FACTORS AFFECTING SEED VIGOUR

To achieve maximal seed vigour of a given

cultivar in seed production, efforts must focus on:

Producing a seed crop in the best possible

environment for development of vigorous seeds

Harvesting as soon as possible after physiological

maturity (PM)

Handling, conditioning, and storing seed to

minimize damage, slow deterioration

INITIAL SEED QUALITY

High initial viability of seeds maintains their quality

in storage longer than those with less initial

viability. Vigorous and undeteriorated seeds can

store longer than deteriorated seeds. Seeds that have

been broken, cracked, or bruised due to handling

deteriorate more rapidly in storage than undamaged

seeds.

Cracks in seeds serve as entrance to pathogens

causing consequent deterioration. Seeds that have

been developed under environmental stress

conditions (such as drought, nutrient deficiency

and high temperatures) become more susceptible to

rapid deterioration.

NUTRITION The structural and textural status of the soil, its fertility

level, pH, microbial environment.

In the nutrition of seed crops, nitrogen, phosphorus,

potassium and several other elements play an important role

for vigorous seed production.

It is advisable to know and identify the nutritional

requirements of seed crops and apply adequate fertilizers.

Adequate fertilization results in good seed development and

maturation.

Adequate supply of nitrogen is very important for a good

healthy seed development.

Severe nitrogen deficiency in carrot, lettuce, and pepper

resulted in poor seed development.

High dose of nitrogen reduces development in seed due to

accumulation of germination inhibitors

A good supply of phosphorus helps in good seed development.

Phosphorus deficiency retards overall growth and development.

It should be applied in the soil before sowing

Excess quantity of nitrogen prolong the growing period and

delays the seed maturity.

Time of application of nitrogen is important.

The second application often leads to an increase in quality seed.

In some crops dressings at flowering tends to delay in seed

ripening.

In certain crops, a side dressing of phosphorus is also

applied at the time of flowering.

The P reserves in the seed in the form of phytic acid and

acts as a antioxidant.

Deficiency of P causes watercress seeds.

Potassium plays an important role in flowering and seed

development.

Helps in synthesis of proteins and fat in oil crops.

Severe deficiency of potassium in pepper resulted in a

higher percentage of abnormal seed production.

TEMPERATURE

Most of the crops require moderate temperatures for flowering

and pollination such that good seeds are formed.

Too high temperatures cause desiccation of pollen resulting in

poor seed set.

If hot dry weather conditions prevail during flowering many

crops such as vegetables, legumes and fruit trees fail to set

vigourous seeds effectively.

Vegetables, legumes, fruit crops require cool conditions to flower

and pollinate normally.

Though oil crops can withstand hot periods during flowering,

very high temperatures result in premature flowering, and

production of poor quality seeds.

Very cold temperatures may also damage seed quality especially

in the early phases of seed maturation

AFFECT OF TEMPERATURE ON SOME CROPS

Very low temperature (0⁰C and below)damages ripening of corn

seed. (Rossman, 1949).

In Lettuce koller (1962) noted that when the seeds matured at

high temperatures, germination was less at 26⁰C in the dark than

the corresponding low temperature matured seed.

Temperature differences during ripening also altered the

dormancy patterns of wheat (Van Dobben, 1947; Kramer, Pest,

Witten, 1952).

In Mungbean, Dharmalingam (1982) showed the late summer

sowing in Tamil Nadu resulted in the production of high % of

hard seeds.

CROP CLIMATIC FACTOR CHANGES IN SEED

CHARACTER

Sorghum Rainfall or high

humidity at maturation

Blackening of seed due

to black mould

Pulses Rainfall at maturation Off coloured seed

Peas High humidity at

maturity

Mottled seed

Groundnut Rainfall at harvest Insitu germination

Brinjal High temperature at

flowering

Pseudo styled flowers

Tomato Rainfall at harvest Insitu germination

Bhendi Rainfall at maturity Off coloured seed

RAINFALL

MOISTURE STATUS OF SOIL

For good-quality seed, a relatively dry climate during the

ripening phase is preferred.

Even for a wetland crop like rice, a dry climate during grain

ripening phase produces seeds of good quality

Adequate soil moisture is essential for good seed development.

Soil with high moisture due to high irrigation or high rainfall

may lead to seeds of low nitrogen and protein content in case of

wheat.

Drought during flowering might interfere with fertilization, thus

seed vigour is reduced.

Weight and size of seed which are usually correlated with

vigour, are reduced by drought during seed development and

maturation.

extreme water deficit stimulates premature

desiccation, and affect the quality of seed.

as such seeds badly affected by pre-harvest rains

should not be stored for planting purposes.

Association of water deficit and thermal stress during soybean seed filling

(Franca Neto and Krzyzanowski, 2010).

PLANT PROTECTION CHEMICALS

Herbicides and pesticides applied to the soil or tothe growing crop may affect the development ofseed and influence its quality. If the concernedherbicide or pesticide is not easily biodegradable.

