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The photosynthesis of young Panicum C 4 leaves is not C 3 ‐like
Author(s) -
Ghannoum O.,
Siebke K.,
Von Caemmerer S.,
Conroy J. P.
Publication year - 1998
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1046/j.1365-3040.1998.00348.x
Subject(s) - photosynthesis , photorespiration , c4 photosynthesis , photosystem ii , vascular bundle , botany , chemistry , chlorophyll fluorescence , stomatal conductance , horticulture , biology
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.