Developmental changes in mesophyll diffusion conductance and photosynthetic capacity under different light and water availabilities in Populus tremula: how structure constrains function

Finite mesophyll diffusion conductance (gm) significantly constrains net assimilation rate (An), but gm variations and variation sources in response to environmental stresses during leaf development are imperfectly known. The combined effects of light and water limitations on gm and diffusion limitations of photosynthesis were studied in saplings of Populus tremula L. An one-dimensional diffusion model was used to gain insight into the importance of key anatomical traits in determining gm. Leaf development was associated with increases in dry mass per unit area, thickness, density, exposed mesophyll (Smes/S) and chloroplast (Sc/S) to leaf area ratio, internal air space (fias), cell wall thickness and chloroplast dimensions. Development of Smes/S and Sc/S was delayed under low light. Reduction in light availability was associated with lower Sc/S, but with larger fias and chloroplast thickness. Water stress reduced Sc/S and increased cell wall thickness under high light. In all treatments, gm and An increased and CO2 drawdown because of gm, CiCc, decreased with increasing leaf age. Low light and drought resulted in reduced gm and An and increased CiCc. These results emphasize the importance of gm and its components in determining An variations during leaf development and in response to stress.