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Canopy gradients in leaf intercellular CO 2 mole fractions revisited: interactions between leaf irradiance and water stress need consideration
Author(s) -
NIINEMETS Ü.,
SONNINEN E.,
TOBIAS M.
Publication year - 2004
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 - 10.1111/j.1365-3040.2003.01168.x
Subject(s) - canopy , irradiance , photosynthesis , stomatal conductance , atmospheric sciences , transpiration , ecophysiology , botany , growing season , horticulture , environmental science , chemistry , biology , physics , quantum mechanics
Intercellular CO 2 mole fractions ( C i ) are lower in the upper canopy relative to the lower canopy leaves. This canopy gradient in C i has been associated with enhanced rates of carbon assimilation at high light, and concomitant greater draw‐downs in C i . However, increases in irradiance in the canopy are generally also associated with decreases in leaf water availability. Thus, stress effects on photosynthesis rates ( A ) and stomatal conductance ( G ), may provide a further explanation for the observed C i gradients. To test the hypotheses of the sources of canopy variation in C i, and quantitatively assess the influence of within‐canopy differences in stomatal regulation on A , the seasonal and diurnal variation in G was studied in relation to seasonal average daily integrated quantum flux density ( Q int ) in tall shade‐intolerant Populus tremula L. trees. Daily time‐courses of A were simulated using the photosynthesis model of Farquhar et al. ( Planta 149, 78–90, 1980). Stable carbon isotope composition of a leaf carbon fraction with rapid turnover rate was used to estimate canopy gradient in C i during the simulations. Daily maximum G ( G max ) consistently increased with increasing Q int . However, canopy differences in G max decreased as soil water availability became limiting during the season. In water‐stressed leaves, there were strong mid‐day decreases in G that were poorly associated with vapour pressure deficits between the leaf and atmosphere, and the magnitude of the mid‐day decreases in G occasionally interacted with long‐term leaf light environment. Simulations indicated that the percentage of carbon lost due to mid‐day stomatal closure was of the order of 5–10%, and seasonal water stress increased this percentage up to 20%. The percentage of carbon lost due to stomatal closure increased with increasing Q int . Canopy differences in light environment resulted in a gradient of daily average C i of approximately 20 µ mol mol −1 . The canopy variation in seasonal and diurnal reductions in G led to a C i gradient of approximately 100 µ mol mol −1 , and the actual canopy C i gradient was of the same magnitude according to leaf carbon isotope composition. This study demonstrates that stress effects influence C i more strongly than within‐canopy light gradients, and also that leaves acclimated to different irradiance and water stress conditions may regulate water use largely independent of foliar photosynthetic potentials.