Increase in the protein content of wheat with subherbicidal doses of Simazine (Ries, Schweizer, andChmiel, 1968).

Ramamoorthy (1990) studied the effect ofFluchloralin, Pendimethalin and Oxyfluoren appliedand observed tat there was no effect on vigour ofgroundnut seeds before storage but after storage theuse of herbicides other than fluchloralin, resulted inbetter seed vigour.

HARVEST FACTORS

Seed quality is highly affected by harvesting and

handling methods.

Harvest and post-harvest deterioration comprises

threshing, processing machinery, seed collection,

handling, transporting and drying.

Mechanical damage is one of the major causes of

seed deterioration during storage. Very dry seeds are

prone to mechanical damage and injuries.

Such damage may result in physical damage or

fracturing of essential seed parts; broken seed coats

permit early entry and easy access for microflora,

make the seed vulnerable to fungal attack and reduce

storage potential (Shelar, 2008).

Soybean seeds germination dropped from 93% for seed harvested on

October 9 to 48% for seed harvested on December 11.

The field emergences of 3 categories of soybean seeds, namely non

broken, lightly broken, and moderately broken, were 96, 72, 52 per

cent, respectively (Moore, 2007) indicating the poor performance of

even lightly injured seed subjected to stress conditions in the field.

The thin coat of flat seeded sesame poses a problem even with manual

harvesting and processing in India and significant reduction of vigour

is encountered following storage.(Atkin, 1998).

Rain soaked and subsequently dried soybean lead to substantial loss of

vigour in storage (Saha and Basu, 1984).

FIELD WEATHERING

Adverse environmental conditions during seed filling and maturation

result in forced seed maturation, which is associated with low yields,

leading to a significant decrease in quality and an extensive reduction

in the crop productivity (Franca- Neto et al., 2005; Pádua et al., 2009).

After physiological maturity if the seeds are retained on mother plant

seeds will deteriorate, physiological changes in seed may lead to

formation of rigid seeds or off colour seeds in pulse crops (Khatun et

al., 2009).

Harvest delays beyond optimum maturity extend field exposure

and intensify seed deterioration.

Weathering not only lowers seed germination, but also increases

susceptibility to mechanical damage and disease infection. Timely

harvesting avoids prolonged exposure to moisture, and is the best

means of avoiding weathering.

POST-HARVEST FACTORS/STORAGE FACTORS

Storability of seeds is mainly a genetically regulated character

and is influenced by quality of the seed at the time of storage, pre-

storage history of seed (environmental factors during pre and

post-harvest stages), moisture content of seed or ambient

relative humidity, temperature of storage environment, duration of

storage and biotic agents (Shelar et al., 2008; Baleseviæ-Tubic et

al., 2005; Khatun et al., 2009; Biabani et al., 2011).

Damage of seed during storage is inevitable (Balesevic-Tubic

et al., 2005).These environmental conditions are very difficult

to maintain during storage. The seed storage environment

highly influences the period of seed survival.

After planting of deteriorate seeds, seedling emergence may be

poor and transmission of pathogens to the new crop may occur.

Lower temperature and humidity result in delayed seed

deteriorative process and thereby leads to prolonged viability

period (Mohammadi et al., 2011).

(Mohammadi et al., 2011).

The rate of seed deterioration is highly influenced by

environmental (temperature, relative humidity and seed

moisture content) and biological factors (such as fungi that

create their own biological niche) (Ghassemi-Golezani et al.,

2010).

Seed longevity is determined by seed moisture,

temperature and seed attributes that are influenced by

genetic and environmental interactions during seed

maturation, harvesting and storage (Walters et al., 2010).

Several other factors such as environmental conditions

during seed producing stage, pests, diseases, seed oil

content, storage longevity, mechanical damages of seed in

processing, fluctuations in moisture (including drought),

weathering, nutrient deficiencies, packaging, pesticides,

improper handling, drying and biochemical injury of seed

tissue can affect vigour of seeds (Krishnan et al., 2003;

Marshal and Levis, 2004; Astegar et al., 2011.

KIND/VARIETY OF THE SEED

The seed storability is considerably determined by the kindor variety of seeds. Some seeds are naturally short-lived,e.g., onion, soybeans, peanuts, etc., whereas some seedslike, tall fescue and annual rye grass, appear very similarbut differ in storability.

Genetic make-up of varieties also influences storability.

Genotypic factors

Some types of seeds are inherently long lived; others areshort lived, while others have an intermediate life spanowing to their differences on genetic makeup.

EFFECT OF TEMPERATURE

High temperature hastened the rate of these biochemical

processes triggering more rapid deterioration that resulted

in rapid losses in seed having high moisture content (Shelar

et al., 2008).

Seeds sensitivity to high temperatures is strongly dependent

on their water content, loss of viability being quicker with

increasing moisture content (Kibinza et al., 2006).

Temperature is important because it influences the amount

of moisture and also enhances the rate of deteriorative

reactions occurring in seeds as temperature increases.

