This model allows a breakdown of the factors that

determine production and provide a means of easy manipulation of these factors for improved

productivity.

E= E (g, r) = Eg Ea Es Eq Ei Ed Er

Eg is determined by the transparency of the earth’s atmosphere

Ea is determined by the cloudiness and aerosol content of the atmosphere

Es is determined by the spectral composition of solar radiation and optical properties of foliage

Eq is related to the number of light quanta needed in photosynthetic process

Ei is determined by the fraction of radiation intercepted by canopy

Ed is determined by the finite rates at which CO2 molecules can diffuse from atmosphere to the

surface of the photosynthetic unit in green cells.

Er is determined by the fraction of assimilates not used for respiration.

The geometric factor, Eg, is the ratio of the amount of radiation received on a plane parallel to

the surface of the earth to the solar constant. This factors which varies with latitude is seen more

or less constant in the tropics with an annual oscillation of ± 0.06 at 200 latitude. The annual

average value of Eg decreases from about 0.3 in the tropics to 0.2 in temperate latitudes.

Eg, atmospheric transmission: this is the radiant energy that would be received, in the presence

of O3 and water vapour and in the absence of clouds and particulates expressed as a fraction of

the extra terrestrial radiation on a horizontal surface, that is as a fraction Eg and solar constant.

Ea at Samaru 110 N, 80 E.

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FIG

Average Ea at Samaru is 0.58. There is a loss of 0.15 of the extra terrestrial flux in the months of November-February because of harmattan.

The spectral factor Es: about 0.40-0.45 of the radiant energy is PhAR. This figure is closer to

0.50 because of irradiation by clouds and particles – sunlight etc.

Of the PhAR, 0.85 of it is absorbed by leaves. This is extremely variable depending on factors

such as amount of chlorophyll per unit area of lamina and leaf orientation. The fraction of the

whole spectrum radiation absorbed by green leaves is therefore about 0.50 × 0.85 = 0.425. this

represents Es.

Photochemical efficiency, Eq: this is the efficiency with which solar radiation is bound in CH2O.

The formation of one molecule of CH2O requires one molecule of CO2 and 10 light quanta. The

average energy content of one quantum of PhAR is 3.6 × 10-19 J. the amount of heat stored in

one molecule of CH2O is 7.7 × 10-19 J. The maximum efficiency of a 10-quantum process is

therefore

Formula equation = =

Westlake (1963) suggested that there are 16.7 KJ of energy in one gram of dry matter. Bearing

in mind the proportion of whole spectrum absorbed and maximum efficiency, 1 KJ of solar

energy is equivalent to of dry matter. It follows that if

P is the rate of dry matter production and I, solar irradiance, we may have this relationship:

P = 20I. This is the maximum photosynthetic efficiency (theoretical limit). Achieved in

principle only when the irradiance and the gross rate of photosynthesis are very small.

Diffusion efficiency, Ed

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When irradiance increases, intercellular CO2 decreases because of the finite rate at which the

molecules are transported to the chloroplasts by diffusion from the external air and from

respiring mitochondria. This decrease in the availability of CO2 is responsible for characteristic

shape of the photosynthesis – light curve.

FIG

As the irradiance tends to 0, the gross rate of photosynthesis per unit incident radiation tends to

a constant value – the limiting slope (for CH2O 20 gh-1KJs-1). As the irradiance increases,

the photosynthetic rate falls further and further below the maximum rate of and approaches

a limiting value, . In this state known as “light saturation”, the rate of photosynthesis is

proportional to the concentration of CO2 in the external atmosphere. Values of determined

for crop plants exposed to air at 300 vpm of CO2 range from about 2 g CH2O m-2h-1 for cotton to

about 8 m-2h-1 for maize and sugar cane (C4 plants).

The total resistance to the diffusion of CO2 from the external air to the chloroplasts is 14 a scm-

1 , where is in g CH2O m-2h-1 so that value quoted for a are equivalent to resistances rnging

from about 7 s cm-1 for the least efficient group of species to about 2 scm-1 for C4 plants. As the

minimum stomatal resistance reported for a wide range of species are about 1- 2 s cm-1, these

figures suggest that stomatal control of photosynthesis is likely to be important in C4 plants.

Interception efficiency, Ei: the interception efficiency of a stand can be defined as the ratio of

the actual rate of gross photosynthesis to the maximum rate estimated for a stand of identical

plants with enough leaves to intercept the entire incident light. Measurements in controlled

chambers have shown that photosynthesis in maize and cotton is proportional to the intercepted

radiation. Intercepted radiation can be measured or inferred from LAI.

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The photosynthetic rate of a crop canopy cannot be strictly proportional to the fraction of

intercepted radiation unless all leaves are working at the same photochemical efficiency. This

implies either all the leaves are:

1) Working on the linear portion of the photosynthetic/light curve; or

2) Exposed to the same radiant flux and respond to the light in the same way.

