NITROGEN NUTRITION AND PHOTOSYNTHESIS IN LEAVES OF PIMA COTTON

The influence of nitrogen (N) on dry matter accumulation and yield in cotton is well documented, but its effects on carbon (C) assimilation and transpiration are less clear. The objectives of this study were to characterize leaf photosynthetic and stomatal responses of Pima cotto n (Gossypium barbadense L., cv. S-6) plants, grown under different N n utritional regimes. Pima cotton was grown in pots under natural enviro nmental conditions. Varying N regimes were imposed on 20-day-old plant s by fertilizing with nutrient solutions containing 0, 0.5, 1.5, and 6 mM of nitrate (NO3) concentrations. Net carbon dioxide (CO2) assimila tion rates, stomatal conductance, internal CO2 partial pressures, tran spiration rates, leaf carbohydrate status, ribulose 1,5-bisphosphate c arboxylase/oxygenase (Rubisco) activities, and leafN concentrations we re determined in the youngest fully expanded leaves. Net photosyntheti c rates, stomatal conductance and transpiration were positively correl ated with leaf N concentration. The maximum-photosynthetic photon flux density-saturated net assimilation rates were about 30 mu mol m(-2) s (-1) when fed with 6 mM NO3 and only about 15 mu mol m(-2) s(-1) when fed with 1.5 mM NO3. Maximum net photosynthetic rates of high N-treate d plants were achieved at approximately 1,800 mu mol photons m(-2) s(- 1). Assimilation rates increased from 15 to 35 mu mol CO2 m(-2) s(-1) as leaf N increased from 16 to 45 g kg(-1) leaf dry weight, respective ly, at the rate of 0.6 mu mol m(-2) s(-1) per unit increase in N. Stom atal conductance increased with increasing leaf N concentration, while the photosynthesis to water use efficiency (mg CO2/g H2O) was not aff ected by leaf-N status. Rubisco activities were proportional to leaf N concentration. Leaf starch decreased with increasing leaf N while suc rose content increased. The ratios of starch/sucrose decreased from 5. 3 to 1.2 as leaf N increased from 16.5 to 45 g kg(-1).