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Review of literature
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Tea [Camellia sinensis (L.) O. Kuntze] is one of the most popular beverages
throughout the world. Indigenous tea was discovered in India during 1823 by
Robert in Assam. After nine years (1832) tea was experimentally planted in Nilgiris
by Dr. Christie. In 1935, tea plants raised from the China seed were sent to Assam
and south India for planting. The tea bushes are pruned on every four or five years
to maintain their vegetative growth as well as to adopt crop-husbanding practices.
This will help to adopt the proper soil conservation practices and prevent further
degradation of soil resources for sustainable productivity of tea (Verma and Adbul
kareem, 1995). Tea being a foliage crop, the shoots is harvested at regular intervals.
In this process, nutrients are removed from the plant-soil system and they should be
replenished to maintain the nutrient status of the soil.
Soil fertility has been described as the capacity of soils to make nutrients
available to plants. The nutrient status of arable soils can be related to yields in
order to develop proposals for fertilization (Tisdale et al., 1985). Nutrient
management in tea plantations is an important aspect and nutrients are supplied
mainly through chemical fertilizers. However, it is widely accepted that a balanced
fertilizer application with efficient use of other inputs is the key to achieve higher
crop production (Karthikeyini, 2002). Repeated and heavy applications of chemical
/ mineral fertilizers lead to the deterioration of soil properties besides causing
environmental damage. The application of organic manures and bioinoculants
could minimize these problems as they are advantageous over chemical fertilizers
to improve soil fertility. Thus, it is felt necessary to integrate three different sources
of nutrients viz., organics, chemicals and bioinoculants, for more efficient and
economical productive system in the long run. Bioinoculants are environmentally
safer and a cost effective supplement to chemical fertilizers (Karthikeyini, 2002).
The literature pertaining to the use of organics and biofertilizers in tea crop is very
much limited. Hence, there is an urgent need to study the influence of straight /
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integrated application of organic manure and biofertilizers with possible reduction
of inorganic fertilizers on yield and quality of tea.
In recent years, the concept of PGPR mediated plant growth promotion is
gaining worldwide importance and acceptance. They are naturally occurring soil
microorganisms that colonize roots and stimulate plant growth. Such bacteria have
been applied to a wide range of plants for the purpose of plant growth enhancement
and disease control (Barka et al., 2000; Chakraborty et al., 2005). They promote
plant growth by several mechanisms including nitrogen fixation, phosphate
solubilization, hormone secretion and suppression of soil borne plant pathogens.
Disease suppression may be due to iron sequestration, production of antimicrobials
or induction of systemic resistance (Chakraborty et al., 2005). Thus, biofertilizers
containing beneficial organisms are cost effective, pollution free and a perennially
renewable source of plant nutrients, making them ideal partners and essential
supplements to chemical fertilizers. To maximize the beneficial plant growth
responses, it is important to identify the efficient strains of PGPRs for the planting
situation. Beneficial effects of PGPRs have been reported by various workers on a
wide range of crops including cereals, pulses, vegetables, oilseeds and plantation
crops (Alagawadi and Gaur, 1992; Bashan and Holquin, 1997; Riggs et al., 2001;
Muthuraju and Jaysheela, 2005; Jayaprakashvel et al 2006). Though scientific
information on the use of PGPRs in tea is meager, their usage should be having
many benefits as indicated by Tennakoon (2007).
Tea rhizosphere
In general, the rhizosphere effect is expressed by greater microbial activity
and bacteria are the group most stimulated by the rhizosphere (Katznelson, 1965).
In the rhizosphere, roots provide shelter to the microbial communities, resulting in
apparently higher rhizosphere: soil values, compared with the normally low
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rhizosphere: soil values recorded for the established tea rhizosphere. Cultivated tea
bushes grow in close proximity to each other and the root exudates may accumulate
in the rhizosphere from all sides, causing a marked inhibitory effect. Leaf litter may
also contain antimicrobial substances which could be slowly released upon
decomposition. (Pandey and Palni, 1996).
The rhizosphere is one of the largely unexplored frontiers in plant-microbes
interaction. The rhizosphere microbial population and communities influenced on
plant nutrition, plant diseases and root architecture (Patten and Glick, 2002). Both
direct and indirect promotion of mineral nutrition through increased root mining
from soil and functions of the root-colonized microorganisms are visible effects of
nursery inoculation. Increased scope of resistance to (escape from) deleterious
microorganisms and pathogens due to increased root density is an invisible effect.
There is also the possibility of carry forward of pathogen escape by niche exclusion
of the heavily colonized roots. Growth-inhibitory relationships or the antagonistic
behaviour of microbial groups growing around established tea roots may also result
in a reduced or smaller microbial population. This includes various categories of
antagonism, such as competition, antibiosis, parasitism, and predation. These
antagonistic activities in a suppressive rhizosphere may maintain a low microbial
(harmful) population in the rhizosphere. Also, the age of a plant should not be
neglected as a factor influencing the microbial population (Pandey and Palni 1996).
The bioinoculants, through natural selection and continued exposure to
antimicrobial metabolites, may have developed a kind of tolerance or resistance to
the inhibitory components of root exudates. Every plant species provides an
individual and specific site of microbial activity in the form of a rhizosphere.
