Neem Leaf Aqueous Extract Induced Growth, Pigments and Photosynthesis Responses of Cyanobacterium Nostoc muscorum
Neem leaf aqueous extract (1, 2, 4, and 8%) induced effects on growth; photosynthetic pigments, and photosynthesis of cyanobacterium Nostoc muscorum were studied. The low concentrations (1 and 2%) of extract produced stimulating effects on growth, photosynthetic pigment contents, photosynthetic electron transport activity (PS II, PS I and whole chain), oxygen yield and 14C-fixation. But 2% extract resulted a marginal decrease in 14C- fixation. Moreover, extract at high concentration (4 and 8%) caused considerable reduction in above parameters. PSI activity was either little or not affected with high concentration of aqueous extract. However, a substantial lowering in PSII activity was observed. The partial recovery of PS II activity by artificial electron donors (DPC, NH2OH, and MnCl2) suggests that the PS II inhibition might have occurred as a result of interruption in electron flow at water oxidation side of PSII as well as reaction centre. Study also reveals that in addition to growth promoting concentrations, the high concentrations (4% and 8%) of extract also support the biomass production of cyanobacterium, when exposed for longer duration.
INTRODUCTION
Rice fields in tropical and subtropical countries are the best habitats for a large number of heterocystous and non-heterocystous cyanobacteria (blue-green algae), which fix atmospheric molecular nitrogen in aerobic and micro aerobic conditions by utilizing light energy. The importance of cyanobacteria in improving soil fertility for sustainable agriculture in submerged and irrigated rice cultivation is well recognized (Venkataraman 1981; Goyal 1993; Saikia & Bordoloi 1994; Tiwari et al. 2000). Since time immemorial, cyanobacteria are exploited for nutrient recycling and maintenance of organic matter in soil. In recent years, the greater exploitation of cyanobacteria as potential biofertilizer for rice cultivation has become very promising. Such inoculation with free living cyanobacteria (algalization technology) has been started in several countries such as the Phillippines, Japan, India, etc., to improve the productivity of paddy and other crops (Martinez et al. 1981,;Watanabe et al. 1981; Singh & Bisoyi 1989; Kulasooriya 1998; Soliman 2000). Despite the congenial condition for the growth of cyanobacteria in paddy fields, the population of cyanobacteria is regulated by a variety of biotic and abiotic factors. Several invertebrate grazers viz. Cypris, Cyclops, Daphnia, Mesocyclops, mosquito larvae etc. feed on cyanobacteria (Roger & Kulasooriya 1980), and limit their growth in rice fields. Decline in the rate of biomass production of cyanobacteria can cause considerable reduction in the level of nitrogen and organic matters in soil, as a consequence of this the paddy productivity declines significantly (Venkataraman & Rajyalakshmi 1971; Kaushik & Venkataraman 1983). In addition, abiotic factors such as synthetic agrochemicals: weedicides, herbicides, insecticides, and chemical fertilizers are used to enhance crop yield, however, indiscriminate application of synthetic agrochemicals had adverse effects on key metabolic activities of cyanobacteria (Anand & Subramanian 1997; Ravindran et al. 2000; Prasad et al. 2005). The chemical pesticides not only create serious threat to the non-target organisms such as crops and beneficial microorganisms, but also enter into the food chain and affect humans. Furthermore, as a result of biological magnification the level of toxic chemicals may reach several times above the acceptable limit.
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