Photosynthesis by Flag Leaves of Wheat in Relation to Protein, RibuloseBisphosphate Carboxylase Activity and Nitrogen Supply

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Abstract
The effects of nitrate supply on the composition (cell numbers, protein and chlorophyll contents) of flag leaves of winter wheat grown with two amounts of N fertilizer and of spring wheat grown in the glasshouse under controlled nitrate supply are described and related to photosynthesis. Nitrogen deficiency decreased the size of leaves, mainly by reducing cell number and, to a smaller extent, by decreasing cell volume. Protein content per unit leaf area, per cell and per unit cell volume was larger with abundant N. Total soluble protein, ribulose bisphosphate carboxylase-oxygenase (RuBPc-o) protein and chlorophyll changed in proportion irrespective of nitrogen supply and leaf age. Photosynthesis per unit area of flag leaf and carboxylation efficiency in both winter and spring wheat were proportional to the amount of total soluble protein up to 7.0 g m−2 and to the amount of RuBPc-o protein up to 4.0 g m−2. However, photosynthesis did not increase in proportion to the amount of total soluble or RuBPc-o protein above these amounts. In young leaves with a high protein content the measured rates of photosynthesis were lower than expected from the amount and activity of RuBPc-o. Carboxylation per unit of RuBPc-o protein, measured in vitro, was slightly greater in N-deficient leaves of winter wheat but not of spring wheat. RuBPc-o activity per unit of RuBPc-o protein was similar in winter and spring wheat leaves and remained approximately constant with age, but increased in leaves showing advanced senescence. RuBPc-o protein from N-deficient leaves migrated faster on polyacrylamide gels than protein from leaves with high N content. Regulation of the rate of photosynthesis in leaves and chloroplasts with a high protein content is discussed. The conductance of the cell to the flux of CO2 from intercellular spaces to RuBPc-o active sites is calculated, from cell surface areas and CO2 fluxes, to decrease the CO2 partial pressure at the active site by less than 0.8 Pa at an internal CO2 partial pressure of 34 Pa. Thus the decrease in partial pressure of CO2 is insufficient to account for the inefficiency of RuBPc-o in vivo at high protein contents. Other limitations to the rate of photosynthesis are considered.