photosynthesis of young Panicum C₄ leaves is not C₃-like

• Published in: Plant, cell and environment cell and environment, 1998-11, Vol.21 (11), p.1123-1131
• Main Authors: Ghannoum, O, Siebke, K, Von Caemmerer, S, Conroy, J.P
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; C4 photosynthesis; chlorophyll fluorescence; Economic plant physiology; Fundamental and applied biological sciences. Psychology; leaf development; Metabolism; Net assimilation, photosynthesis, carbon metabolism. Photorespiration, respiration, fermentation (anoxia, hypoxia); Nutrition. Photosynthesis. Respiration. Metabolism; Panicum; photorespiration; photosynthesis modelling; Photosynthesis, respiration. Anabolism, catabolism; Plant physiology and development
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1046/j.1365-3040.1998.00348.x

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.