(Shelar et al., 2008).

EFFECT OF MOISTURE CONTENT

Deteriorative reactions occur more readily in seeds athigher moisture content and subsequently, this conditionconstitute hazard to the longevity of seed survival(Vashisth and Nagarajan, 2009).

Seeds stored at high moisture content demonstrateincreased respiration, heating, and fungal invasion resultingin reduced seed vigour and viability.

After physiological maturity the rate of seed quality lossdepends on the degree of unfavourable environmentalconditions surrounding the seed.

Environmental moisture, predominantly intermittent orprolonged rainfall, during the post maturation and pre-harvest period, is quite detrimental to seed quality andcause rapid deterioration.

When exposed to humid conditions (heavy rain), dried seedscan absorb enough moisture to reach 27% and subsequentlyexpand in volume. At this moisture level, seed respiration ishastened.

Cotyledonary reserves will be consumed, not only by theseed itself, but also by fungi allied with the seed.

It has been reported that seed moisture content of about 6-8% is optimum for maximum longevity of most crop species.

Below 4-6% seed moisture content lipid autoxidation becomesa damaging factor and seeds become more susceptible tomechanical damage.

The moisture content of seed during storage is the mostpersuasive factor affecting the longevity.

Storing seeds at high moisture content enhances the risk

of quicker deterioration at shorter time.

Seeds are hygroscopic in nature; they can pick up and

releases moisture from and to the surrounding air.

They absorb or lose moisture till the vapour pressure of

seed moisture and atmospheric moisture reach equilibrium

(Shelar et al., 2008).

Control of relative humidity is the most important

because it directly influences the moisture content of

seeds in storage as they come to equilibrium with the

amount of moisture surrounding them; a concept known as

equilibrium moisture content.

The lower the moisture content, the longer seeds can be

stored provided that the moisture level can be controlled all

through the storage period.

(Vashisth and Nagarajan, 2009).

EFFECT OF ORGANISMS ASSOCIATED WITH

SEEDS Organisms associated with seeds in storage are bacteria,

fungi, mites, insects and rodents. The activity of theseentire organisms can lead to damage resulting in loss ofvigour and viability or, complete loss of seed.

Bacteria and Fungi:

There are several factors which favour infection fungi and promote their infestation such as moisture content of seed and interspace relative humidity, temperature, prestorageinfection and storage pest.

Most storage fungi belong to Penicillium and Aspergillusgenera. They induce seed deterioration by producing toxic substances that destroy the cells of seeds.

Mechanically damaged seed allow quick and easy accessfor micro flora to enter the seed (Shelar et al., 2008).

To minimize the risk of fungi invasion, seeds have to be

stored at low moisture content, low temperature, and RH.

Researches show that all storage fungi are completely

inactive below 62% relative humidity and show very little

activity below about 75% relative humidity upwards, the

amount of fungi in a seed often shows an exponential

relationship with relative humidity.

The storage bacteria require at least 90% relative humidity

for growth and therefore only become significant under

conditions in which fungi are already very active.

INSECT AND MITES:

There is no insect activity at seed moisture contents

below 8%, but if grain is infected, increased

activity may generally be expected up to about

15% moisture content.

The optimum temperature for insect activity of

storage insects ranges from 28 to 38°C.

The temperatures below 17 to 22°C are considered

unsafe for insect activity. Although it is usually

preferable to control insect and mite activity by the

manipulation of the seed environment, i.e., use of

fumigants and insecticides.

The main problem of chemical control is the adverse

effect of chemicals on seed viability and vigour, and

some of them are dangerous to handle.

However, fumigants which have been used

successfully include methyl bromide, hydrogen

cyanide, phosphine, ethylene dichloride and

carbon tetrachloride in 3:1 mixture, carbon

disulphide and naphthalene.

Insecticides – used in seed storage include DDT,

lindane and Malathion.

PROVENANCE

Seeds obtained from different sources may show

differences in viability and storability.

Nevertheless, the seed begins its existence before

it harvest and it is expected that seeds harvested in

different pre-harvest condition.

Fluctuating environmental conditions

Fluctuating environmental conditions are harmful

for seed viability.

Rapid changes in seed moisture content and

temperature cause deleterious effect.

OXYGEN PRESSURE

Recent researches on the role of a gaseous environment on

seed viability indicate that increases in pressure of oxygen

incline to decreases the viability period.

Denaturation of cell constituents (membranes, enzymes, DNA)

only occurs under aerobic conditions (Roberts 1972)

Accordingly, high oxygen pressure promotes and low pressure

represses denaturation of these constituents. Storage under low

oxygen pressure, e.g. in vacuum or in CO2 at temperatures where

insects, fungi and micro-organisms are inactive prevents their

development.

Seeds stored at high moisture content (e.g. recalcitrant) do not

tolerate low oxygen pressure because oxygen is necessary for

respiration to sustain and for repair and turnover processes within

cells (Roberts 1983).