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Biofertilizers
The concept of biofertilizers was developed with the discovery of nitrogen
fixing Azospirillum by Dobereiner and Day (1976) and phosphate solubilizers by
Pikovskaya (1948). Their population level naturally varies with soil and
agroclimatic zones. Further, the population level most often may not be enough to
bring out significant contribution to the plant nutrition as their efficiency varies
with strains. Biofertilizers are most useful and they are not yet explored much in tea
(Baby, 2002). Biofertilizer is a wide term which includes a diverse category of
bioinoculants such as nitrogen fixers, phosphate solubilizers, phosphate mobilizers
and plant growth promoting rhizobacteria (Jayaraj et al., 2004).
Since most of the plantation crops are long duration crops use of
biofertilizers may have some limitations. For example, in plantation crops
biofertilizers could be used more effectively during the plant establishment phase
either in the nursery or field to increase the health of the planting stock to enable
subsequent successful establishment in the field. In most of the plantation crops,
initial experimentation has been carried out to explore the possibilities to use
biofertilizers, including manures/composts, for enhancing growth and productivity.
Performance has been either equally good or better than inorganic fertilizers. The
mineral mobilizer, arbuscular mycorhizal fungus has been found to be quite
effective for growth improvement and 25% saving in phosphorus nutrition of
plantation crops and spices such as pepper, cardamom, cashew, coconut, ginger and
turmeric under field conditions (Nautiyal 2006). Similarly application of P –
solubilizing bacteria, Bacillus megaterium along with inorganic P improved P
nutrition of cashew plants and continual application of AMF, Azospirillum and
phosphate solubilizers has shown synergistic effect among themselves (Sharma,
2006).
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However, recent studies in tea microbial inoculants illustrated the efficient
strains of nitrogen fixing, phosphate solubilising or cellulolytic microorganisms
used for application in soil or composting organic matter (Verma et al., 2001).
Biofertilizers may supplement to the organic fertilizers too in recent years or it can
be integrated with chemical fertilizers to reduce the cost of production and
conserving the soil health in tea plantation. Following the initial research on
biofertilizers, there are several reports appeared on microbes in relation to fertilizer,
pest and disease management in tea (Baby et al., 2004 a, b; Ajay et al., 2005 and
2007; Ponmurugan and Baby, 2005).
Plant growth promoting activity by biofertilizers
Several soil microorganisms including nitrogen fixers and phosphate
solubilizers are known to produce plant growth promoting substances (PGPS). The
beneficial effect of bioinoculants is attributed to increase the nitrogen input from
biological nitrogen fixation, higher phosphate solubilization, production of plant
growth promoting hormones like auxins, gibberellins and cytokinins and reduction
of plant diseases and nematode infection. Beneficial effects reflect as direct plant
growth promotion and or including host resistance. Specific PGPR strain brings
about Induced Systemic Resistance (ISR) against multiple pathogens attacking the
same host. Broad spectrum of diseases control using PGPR strains would be an
effective and economical way to plant protection measures. Moreover, certain
PGPR strain mixture showed the synergistic action in growth promotion as well as
plant protection (Megha, 2005).
Phytohormones or plant growth hormones in the rhizosphere are originated
from plants through root exudates and also by microbial synthesis in situ, of which
microbial production is considered as the primary source. The phytohormones
induce plant growth as well as dry matter production. Inoculation with Azospirillum
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resulted in better growth and higher dry production in maize and was mainly
attributed to nitrogen fixation and the production of plant growth regulators
(Arshad and Frankenberger, 1991). The major hormone produced was indole – 3-
acetic acid (IAA) (Fallik et al., 1989). Other hormones detected at much lower, but
biologically significant level were indole -3 –lactic acid (Xie et al., 2005), indole -
3- methanol (Crozier et al., 1988), unidentified indole compounds (Hartmann et al.,
1983), abscissic acid (ABA) (Kolb and Martin, 1985), cytokinins and indole – 3-
butyric acid (IBA) (Fallik et al., 1994) and several gibberellins (Bottini et al.,
1989). Frietas and Germida (1990) showed that the inoculation of P. aeroginosa, P.
cepacia, P. Putida and P. fluorescens strains on winter wheat increased the plant
height, root and shoot mass and number of tillers in growth chamber. Govindarajan
and Kavitha (2004) observed higher IAA production by Azospirillum isolates in the
presence of tryptophan in the medium. Radwan et al. (2005) observed enhanced
growth and production of indole compounds by Azospirillum brasilense Cd and A.
lipoferum Br17 due to aeration of the medium.
In recent years, more attention is being given for searching early root
colonizers which directly or indirectly benefit plant growth. Beneficial root
associating soil bacteria are usually referred to as Plant Growth Promoting
Rhizobacteria (PGPR) (Piao et al., 1992; Ona et al., 2005; Somers et al., 2005).
Among the root associated bacteria Pseudomonads are the early root colonizers,
which contribute considerably to plant production and protection. Plant growth
promoting rhizobacteria belong to several genera viz., Azospirillum, Azotobacter,
Bacillus, Bradyrizobium, Erwinia, Pseudomonas, Rhizobium and Serratia.
Nitrogen fixers
Nitrogen is one of the major element and which is essentially involved in
most of the metabolic pathway in the tea crop. The applied nitrogen has the
possibilities to leach, converted to many other forms then the chance to avail to tea
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crop is less. When nitrogen fixers are incorporated, the fixation process could be
activated so as to improve the nitrogen content in the soil may be improved.
Nitrogen fixers are organisms which assimilate molecular nitrogen present in the
atmosphere to ammonical nitrogen. Biological nitrogen fixers offer economical,
attractive and ecologically sound means of reducing nitrogen fertilizers. The most
important nitrogen fixers are Azospirillum, Rhizobium, Azotobacter and Azolla.
