Leaf Architecture and Net Photosynthetic Rate During Ontogeny of Primary French Bean Leaves

Abstract

Anatomical characteristics (mesophyll thickness, thickness of palisade and spongy mesophyll layers, internal leaf surface area, the mean distance of chloroplasts from the mesophyll cell surface, and stomata density and sizes) of primary bean leaves (Phaseolus vulgaris L. cvs. Jantar and Harzgruss) were followed during leaf ontogeny with the aim to evaluate how changes in leaf structure may limit the photosynthetic performance of the leaf. During leaf development leaf area, mesophyll thickness and internal leaf surface area increased to a maximum size. The mean distance of chloroplasts from the cell wall surface increased to a maximum on about the 5th day after leaf unfolding followed by a slow decline. Stomata density decreased whereas stomata sizes increased during leaf ontogeny. These changes were reflected in changes in the conductances for CO2 transfer calculated from the anatomical parameters: thus, the intracellular conductance decreased shortly after leaf unfolding and increased slowly thereafter; stomatal conductance increased to a maximum on about the 5th day after leaf unfolding and then slowly decreased. The comparison of the ontogenetic course of net photosynthetic rate, i.e. a rapid increase after leaf expansion to a maximum, followed by a slow decrease in senescent leaves, with that of the anatomical characteristics showed that the compact leaf structure (tightly packed mesophyll cells, small intercellular spaces) of the young leaf might limit the CO2 supply to photosynthetic centres.