The combined effect of a hormone weed killer 2,4-dichlorophenoxyacetic acid (2,4-D) and an amino acid (tryptophan) has been studied on growth and heterocyst differentiation in the cyanobacterium Nostoo linckia. 2.4-D a t 100 pg/ml stimulated growth and heterocyst frequency in combined nitrogen-free medium while its higher concentrations inhibited both. Tryptophan under similar conditions promoted much growth yield with 3-4 fold enhanced heterocyst frequency than the control. Such heterocysts were immature and showed germination under in situ condition. The concentrations of 2,4-D (100 (*g/ml) and tryptophan (50 pg/ml), stimulatory to growth and heterocyst formation, caused additive effect while herbicide inhibition of nitrogen-fixing growth a t higher doses was partially relieved by tryptophan but tryptophan-induced heterocyst frequency was completely suppressed under this condition. The possible role of interaction of these two chemicals on growth and heterocyst formation has been discussed. Photosynthetic nitrogen-fixing cyanobacteria are known to maintain the fertility of the soils and water-logged paddy fields by fixing atmospheric nitrogen (SINGH 1961, STEWART 1973). Such ecosystems provide most suitable conditions for luxurient growth of these cyanobacteria as principal components. The increasing use of herbicides to paddy fields, particularly 2,4-dichlorophenoxyacetic acid (2,4-D), is likely to create microbial imbalance by affecting their growth and development and thus, causing a great setback to the nitrogen status of the soil as has been previously reported (LUND-KVIST 1970, DAS and SINGH 1977, SINGH 1974). We have previously shown stimulation in growth and heterocyst differentiation in the paddy soil cyanobacterium by lower concentrations of 2,4-D (TIWARI et al. 1981). I n this paper we report the influence of tryptophan on 2,4-D toxicity of growth and heterocyst formation in the nitrogenfixing cyanobacterium Nostoc linckia.
Publication, New Delhi (India).