Among these, the most promising organism capable of colonizing roots and
exerting this miracle process is the bacterium belonging to the genus of
Azospirillum. Azospirillum is microaerophillic, gram negative and spiral shaped
bacteria, which fixes atmospheric nitrogen asymbiotically. Azospirillum brasilense,
A. lipoferum, A. amazonense, A. halopraeferans, A. irakense and A. dodereinera
are the different species of the genus. Azospirillum used to be present
predominantly in acidic soil environment.
Azospirillum population was reported from 104 to 10
6 cells per gram of soil
(Magalhaes et al., 1981). Rao and Venkateswaralu (1985) reported that the most
portable number of Azospirillum varied in the root zone of pearl millet with the
maximum in the rhizosphere compared to rhizoplane and inside roots. Further, its
population was found to be maximum in laterite soil and the minimum in extremely
acid sulphate saline kari soil (Charyulu and Rao, 1980). Attachment of A.
brasilense to wheat (Michiels et al., 1990) and maize roots (Jofre et al., 1996)
taken place in two steps. First, bacteria adsorbed rapidly on the root surface. In this
step, a protein cell surface component was used for adsorption. In the second step,
the adsorption was mediated by surface polysaccharides (exopolysaccharides) or
lipopolysaccharides (Schloter et al., 1984). Some Azospirillum strains penetrate the
roots of their host and become endophytes. In such cases, they establish in the
intercellular spaces between the epidermis and the cortex (Dobereiner, 1983b;
Whallon et al., 1985) and even in the vascular system (Baldani et al., 1983).
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Bashan and Levanony (1988) reported the colonization of A. brasilense in the
endodermis of wheat roots.
Greenhouse and field trials have been conducted in many places with
different crops which revealed that biological nitrogen fixation by root associated
Azospirillum contribute significant amounts of nitrogen to the plants thereby saving
inorganic nitrogenous fertilizers. Azospirillum inoculation benefits plant growth
and increases yield of crops by improving root development, mineral uptake and
plant water relationship (Okon, 1985). The effect of Azospirillum inoculation on the
total yield increases of field grown plants generally ranged from 10 to 30 per cent.
The responses varied with crops, cultivars, locations, seasons, agronomic practices,
bacterial strains, levels of soil fertility and interaction with native microflora (Wani,
1990). Significant yield increase was observed in tomato plants with the
inoculation of Azospirillum spp. (Bashan and Holquin, 1997). Inoculation of
Azospirillum sp. to wetland rice under acidic condition improved shoot growth,
straw yield and N uptake (Govindan and Bagyaraj, 1995). Moreover, Salomone and
Dobereiner (1996) observed significant increase in grain yield of maize on
inoculation with Azospirillum spp. Hemawathi (1997) observed improved plant
growth and flower yield of Chrysanthemum on inoculation with Azospirillum spp.
over the uninoculated control. Alagawadi and Krishnaraj (1998) studied the effect
of two Azospirillum strains viz., ACD-15 and ACD-20 on growth and yield of
sorghum under field condition. They observed significant increase in grain yield of
sorghum over the control. Dobbelaere et al. (2001) observed a significant increase
in plant dry weight due to inoculation of wheat plants with A. brasilense SP-245
and A. irakense KBC1 in Belgium.
Baby et al. (2002) reported that inoculation of Azospirillum bioformulation
or liquid near the rhizosphere of tea seedlings significantly increased growth.
Polyanskaya et al. (2002) studied the growth promoting effect of two new strains of
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Beijerinickia mobilis and Clostridium sp. isolated from pea rhizosphere on some
agricultural crops and reported that application of B. mobilis and Clostridium sp.
cultures in combination with mineral fertilizers increased the crop yield by 1.5 to
2.5 times. Significant growth and yield increase was observed in rice plants due to
the inoculation of Azospirillum lipoferum by Govindan and Varma (2004).
Enhanced shoot growth and nitrogen content of the whole tomato plant was
observed by Meunchang et al. (2006a) in soil amended with sugar mill by product
compost inoculated with N2 fixing bacteria, Azotobacter vinelandii, Bejerinickia
derxii and Azospirillum sp. than uninoculated control. Inoculation of foxtail millet
with three strains of Azospirillum lipoferum either alone or in combination with N
fertilizer increased the plant height, dry weight and total N content of shoot and
root (Rao and Charyulu, 2006).
Mechanism of Nitrogen fixation
Biological nitrogen fixation by non-symbiotic nitrogen fixing bacteria like
Azotobacter and Azospirillum which requires a complex enzyme system since the
reaction is highly endergonic and it is widely being exploited all over the world for
non-leguminous crops. Azospirillum is a rhizospheric bacterium colonized the roots
of crop plants in large numbers, making use of root exudates and fixes substantial
amount of atmospheric nitrogen. The protons and electrons required for this
process are generated in metabolic reactions and catalyzed by an enzyme
nitrogenase. It is found only in prokaryotic microorganisms and so eukaryotes can
benefit from nitrogen fixation only if they interact with nitrogen fixing prokaryotes
to obtain the fixed nitrogen after their death and decomposition. Nitrogenase
enzyme is highly essential for reducing nitrogen to ammonia and is composed of Fe
(dinitrogenase) and Mo-Fe protein of nitrogenase have been sequenced and
characterized from a variety of nitrogen fixing bacteria (Burgess and Lowe, 1996).
Fixation of atmospheric nitrogen by Azospirillum was evaluated mainly by
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acetylene reduction assay and this method was useful for quantitative evaluation of
nitrogen fixation by Azospirillum and their screening (Van Berkum and Bohlool,
1980).
