Photosynthetic Characteristics of Spinach Leaves Grown with Different Nitrogen Treatments

Spinach plants (Spinacia oleracea L.) were grown hydroponically with different concentrations of nitrate nitrogen, ranging from 0.5 to 12 mM, in a glasshouse under full sunlight. Using an open gas exchange system, the rate of CO2 assimilation, A, was determined as a function of intercellular partial pressure of CO2, Pi, with a constant amount of absorbed light per unit Chl. When expressed on a leaf area basis, A measured at high irradiance and at pi=500 μbar, was proportional to the in vitro rate of uncoupled whole-chain electron transport as well as to Chl content. There was a curvilinear relationship between the mesophyll conductance (the slope of the A : Pi curve near the CO2 compensation point) and the in vitro RuBP carboxylase activity. The curvature did not appear to be due to enzyme inactivation in vivo in leaves with high nitrogen contents. The curvature suggested the presence of a CO2 transfer resistance between the intercellular spaces and the site of carboxylation of 2.2 m2 s bar mol−1 CO2, which is similar to that previously observed in wheat. This implied that, while nitrogen deficiency increased the ratio of in vitro activity of electron transport to that of RuBP carboxylase, the two activities remained balanced in vivo. Irradiance response curves were determined by both net CO2 and O2 exchange. The two methods gave reasonable agreement at light saturation. The quantum yield measured by O2 evolution was 0.090±0.003 mol O2 mol−1 absorbed quanta, whereas after correcting for pi = 500μbar, the quantum yield for CO2 assimilation was only 82% of that measured by oxygen evolution.