Condition (1) is satisfied under weak light conditions - dull days or at sunrise or sunset.

FIGInterception efficiency as a function of LAI and the geometric factor respiration factor Er:

The respiration factor can be defined as the ratio of (P-R): P = 1 – R/P where R is the weight of

CH2O used for respiration per day calculated by multiplying the mean dark respiration rate (gm -

2h-1) by 24; and P is the weight of CH2O produced by photosynthesis over the same period

calculated by adding the dark respiration rate to the uptake of photosynthesis hour by hour

throughout the day. In any plant community, the ratio of reparation to photosynthesis over the

same period depends on many internal and external factors, notably:

1) The growth rate of the stand.

2) The fractions of the total dry weight represented by leaves and other photosynthetic organs.

3) The temperature of the respiring tissue. The traditional figure quoted for E r is 0.20 – 0.25 but

figures up to 0.75 have been quoted.

The proportion of the assimilate used for the respiration is expected to be larger in tropical than

in temperate climates because the temperature coefficient for respiration of green tissue exceeds

the coefficient for photosynthesis over the range of temperature in which plants usually grow. It

has been shown that respiration is a linear function both of the photosynthesis rate and the

standing dry weight (W) and of tropics at 250 C. Er = 0.65 – 0.021 W/P

An arbitrary value of 0.50 is usually adopted for the tropics.

In subsistence agriculture, crop growth is limited by a serious shortage of nutrients and water.

The main effect of these shortages is to restrict leaf development so Ei is set at 0.05 and

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assuming that photosynthesis is also checked, Ed is set at 0.20. a respiration factor is assumed

for a temperate climate to allow for lower mean temperatures.

The interception factor Ei emerges as a major factor or discriminator of dry matter production

accounting for:

(i) Differences of productivity under different conditions of climate and management and

(ii) Differences between the mean and maximum rates of production within a particular

stand.

The main limitations of the models currently used to predict crop growth from photosynthesis

rates are:

(i) Ignorance about whether the decrease of Ed as a leaf ages is a result mainly of an

increase in the resistance to CO2 diffusion within the mesophyll tissues or a decrease in

the concentration of carboxylating enzymes.

(ii) Ignorance about the way in which rates of respiration are related to the contemporary

rate of photosynthesis and to the accumulated dry weight of the stands and

(iii) Ignorance about ways in which the partitioning of assimilates may be determined by an

interaction of environment factors and endogenous controls. The need to use

measurements instead of prediction of LAI is a weakness of most models (Kowal and

Knabe: p.12 page 13-23).

NB: The role of temperature in the life of a plant

Seed germination: dry seeds are extremely tolerant of temperature for a short time

without ill effects. Some have survived temperature close to absolute zero (-2730 C) and

others remain alive after heating to 1200 C for 30 min. the ecological significance of this

response to diurnal alternation of temperature may be that it promotes germination of

those seeds that are close to the soil surface. This is because alternation of temperature

may increase the permeability of membrane and seed coats.

Bud dormancy: plants commonly become dormant before the onset of winter and growth

occurs in the following season from resting buds. The breaking of bud dormancy may be

controlled by photoperiod, temperature or, more usually, a combination of the two.

Cell growth: the rate of elongation in plants increases almost with increase in temperature

between about 13-300 C and above this rate decreases sharply. Cell elongation is sensitive

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to temperature. Water uptake is basically a physical process and so rapid osmotic

temperature. Temperature affects both mitosis and cytokinesis.

Vernalization: some plants need to be exposed to a period of low temperature before they

will flower.

Fruit ripening and storage: fruits tend to grow fast and ripen as temperature increases up

to an optimum level.

Thermotropism and thermonasty

Temperature is loosely connected to light. Most of the light is converted to heat. In the equatorial

zone, the light shines directly.

Energy is neither created nor destroyed but can be changed from one form to another.

Conduction is actually more noticeable in gas and liquids. These a generation of energy to plants.

A plant looses heat by the process of conduction and convection if a plant looses water through

latent heat of evaporation.

Qa = Qr + Ql + Qc + Qm + Qs

Qa total incident radiation

Qr = total re-radiated as infra-red

Qe = latent heat of evaporation

Ql = heat of conduction and convection

Qm = energy balance for the metabolism

The heat balance varies with the community

Qi + Qm + Qp + Qs + Qh + Qe = 0

The magnitude will depends on the magnitude of others

The role of temperature in the life of a plant

A lot of energy is required to evaporate water. In plants, water shortage is a perennial problem.

Some plants, just to avoid this shortage, close their stoma. One of the ways of avoiding this

shortage is by CAM

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The best adaptation for water shortage is therefore the CAM. In terms of aridity, most plants that

occur in tropical rain forests are C3 plants. So even if the stomata are closed, the plants are still

able to photosynthesize

In the desert, plants follow the CAM way.