Okon and Vanderleyden, (1997) exerted beneficial effects of Azospirillum
on growth and yield of many economically important crops. They are extensively
used in rice and other cereal crops as a biofertilizers (Singh, 2010). These are free-
living bacteria and fix atmospheric nitrogen in cereal crops without any symbiosis.
Many Azospirillum strains produce plant hormones both in liquid culture and
natural situation. Colonization of Azospirillum in roots was known to be non-
homogenous. The bacterial cells were observed throughout the entire root system
of many plant species, however it preferred root tips and zone of elongation
(Levanony et al., 1989). The colonization of root tips was advantageous for
Azospirillum cells because when the roots penetrate deeper into the soil layer, the
oxygen supply was lower and competition with the aerobic bacterial population of
the rhizosphere was therefore reduced, however analysis of Azospirillum sites along
the roots revealed that they are found particularly on young roots and much less
frequently on the older parts of the roots (Cohen et al., 2004). They have the ability
to produce antifungal compounds against many plant pathogens. They also increase
germination and vigour in young plants leading to an improve stand in crops.
Phosphate Solubilizers
Phosphorus is another major element required for plant growth and higher
yields. Rock phosphate is one of the basic raw materials for phosphatic fertilizers.
Direct application of rock phosphate is limited to acidic soil, while in other types of
soil the applied phosphate becomes insoluble within a short time. Monocalcium
phosphate are converted to dicalcium phosphate which is slowly available to plants.
Under such conditions large amount of phosphorus is fixed in the soil which is
unavailable to the plants. The most efficient phosphate solubilizing bacteria
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includes of Bacillus and Pseudomonas and that of fungi include species of
Aspergillus and Penicillium (Baby, 2002) make available insoluble phosphorus to
the plants. The main mechanism in solubilizing insoluble phosphate by soil
microbes is on their ability to secrete organic acids. The organic acids bring down
soil pH resulting in the dissolution of immobile forms of phosphate (Hedge and
Dwivedi, 1994) and production of organic acid by Pseudomonas strains decreases
soil pH reported by Rashid et al., (2004). The effect of phosphate solubilizers on
plants are attributed to P solubilization plus other factors like release of
phytohormones, supporting nitrogen fixation, mineralization and mobilization of
other nutrients, antagonism to plant pathogens and promotion of plant growth
promoting rhizosphere microorganisms (Gaur, 1990). Further, the potential of
phosphate solubilizers in solubilizing P and mycorrhizae in mobilizing P made
agricultural scientists to think over the possibility of exploiting these organisms in
integrated nutrient management programme.
Pikovskaya (1948) made a pioneering attempt in isolating an organism
capable of actively solubilizing tricalcium phosphate and coined the name
"Bacterium P". Later he had formulated a medium having glucose as carbon source
and ammonium sulphate as nitrogen source with enrichment technique and special
media for the isolation of acid producing and phosphate dissolving microorganisms
from soils and rhizosphere were designed by Katznelson and Bose (1959).
Mahmoud et al. (1973) made a comparative study of the population of PSB in the
rhizosphere of board bean and wheat. Broad bean had more population of
phosphate solubilizers than the wheat plants. Phosphate solubilizing
microorganisms were found in all soils but their number varies with soil climate as
well as history (Gupta et al., 1986). Soil samples collected from sugarcane growing
belt of north Bihar indicated that the population level of phosphate solubilizers
range from 27-112 x 103 per gram of soil. This large variation in their distribution
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in different soils might be due to the differences in organic carbon content (Yadav
and Singh, 1991).
Bacteria, fungi, actinomycetes are active in solubilizing insoluble inorganic
phosphate with high efficiency (Kapoor et al., 1989; Narsian and Patel, 1995).
Parameters affecting the ability of PGPR to express different attributes include soil
and environmental conditions, microbes–plant host interactions, and microbes–
microbes interactions (Dey et al., 2004). De Freitas et al., (1997) assessed the
potential use of P solubilizing Bacilli and other rhizobacteria as biofertilizers for
canola and reported that Bacillus thuringiensis isolate was the most effective
inoculant which significantly increased the number and weight of pods and seed
yield without rock phosphate. Kundu et al., (2002) studied host specificity of
phosphate solubilizing bacteria isolated from different crop rhizospheres and
observed greater establishment of the isolates in the rhizosphere of their respective
crop plants than other plants.
Afzal et al., (2005) reported increased yield and P uptake of wheat plants
due to inoculation of mixture of Pseudomonas and Bacillus spp. Frietas and
Germida (1990) isolated the phosphate solubilizing microorganisms such as
Pseudomonas aeruginosa, P. cepacia, P. fluoresence and P. putida from the
rhizosphere of wheat and Bacillus licheniformis, B. mycoides, B. megaterium from
the rhizosphere of paddy (Watanabe and Hayano, 1993). Phosphate solubilizing
microorganisms are also known to produce plant growth promoting substances
(PGPS). P-solubilizing bacteria isolated from the rhizosphere of wheat and rye
plants produced auxin type of PGPS, when they were grown in liquid medium
supplemented with tryptophan (Leinhos and Vasek, 1994). Production of IAA and
GA to a considerable extent by P-solubilizing Bacillus polymyxa (Sattar and Gaur,
1987) and Erwinia, Pseudomonas and Serratia from bamboo rhizosphere was
observed by Maheshkumar (1997). Veena (1999) recorded IAA and GA3
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production by P-solubilizing Enterobacter, Xanthomonas and Pseudomonas
isolated from rhizosphere of sorghum plants. Megha (2005) examined 52
fluorescent pseudomonads for the production of IAA and GA and found them to
produce IAA in the range of 80 to 760 μg per L of broth and GA in the range of
24.8 to 262.8 μg per L of broth.
