The photosynthesis of young Panicum C4 leaves is not C3‐like

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Abstract
Evidence is presented contrary to the suggestion that C4 plants grow larger at elevated CO2 because the C4 pathway of young C4 leaves has C3‐like characteristics, making their photosynthesis O2 sensitive and responsive to high CO2. We combined PAM fluorescence with gas exchange measurements to examine the O2 dependence of photosynthesis in young and mature leaves of Panicum antidotale (C4, NADP‐ME) and P. coloratum (C4, NAD‐ME), at an intercellular CO2 concentration of 5 Pa. P. laxum (C3) was used for comparison. The young C4 leaves had CO2 and light response curves typical of C4 photosynthesis. When the O2 concentration was gradually increased between 2 and 40%, CO2 assimilation rates (A) of both mature and young C4 leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO2 assimilation (ΦPSII/ΦCO2) increased more in young (up to 31%) than mature (up to 10%) C4 leaves. A of C3 leaves decreased by 1·3 and ΦPSII/ΦCO2 increased by 9‐fold, over the same range of O2 concentrations. Larger increases in electron transport requirements in young, relative to mature, C4 leaves at low CO2 are indicative of greater O2 sensitivity of photorespiration. Photosynthesis modelling showed that young C4 leaves have lower bundle sheath CO2 concentration, brought about by higher bundle sheath conductance relative to the activity of the C4 and C3 cycles and/or lower ratio of activities of the C4 to C3 cycles.