Dinâmica da deficiência de nitrogênio em algodoeiro: relações entre assimilação de nitrato, fotossíntese e níveis de açúcares

Abstract

Nitrogen deficiency (N) is a limiting factor to plant growth and development, and the mechanisms involved are few understood. The objective of this work was to evaluate the dynamics of N deficiency and its relationships with changes in nitrate assimilation, photosynthesis and sugar levels in cotton plants. For this, the plants were germinated in autoclaved sand, grown in a greenhouse and subjected to nutrient solution adapted from Hoagland & Arnon with nitrate as the only source of N. After 22 days, plants previously supplied with 10 mM NO3-, were exposed to two levels of NO3-: 0 mM and 10 mM for 8 days, aiming to induce contrast in the assimilation rate of NO3-. The progressive N deprivation (as nitrate) induced a decrease in biomass of roots (15%) and shoots (41%). Nitrate deprivation induced changes in the following order in the parameters related to nitrogen nutrition: NO3- content > nitrate reductase activity > fructose concentration > free amino acid content > total N content > total chlorophyll content > glucose concentration. Interestingly, the parameters related to the efficiency of photosynthesis (CO2 assimilation and photochemical efficiency), in addition to the sucrose concentration, were less affected during the period of nitrate deprivation and N deficiency in cotton. These results show that young cotton plants display high phenotypic plasticity of N use or efficiency for photosynthesis, that is, they are able to perform high rates of photosynthesis with low N concentrations. However, the best markers for indicating the N status in the leaves (nitrate, amino acids, total N and chlorophylls) are not necessarily best to indicate important physiological disturbances such as in photosynthesis. In conclusion, young cotton plants previously supplied with high levels of nitrate are able to mobilize nitrogen reserves for photosynthetic activity. In these conditions nitrate content and nitrate reductase activity are most sensitive indicators to indicate earlier N mobilization from leaves, as compared to N-total and free amino acid contents.