The use of P-solubilizers as bioinoculants has been found to increase
growth, yield and phosphorus uptake by many crop plants. The field and pot trials
with phosphate solubilizers with or without phosphatic fertilizers, tri calcium
phosphate, pyrite or hydroxy apatite showed increase in yield and P uptake from
marginal to significant levels (10 – 27%) (Altomare et al., 1999). Alagawadi and
Gaur (1988) reported the increase in nodulation, nitrogenase activity, dry matter
yield, P uptake and grain yield of chickpea plants as well as available P content in
soil due to inoculation of P. striata and B. polymyxa as compared to uninoculated
control. Increase in grain and straw yield of sorghum under field condition and
nutrient uptake as well as available P content in soil has also been reported by Jisha
and Alagawadi (1996) due to inoculation of the bioinoculants.
Mechanism of phosphate solubilization
Phosphate solubilizing microorganisms were found to produce mono
carboxylic acids (acetic acid, formic acid), monocarboxylic hydroxyl acids (lactic,
gluconic) dicarboxylic acids (oxalic, succinic), dicarboxylic hydroxyl acids (malic,
maleic) and tricarboxylic hydroxyl acids (citric) in liquid medium from simple
carbohydrates (Sperberg, 1958a). A fall in pH accompanied phosphate
solubilization due to the production of organic acids, but no correlation could be
established between acidic pH and quantity of P2O5 liberated (Dave and Patel,
1999). Kapoor et al., (1989) reported that organic acid produced and their quantity
differ with different microorganism. Tri and dicarboxylic acids are more effective
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compared to mono basic acids and aromatic acids. Aliphatic acids are also found to
be effective in P-solubilization than phenolic acids while citric acid and fumaric
acids had highest P-solubilization ability. The organic acid production by PSB is
capable of solubilizing the inorganic phosphorus in to available state so as to
nourish the crop. This is the main mechanism to bring acidic soil environment
which retains higher phosphorus content was reported by Rashid et al 2004.
Arbuscular Mycorrhizae Fungi (AMF)
The symbiotic association between plant roots and fungal mycelia is termed
as mycorrhiza (Fungal roots). AM fungi are known for their mutualistic association
with most of the vascular plants and for helping in the absorption and assimilation
of the elements, which are less soluble and non-available to the plants, i.e., P, Zn,
Cu, etc., from the rhizosphere thereby increasing growth and productivity of the
plant (Hayman and Mosseae, 1972; Neelima et al, 2002). These fungi are obligate
symbionts and have not been cultured on nutrient media. AMF fungi infect and
spread inside the root system. They possess special structures known as vesicles
and arbuscules. The arbuscules help in the transfer of nutrients from soil to the root
system and the vesicles, which are saclike structures, store P as phospholipids. AM
fungi colonize the root cortex of plants and develop an extrametrical hyphal
network that can absorb nutrients from the soil. Enhanced plant growth due to
arbuscular mycorhizae (AM) association was well documented by Bagyaraj (1984).
Also he reported that improved plant growth is attributed to increased nutrient
uptake, especially of phosphorus, tolerance to water stress, root pathogens and
adverse soil environments and production of growth-promoting substances.
Mycorrhizal fungi enhancing the numerous and activity of beneficial soil
organisms like nitrogen fixers and phosphate solubilizers with consequential
beneficial effect on plant growth have also been substantiated by Linderman
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(1992). It has also been suggested that AMF stimulate plant growth by
physiological effects other than the enhancement of nutrient uptake or by reducing
the severity of diseases caused by the soil pathogens.
Conversely, soil microorganisms can affect AM formation and function
(Azco´n-Aguilar and Barea, 1992; Zapata and Axmann, 1995; Barea et al., 1997).
The mycorrhiza helper bacteria are known to stimulate mycelial growth of
mycorrhizal fungi or to enhance mycorrhizal formation (Garbaye, 1994; Frey-
Klett,1997). The microbiologically solubilized phosphate could, however, be taken
up by a mycorrhizal mycelium, thereby developing a synergistic microbial
interaction (Barea et al., 1997). Phosphorus (P) is added in the form of phosphatic
fertilizers, part of which is utilized by plants and the remainder converted into
insoluble fixed forms (Narsian, and Patel, 2000). The contribution of their process
to plant nutrition is unclear and because of the possible refixation of solubilized
phosphate ions on their way to the root zone. Mycorrhizal colonization may alter
the host root physiology which may in turn influence the microbial population.
The flow of carbon from shoot to root may be increased by AM
colonization (Muthukumar and Udaiyan, 2000) which may alter the carbon
availability for bacteria in the rhizosphere. Furthermore, it is well known that root
exudates strongly modify microbial composition and activity in the rhizosphere and
AM fungi can modify the quantity and quality of root exudates (Andrade et al.,
1997). Mycorrhizal dependency of plant species is often related to the
morphological properties of the root; the root system of neem, with short sparse
root hairs, tends to make it more mycorrhiza- dependent like several other tree
species. The enhanced uptake of P in AM-fungi-inoculated seedlings may be due to
an increase in the number of uptake sites per unit area of roots and a greater ability
of these roots to exploit the soil nutrients (Bolan, 1991). AM fungi allow the root
system to exploit a greater volume of soil P by (i) extending away from the roots
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and translocating P away from the root zone, (ii) exploiting smaller soil pores not
reached by the root hairs and (iii) effective acquisition of organic phosphates by
production of extracellular acid phosphatases (Marschner and Dell 1994).
