EFFECT OF SOIL-APPLIED L-TRYPTOPHAN ON GROWTH AND CHEMICAL-COMPOSITION OF COTTON

Addition of L-tryptophan (L-TRP) to soil can have an ecological impact on the growth and development of some plants through its catabolism i nto auxins by rhizosphere microflora. A pot experiment was conducted t o assess the influence of L-TRP on growth and chemical composition of cotton (Gossypium hirsutum L.). Seven L-TRP levels (10(-7) to 10(-1) g /kg soil) were applied as a soil drench in addition to a control. Data obtained revealed that specific growth parameters were significantly promoted, such as plant height (27.3%), dry weights of shoot (45.7%) a nd root (35.8%), biomass (43.3%), and number of branches (37.5%), flow ers (63.3%) and bells per plant (22.4%) in response to L-TRP treatment s. Similarly, the nitrogen (N), phosphorus (P), and potassium (K) conc entrations in plant tissues and their uptake were also significantly a ffected by the exogenous application of L-TRP. Significant linear or q uadratic dose-response relationships were found when root mass, branch es, flowers, bells, and NPK concentrations in plant tissues and their uptake, were regressed against log[L-TRP] excluding the control. The e ffects of L-TRP could be most likely attributed to its conversion into auxins by the rhizosphere microflora upon direct uptake by plant; how ever, other mechanisms such as direct uptake of L-TRP by plant roots w ith subsequent auxin production within plant tissues or a change in th e balance of rhizosphere microflora affecting plant growth cannot be e xcluded.