Heat injuries in plants (high temperature effect)

If we have a plant and the plant is freshly, it will be shown that the temperature will rise by 110 C

higher than that of its environ.

The quantity of water in the leaf is another factor. Leaves that are wilted able to survive high

temperatures than those that are turgid. The temperature of the soil is higher than that of the

plant.

Four ways by which heat injury can be shown in a plant/ causes of heat injury

1. Starvation

2. Chemical toxicity

3. Biochemical lesion

4. Protein breakdown

5. Effect on metabolism

6. Injury by desiccation

Starvation: this is obvious because if the temperature is increased in plants, the rate of

photosynthesis is not equal

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PHYSIOLOGICAL PLANT ECOLOGY

Physiological Plant Ecology deals with function of plants in their natural environment of habitat; as well as their interaction with each other and their environment. Another name for Physiological Plant Ecology is Physio-ecology or Eco-physiology. It is concerned with physiological processes of a particular plant/organism as affected by quantified environment.

There are two basic approaches when studying Physiological Plant Ecology (PPE). These are:

1. To assume that the performance of a particular organism, no matter where it is found, is genetic. This type of approach is referred to as Teleological. This is to say that the study is concerned with the adaptive characters of the organism during the Ontogeny and Phylogeny of that particular organism.

2. The second approach is descriptive, that is, it is concerned with the quantification of the energy relationships between the organism and its environment.

If we follow the teleological approach, we will end up with a feeling that the environment modifies the adaptive ability of the organisms e.g. organisms that live in the dark and those that live in illuminated environments. Physiological Plant Ecology in this sense can be taken to mean the study of the distribution of life over earth’s surface. By this procedure, the Physiological Plant Ecology becomes Autecology.

The second approach is described as being advantageous because it works with a few equations that describe the mono-climate.

ENERGETICS, ECOLOGY AND EVOLUTION

Here we are concerned with the relationships between optimization theory, strategies of resource use and fitness as well as out-lining Phylogenetic trends in bioenergetics processes via resource acquisition and allocation. It also points out what fitness means with respect to physiological context and how it could be applied to the more traditional and genetical ones.

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Mayr (1963) argued that evolution by natural selection was ‘differential perpetuation of genotypes’. This attracted the attention of many critics who proposed that the theory of evolution is irrefutable and therefore unscientific. To clear the air, Mayard Smith (1969, 1978a) upheld that a hypothesis can be tested by comparing its prediction or by a direct test of validity of the assumptions it incorporates. The assumptions of Neo-Darwinism are concerned with heredity and variability. Both assumptions had no solid scientific backings. But from a wide variety of evidence derived from molecular biology, cytology and breeding experiments, we are convinced that genes are particulate and can only be modified through random mutation. Consequently, the fitness of a genotype refers to the relative contributions which carriers of theat genotype make to the gene pool of future generations. The word fitness therefore can be viewed and used in the following ways:

Firstly, consider the Neo-Darwinism approach otherwise called the Neo-Darwinism fitness. This measure the effects of a selective advantage on the abundance of genes (s) coding for it; relative to other gene frequency or more technically as a coefficient of selection (Haldare, 1924), Malthusian parameter (Fisher, 1930) or fitness space (Wright, 1968).

The second approach is called the adaptationist’s fitness. This is concerned with the casual basis of selective advantage in terms of the interaction between the phenotype and its environment. This is a more difficult approach.

There is yet another type of approach called the phenotypic measure of fitness. This approach believes that natural selection will tend to produce organisms which are maximally effective at propagating genes.

METHODS OF MEASURING FITNESS

The methodology of measuring fitness involves two approaches. These are Posteriori Approach and Priory Approach. The posteriori approach is based on inductive philosophy (Propper, 1959). The problem associated with this type of approach is that it often confuses correlation with cause. This affects the relationships and results in the formulation of the law-like statements when less strong inferences are more appropriate. This approach though with its problems, throws light on how organisms actually meet the challenge posed by different kinds of ecological circumstances. On the other hand, the priori approach is based on the assumption that natural selection is an optimizing process.

The most unequivocal measure of the effect of a particular selective advantage is in terms of the spread of genes while the cause of a particular selective advantage could be understood in terms of the way the phenotype interacts with the environment. This interaction involves the uptake and utilization of resources and depends on the physiological and behavioural functioning of the organism. It is therefore assumed that the qualitative form of resource input puts no constraints on the way that it is allocated between growth, storage, reproduction etc.

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RELATIONSHIP BETWEEN PLANT PRODUCTION AND LIGHT

How does light affect the life of plants?

In discussing this question, emphasis will be placed on radiation uptake. The amount of radiation received by the plant is equivalent to 1.36KJm-2s-1. But in terms of physiological process, such as photosynthesis, the light comes in form of quanta (m-2s-1).

About 45% of the radiation goes to the environment

30