The photosynthesis of young Panicum C 4 leaves is not C 3 ‐like

• Published in: Plant, cell and environment Vol. 21; no. 11; pp. 1123 - 1131
• Main Authors: Ghannoum, O., Siebke, K., Von Caemmerer, S., Conroy, J. P.
• Format: Journal Article
• Language: English
• Published: 01.11.1998
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1046/j.1365-3040.1998.00348.x

Evidence is presented contrary to the suggestion that C 4 plants grow larger at elevated CO 2 because the C 4 pathway of young C 4 leaves has C 3 ‐like characteristics, making their photosynthesis O 2 sensitive and responsive to high CO 2 . We combined PAM fluorescence with gas exchange measurements to examine the O 2 dependence of photosynthesis in young and mature leaves of Panicum antidotale (C 4 , NADP‐ME) and P. coloratum (C 4 , NAD‐ME), at an intercellular CO 2 concentration of 5 Pa. P. laxum (C 3 ) was used for comparison. The young C 4 leaves had CO 2 and light response curves typical of C 4 photosynthesis. When the O 2 concentration was gradually increased between 2 and 40%, CO 2 assimilation rates ( A ) of both mature and young C 4 leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO 2 assimilation ( Φ PSII / Φ CO2 ) increased more in young (up to 31%) than mature (up to 10%) C 4 leaves. A of C 3 leaves decreased by 1·3 and Φ PSII / Φ CO2 increased by 9‐fold, over the same range of O 2 concentrations. Larger increases in electron transport requirements in young, relative to mature, C 4 leaves at low CO 2 are indicative of greater O 2 sensitivity of photorespiration. Photosynthesis modelling showed that young C 4 leaves have lower bundle sheath CO 2 concentration, brought about by higher bundle sheath conductance relative to the activity of the C 4 and C 3 cycles and/or lower ratio of activities of the C 4 to C 3 cycles.