Canopy gradients in leaf intercellular CO2 mole fractions revisited:: interactions between leaf irradiance and water stress need consideration

Intercellular CO2 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,C- 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 mumol mol(-1). The canopy variation in seasonal and diurnal reductions in G led to a C-i gradient of approximately 100 mumol 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.