Enhancement of uptake of phosphorus and other nutrients by fungal hyphae
is the primary mechanism responsible for plant growth stimulation by arbuscualr
mycorrhizal fungus (AMF) (Bolan, 1991) and improved shoot and root length
(Hayman, 1980). However, AM fungi also helps in the production of plant
hormones such as cytokinins, IAA and IBA, all of them have a role in plant
metabolic process (Barea and Azcon-Aguilar, 1982). Inoculation with AM fungi
(Phavaphutanon et al., 1996) or Azospirillum (Okon and Labandera-Gonzalez
1994) can reduce fertilizer requirement in plant production. Studies of
Muthukumar et al., (2001) indicated that microbial inoculations can substantially
reduce fertilizer requirement in neem seedling production. Moreover, mycorrhizal
colonization may also alter the pH of the substrate through release of certain
substances, which are not well-documented (Filion et al., 1999). Results of several
workers indicated that a drop in soil pH with phosphobacteria inoculation and in
certain treatments involving Glomus intraradices (Kim et al., 1998). In addition, A.
brasilense and phosphobacteria might affect plant growth as a result of their ability
to synthesize plant hormones (Rodríguez and Fraga, 1999). The colony forming
units of B. coagulans and T. harzianum were also highest when all three organisms
were inoculated together and the least when these were inoculated alone (Jayanthi
et al., 2003).
Influence of bioinoculants consortium
Group of bioinoculants than a single perform better and proves high yield,
productivity, nutrient uptaken and soil health improvement in various crops.
Inoculation with asymbiotic nitrogen fixers like Azospirillum may improve plant
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growth and yield due to supplementing the growing plants with fixed nitrogen and
growth-promoting substances (Sumner, 1990). Phosphate- solubilizing bacteria
(PSB) on the other hand solubilize insoluble phosphorus by producing organic
acids which are taken up by plants (Rodríguez and Fraga, 1999). VAM fungi
enhance the uptake and translocation of phosphorus (P) and other minerals from the
soil solution to the root cells (George et al., 1995). So the inoculation along with
AM fungi, PSB and Azospirillum could enhance the growth of tree seedlings in
nurseries, the synergistic effect of indigenous AM fungi, PSB and Azospirillum on
growth, nutrient status and seedling quality of neem has been reported by
Muthukumar et al (2001).
Dual inoculation increased yields in sorghum (Algawadi and Gaur, 1992),
barley (Belimov et al., 1995), black gram (Tanwar et al., 2002), soybean (Abdalla
and Omer, 2001) and wheat (Galal, 2003, Aftab and Asghari 2008). The most
efficient and dominant solubilizers were Bacillus and Pseudomonas and these
bacteria behaved as mycorrhiza helper bacteria (Garbaye, 1994; Frey-Klett et al
1997; Sabannavar and Lakshman, 2009). Because they promoted root colonization
when associated with mycorrhizal fungi and other phosphate solubilizing bacteria
combinations (Azco´n-Aguilar, and Barea, 1992). Gaur (1985) observed that
increased grain yield and phosphorus uptake by response of crops to Pseudomonas
striata. Microorganisms in the mycosphere of AM fungi may affect mycorrhizal
functions such as nutrient and water uptake carried out by the external hyphae of
AM fungi (Duponnois et al., 2005). Synergistic effects of PGPRs during combined
inoculation have also been reported in various crops, for examples potatoes (Kundu
and Gaur, 1980a), rice (Tiwari et al., 1989) and sugar beet and barley (Cakmakci et
al., 1999). In case of co-inoculation of P solubilizing and K solubilizing bacterial
strains synergistically solubilized rock P and K sources of fertilizers which were
added into the soil and make them much more available for uptake by plant roots.
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27
Higher N, P and K uptake may subsequently promote the plant growth. So the
combined inoculation with N2
fixing and phosphate solubilizing bacteria was more
effective for providing a more balanced nutrition for plants (Belimov et al., 1995).
Sensitivity of bioinoculants to organic and inorganic fertilizers
Bioinoculants are able to compatible with possible reduced dose of either
organic and/or inorganic fertilizers. They act on external sources of fertilizers so as
to get energy for their establishment by producing certain organic acids, enzymes
and other secondary metabolites. When the concentration of such inorganic and/ or
organic manures may influence the growth of bioinoculants. In case of organic
based materials, they survive very well by utilizing it and later the minerals could
be released from the organic sources of fertilizers in the soil. Whereas the inorganic
fertilizers adversely affect them but at the minimized doses may have the
possibility to support the bioinoculants. Interactions of A. fumigatus with K –
bearing minerals release potassium through three different reaction pathways. The
first involves the smaller and soluble components of the secretion, the second
concerns the insoluble part of the macromolecules and membrane-bound
biopolymers, and the third may be related to the physical activities of the cells
including direct ingestion of mineral particles (Adeleke et al., 2010). While the
interactions of free aqueous biomolecules with minerals do not seem to require
active involvement of fungal activities, the routes that entail the participation of
immobile organic matrices and cell-mineral physical contact appear to have a
strong dependence upon the vitality of the microbes and therefore are correlated to
the optimal living conditions of the organisms.
