ABSTRACT

The influence of nitrogen deficiency on dry matter accumulation, total production and yield in cotton is well documented, but the effects of the basic processes of carbon assimilation and transpiration is less clear. The main objective of this study was to characterize leaf photosynthetic and stomatal responses of Pima cotton (Gossypium barbadense L., cv S-6) plants grown under four nitrogen nutritional regimes. Pima cotton was reared in pots under natural environmental conditions. Varying N-regimes (0, 0.5, 1.5 and 6 mM) were imposed on 20-day-old plants by fertilizing with nutrient solutions containing different nitrogen concentrations. Net CO₂ assimilation rates, stomatal conductance, internal CO₂ partial pressures, transpiration rates, leaf carbohydrate status, ribulose 1,5-bisphosphate (RUBP) carboxylase activities and the amounts of chlorophyll and leaf nitrogen were determined for the third or fourth leaf from the top.

Net photosynthetic rates, stomatal conductance and transpiration were uniquely related to total leaf nitrogen at all times. The maximum Photosynthetic photon flux density (PPFD)-saturated net assimilation rates and the PPF required for saturation decreased with decreasing applied nitrogen. Maximum net photosynthetic rates of N-sufficient leaves were achieved at approximately 1800 ìmol photons m^-2s^-1. Assimilation rates increased from 15 to 35 ìmol m^-2s^-1 with increasing leaf nitrogen from 1.61 to 4.56 per cent leaf dry weight respectively. Stomatal conductance and transpiration increased positively with increasing leaf nitrogen concentration, while the photosynthetic water use efficiency (mg CO₂/g H₂O) was not significantly affected by leaf nitrogen status. The lower transpiration rates under low nitrogen regimes were believed to be due to partial stomatal closure. The amounts of leaf starch decreased with increasing leaf nitrogen while sucrose content increased. The ratios of starch/sucrose decreased from 5.3 to 1.2 with increasing leaf nitrogen. Chlorophyll content and RUBP carboxylase activities were proportional to leaf nitrogen content. It appears that in this experiment leaf nitrogen controlled the gas exchange responses of cotton leaves in a manner similar to leaf responses to water deficits.