Rice is the staple food for half of the world's population especially in oriental countries. In India, about 2500 varieties of rice are being cultivated, from which more than 1500 varieties are in southern India which are preferred over others, owing to their high yield, good quality and quantity of grain, short duration of growth and resistance against pest and diseases. A large number of experiments have been conducted in several countries to investigate the effect of inoculation of various strains of Azospirillum spp. on cereals and grasses (Smith et al., 1976; Watanabe et al., 1981). The aim of the application of Azospirillum is to get fast growth, better health of the plant and higher yield. It is known to be a very active nitrogen fixer under laboratory as well as soil conditions. Various kinds of cereals were tested by using a member of nitrogen fixing bacteria viz., Azotobacter, Nitrosomonas and Azospirillum to increase yield under controlled conditions. Balasubramanian and Kuamr, 1987; Wani, 1990; Bashan and Holgain, 1995 investigated that Azospirillum treatment showed remarkable increase in the grain and the straw yield in sorghum, wheat, maize, paddy and other food and fodder crops.
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The yield responses caused by Azospirillum inoculation may be due to biological nitrogen fixation (Hartmann et al., 1983). Split application of biofertilizer inoculation through seed, seeding and soil gave the highest grain, straw yield, plant height and number of productive tillers in rice (Gopalswamy and Vidhy asekaran, 1988). The objective of the present study was to investigate the effect of Azospirillum inoculation on the different paddy varieties to find out the best variety. The observations were made to note the seedling characters such as seed germination, biomass and phytomass yield of paddy varieties under controlled conditions.
Materials and Methods
Healthy, viable paddy seeds of seven varieties viz, CO43, ASD16, ASD18, AS89044, ADT36, ADT37 and IR64 were procured from Tamil Nadu Agricultural University, Coimbatore. The seeds were sterilized with 2% mercuric chloride solution before treatment. After sterilization, the seeds were washed well with sterile distilled water. Twenty five seeds were selected from each variety and dressed well with the paste of 250mg of Azospirillum brasilense in water. These seeds were dried under the shade condition and transferred after sowing to a plastic trough containing 2kg of sterilized garden soil. The troughs were watered regularly and being maintained under controlled condition. A control set up was also made by following the same conditions except the addition of biofertilizer. Five seedlings were selected at random from each trough and the following observations were made on the 10th, 20th and 30th day of sowing.
The seedlings were uprooted gently without causing any damage to the root and shoot systems and washed well with water. The shoot and root lengths were measured with a metric scale. The shoot and root fresh weights were determined using an electronic balance. Net photosynthesis rate (Pn rate) was measured at 10th, 20th and 30th day of sowing using infra red gas analyzer (model: ADCLCA-3) fitted with a Parkenson leaf chamber (PLL-3). The same leaf was removed and processed for the estimation of total chlorophyll and carotenoid (Horborne, 1973), protein (Lowry et al., 1951) and sugar (Dubois et al., 1956).
Results and Discussion
The results indicated that the growth of Azospirillum treated paddy seedlings excelled over the untreated ones. The seed germination studies revealed that the percentage germination of seeds were higher in Azospirillum treated seeds than in control (Table 1). The inoculation of the biofertilizer in the varieties CO43 and AS89044 showed a considerable increase in the seed germination than the other varieties under same experimental conditions. The reason for this may be due to the tremendous pressure developed inside the seeds, which is responsible for breaking of the seed coat quickly (Sifton, 1959). This pressure may be induced by phytohormones especially auxin, indole acetic acid, cytokinin and gibberelic acid like substances secreted by Azospirillum (Okon, 1985; 1986).
The observations made on 10th, 20th and 30th days of sowing revealed that Azospirillum treated seeds had higher productivity than control. The seedlings from this particular biofertilizer treated seeds had longer shoot and root lengths than the untreated ones. Similar results were observed in other plant species. The seed dressing by this biofertilizer induces the production of plant growth promoting substances and leads to the increase of shoot and root length (Table 2). Secretion of plant growth hormones by Azospirillum was reported in several cereals and grasses Balasubramanian and Kuamr, 1987: Bashan and Holgain, 1995). This also reflects a specific capability of the host plant to attract the bacteria and modify the rhizosphere and/or to respond to some bacterial activity and benefit from it (Bottini et al., 1989).
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The fresh and dry weights of root and shoot system of paddy varieties were also found to be increased to a considerable extent in Azospirillum treated seedlings (Table 3 and 4). This may be due to the formation and development of numerous root branching, root hairs and primary and secondary lateral roots which increases the nutrient uptake capacity of roots (Gopalswamy and Vidhy asekaran, 1988; Hartmann et al., 1983). This effect on the root system as well as more root colonization and root proliferation are probably due to the growth hormones secreted by the bacteria and also nitrogen fixation by it. The increased nitrogen uptake from the soil might have correspondingly increased the biomass to some extent. The changes in root functions due to Azospirillum treatment in different wheat cultivars were also reported (Kapulnik et al., 1981). These growth enhancing effects are of interest because of their potential significance for yield increases in agronomic systems in which the use of fertilizers is the limiting factors for their development (Sarig et al., 1984).
Net photosynthetic rate was higher in Azospirillum inoculated plants than in control plants (Table 5). It may be due to the absorption of nutrients from the soil and stimulate the metabolism of photosynthesis. Photosynthetic activity plays an important role in the increase of leaf area leading to more biomass accumulation. The leaf area was also increased to some extent in Azospirillum treated seedlings than untreated ones. The plants like Digitaria decumbens, Panicum maximum and Pennisetum americanum were subjected to Azospirillum inoculation and observed that the photosynthetic rate and dry matter contents were increased to a limited extent (Smith et al., 1976; Sarig et al., 1984;). The biochemical parameters such as total chlorophyll, carotenoid, soluble protein and sugar were increased to varying level in Azospirillum treated plants when compared to control ones (Fig.1). Very high contents of biochemical constituents in CO43 and very less in AS89044 were observed. The increased chlorophyll content could be correlated with the high level of photosynthesis this might be due to uptake of more nitrogen from the soil, which is fixed by this bacteria. The increased protein content may be due to the presence of kinetin which promotes the amino acid content which in turn helps in active protein synthesis (Tien et al., 1979). Similarly, the increased sugar content in the leaves might also be due to active role of Azospirillum in sugar metabolism (Watanabe et al., 1981).
The results clearly showed that paddy variety, CO43 accounted well followed by ADT36 and ADT37. Other varieties showed poorer response. From these observations, it can be concluded that among the paddy varieties tested in response to Azospirillum inoculation, CO43 had high phytomass and biomass accumulation, physiological and biochemical parameters. It may be due to nitrogen uptake from the soil and proper utilization. The beneficial effect of Azospirillum brasilense varies itself which depending upon the plant varieties, microbial strains, method of inoculation and environmental factors particularly soil temperature, pH, Ec, moisture content and water holding capacity.