Biomolecules can penetrate hard mineral materials for the purpose of
nutrient absorption (Van Breemen et al., 2000). Their primary role is to search for
and absorb nutrients under severe soil conditions on behalf of their host plants
Review of literature
28
(Smith and Read, 2008). Both ecological and weathering roles of ectomycorhiza
fungi have been investigated and linked to their ability to produce metabolites such
as organic acids. These acids are low molecular weight compounds and confirmed
to have great potential in the solubilisation of complex or hard mineral materials
(Paris et al., 1995; Gadd, 1999; Van Breemen et al. 2000). It suggests that
microbial mediated mineral weathering cannot be approximated by simple
dissolution reactions with the participation of organic or biochemical molecules.
While the pH decrease resulting from metabolic activities and the complelx
reactions involving organic ligands derived from metabolites and biomass
decomposition are active participants in microbe-mineral interactions, the immobile
biopolymers (either attached to the cell surfaces or insoluble secretion), combined
with cell-mineral mechanical interactions, may be a more forceful agent, leading to
an effective destruction and transformation of minerals in biogeochemical
processes (Adeleke et al., 2010).
Integrated Nutrient Management
The supplementary and complementary use of organic manures and
inorganic chemical fertilizers integrated with beneficial proven bioinoculants
augment the efficiency of both the substances to maintain a high yield and soil
productivity (Thakuria et al., 1991). The beneficial effects of combined application
of chemical fertilizers with organic manures viz., farmyard manure, vermicompost,
biofertilizers, panchagavya and many more of such materials are universally
known. Application of organic manures in general improves the availability of
micronutrients like zinc, iron, manganese and copper. A balanced application of
both organic, inorganic and bio-fertilizers appear to be an ideal proposition to meet
nutrient requirements of dry land crops rather than single application. In view of
this, henceforth research on integrated nutrient management involve intensively to
assess the effect of organic and inorganic sources of nutrients integrated with
Review of literature
29
biofertilizers on the nutrient uptake and residual soil fertility (Kondapa naidu. et al.,
2009). Possibilities of INM were tried in tea with inclusion of biofertilizers like
Azospirillum and phosphobacteria in manuring programme of tea and results
revealed that bacterial fertilizers in tea can be good supplement. It was found that
integrated approach of inorganic and organic sources like 100Kg nitrogen + 5Kg
farmyard manure / hectatre provided yield significantly higher than when 150 Kg
nitrogen from purely inorganic source was given and which has eco-friendly effect
with the environment (Saikia, 2006). Application of AM fungi, phosphobacteria,
Azotobacter, Azospirillum are beneficial in nutrient management to the plants and
reducing the consumption of inorganic fertilizers (Jamaludin, 2006). It has now
become possible to meet a large part of our total nitrogen demand through proper
husbandry of biological nitrogen fixation by microorganisms in crop production
system.
Biofertilizers are capable of providing an economically viable level for
achieving the ultimate goal of enhanced productivity. The crop microbial soil
ecosystem can, therefore, be energized in sustainable agriculture (Nautiyal, 2006).
The use of chemical fertilizer, organic fertilizer or biofertilizer has its advantages
and disadvantages in the context of nutrient supply, crop growth and environmental
quality. The advantages need to be integrated in order to make optimum use of each
type of fertilizer and achieve balanced nutrient management for crop growth (Chen,
2006). The integrated nutrient management favourably affects the physical,
chemical and biological environment of soils (Singh et al., 2011). Increasing
importance of INM in maintenance of soil fertility and of plant nutrient supply for
sustaining desired crop productivity can be achieved through optimization of
benefits from all possible sources of plant nutrients in an integrated manner
(Nautiyal, 2006). The development of biofertilizers formulation would be
immensely exploited in crop production to reduce the chemical fertilizers inputs
Review of literature
30
and to develop sustainable production systems. Plantation crops did not receive the
same attention as that of field crops. Balasubramani et al. (1997) reported that the
soil and seed treatment of Azospirillum alone recorded the higher number of fruits
per plant followed by the Azospirillum treated seeds and soil application of nitrogen
at 30 kg per ha. Kundu and Gaur (1980 b) observed a synergetic interaction
between Azotobacter and phosphate solubilizing bacteria when the two organisms
were inoculated together in cotton. In the combined inoculation treatments, the
population of both the organism was enhanced in addition to increase in yield of
cotton.
Kalyani et al., (1996) studied the interaction of Azospirillum and fertilizer
nitrogen on cauliflower Cv. Jawahar moti and reported that soil inoculation of
Azospirillum coupled with less nitrogen (80 Kg/ha) had beneficial effect in
improving the growth and yield, besides saving of recommended nitrogen upto
50%. Balasubramani (1988) reported that the seed and soil treatment of
Azospirillum with 75% recommended dose of nitrogen per ha recorded the higher
yield (17.5 t/ha) compared to control (9.6 t/ha) in bhendi. Thamizh and Nanjan
(1998) stated that the combined application of Azospirillum, phosphobacteria and
VAM with 75% of recommended NPK (90:90:90 Kg/ha) recorded higher yield
(14.96 t/ha) which was 21 per cent higher than uninoculated control (11.93 t/ha) in
potato. Nanthakumar and Veeraghavathatham (1999) noticed that combined
nutrition of organic manure through FYM (2.5 t / ha.) Azospirillum (2 Kg / ha) and
phosphobacteria (2 Kg / ha) along with 75% of recommended dose of inorganic
nitrogen and phosphorus increased the yield and yield components in brinjal. So
there is a wide scope to develop biofertilizers to supplement/substitute the use of
inorganics to enhance productivity of plantation crops (Thomas 2006). Ragland et
al. (1989) observed that 75 per cent of the recommended N and P levels (112.5:
112.5 Kg N and P/ha) along with Azospirillum and AM produced significantly
Review of literature
31
higher bulb yield of Bellary onion as compared to the uninoculated control, which
was on par with 100 per cent recommended dose of NPK and biofertilizers (Mog,
2007). Prabhu et al. (2002) reported that, application of biofertilizer and FYM with
reduced dose of inorganic fertilizers increased yield and yield attributes in okra.
