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 cotton (Gossypium barbadense L., cv. S-6) plants, grown under different N nutritional regimes. Pima cotton was grown in pots under natural environmental conditions. Varying N regimes were imposed on 20-day-old plants by fertilizing with nutrient solutions containing 0, 0.5, 1.5, and 6 mM of nitrate (NO3) concentrations. Net carbon dioxide (CO2) assimilation rates, stomatal conductance, internal CO2 partial pressures, transpiration rates, leaf carbohydrate status, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities, and leafN concentrations were determined in the youngest fully expanded leaves. Net photosynthetic rates, stomatal conductance and transpiration were positively correlated 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-treated 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, respectively, at the rate of 0.6 mu mol m(-2) s(-1) per unit increase in N. Stomatal conductance increased with increasing leaf N concentration, while the photosynthesis to water use efficiency (mg CO2/g H2O) was not affected by leaf-N status. Rubisco activities were proportional to leaf N concentration. Leaf starch decreased with increasing leaf N while sucrose 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).