The treatment combination of FYM (10 t/ha) + 2/3 RDF + Azospirillum + VAM
resulted in higher yield, Kropisz (1992) observed that application of FYM (25 t/ha)
+ NPK recorded significantly higher yield compared to FYM alone and NPK in
cabbage, onion and carrot.
Mandal and Mazumdar (1986) indicated application of RDF in combination
with FYM (15 t/ha) resulted in higher potato tuber yield compared to control.
Luzzati et al. (1975) found that organic manures alone did not generally increase
the yield of carrot. Kropisz and Wojciechowski (1978) reported that compost in
combination with NPK had beneficial effect in carrot. Patil (1995) reported that
application of vermicompost (4 t/ha) with 50% RDF increased potato yield (34
t/ha) as compared to control (14.2 t/ha). Tomar et al. (1998) indicated that brinjal
and carrot plants were grown in pots and addition of FYM or urea, vermicompost
and vermicompost + FYM (1:1) recorded maximum yield with soil amended with
FYM and vermicompost compared to unamended soil. Integrated use of inorganic
nitrogenous and phosphatic fertilizers and biofertilizers is the most efficient way of
economizing the fertilizer use and improving agronomic efficiency besides
improving physical, chemical and biological properties of the soil (Ramanjaneyulu
et al., 2010).
Influence of INM on biochemical and quality parameters
Mineral nutrients are essential to plant growth and development which also
influence the quality attributes of plant products. The beneficial effect of adding
mineral elements to soils to improve plant growth and it has been known well in
Review of literature
32
agriculture for more than 2000 years. The growth, development and quality of tea
plants, also require the continuous supply of mineral nutrition, especially N, K, Mg
and S that are essential to tea plants. This is because such minerals tend to
influence the level of polyphenols and N-containing compounds and thus affect the
quality of made tea. Nutrient deficiency in soils and poor fertilization are possibly
two important issues and reasons for low yield and quality of tea (Malenga and
Wilkie 1994). With the fast development of tea production in China, great attention
has been paid to the balanced fertilization in tea cultivation for good quality and
high yield of tea in recent years. It is necessary to study the background of nutrients
and the soil properties in tea-grown soils, and their effects on tea quality and yield
in order to develop efficient fertilization (Jie, 2005). Panwar and Singh (2002)
studied the role of biofertilizers in wheat and revealed that N2 fixing bacteria
(A.brasilense) and phosphate solubilizing bacteria (B.subtilis) significantly
increased chlorophyll content, nitrate reductase activity and achieved better quality
over other treatments. The application of increasing levels of chemical fertilizers
will lead to increasing crop yields but quality of its products may get deteriorated.
Continual and excessive use of chemical fertilizers may adversely affect the
soil health and fertility resulting in environmental and ground water pollution. The
application of chemical, organic and biofertilizers in a balanced manner can meet
the nutrient requirement of the tea plant for sustainable productivity. In this
context, possible reduction of chemical fertilizers and regaining of soil health can
be achieved by incorporating the organic and/or biological fertilizers. The organic
and biofertilizers provide not only nutrient supply to the plants but also impart
resistance to plants against pests and diseases. Utilization of biofertilizers and
organic manures becomes essential at this juncture to maintain soil productivity and
to supply nutrients to the plants due to its plant growth promoting and disease
control abilities by keeping ideal quality too. The use of biofertilizers in
Review of literature
33
combination with chemical fertilizers and/or organic manure offers a great
opportunity to increase the crop productivity with greater quality at different
growth stages of tea crop (Baby 2006). Integrated schedule of biofertilizers with
possible reduction of inorganic fertilizers in plantation crop like tea has not yet
been explored. Inoculation with AM fungi, PSB and Azospirillum enhanced the
growth of tea seedlings and improved the biochemicals related to quality of tea
which were comparable to 100% inorganic fertilizers treated and much higher over
un inoculated seedlings under nurseries (Balamurugan et al., 2011). The present
study was undertaken to evaluate the synergistic effect of indigenous AM fungi,
PSB and Azospirillum on growth, nutrient status and seedling quality of tea.
The present study was focused on the standardization of the ideal
combinations of inorganic, organic fertilizers and biofertilizers to meet the nutrient
requirements of tea plant. The possible reduction of nitrogenous and phosphatic
fertilizers as well as exploiting the indigenous biofertilizers strains were attempted
for getting better yield and quality of tea. The advantages and beneficial effects of
bioinoculants on tea seedlings were also studied under nursery conditions. The
physico-chemical parameters and microbial studies, influence of root extracts and
synergistic effects of manuring chemicals on the survival of bioinoculants, were too
studied in detail. Apart from this, studies on beneficial bioinoculants for their
growth promoting abilities and their role in controlling diseases were also
undertaken. This study on integrated nutrient management in tea by exploiting of
bioinoculants would be useful to the southern Indian tea industries and which may
immensely get benefited from the outcome of these present findings.