Changes in stomatal conductance along grass blades reflect changes in leaf structure

Abstract

Identifying the consequences of grass blade morphology (long, narrow leaves) on the heterogeneity of gas exchange is fundamental to an understanding of the physiology of this growth form. We examined acropetal changes in anatomy, hydraulic conductivity and rates of gas exchange in five grass species (including C 3 and C 4 functional types). Both stomatal conductance and photosynthesis increased along all grass blades despite constant light availability. Hydraulic efficiency within the xylem remained constant along the leaf, but structural changes outside the xylem changed in concert with stomatal conductance. Stomatal density and stomatal pore index remained constant along grass blades but interveinal distance decreased acropetally resulting in a decreased path length for water movement from vascular bundle to stomate. The increase in stomatal conductance was correlated with the decreased path length through the leaf mesophyll. A strong correlation between the distance from vascular bundles to stomatal pores and stomatal conductance has been identified across species; our results suggest this relationship also exists within individual leaves. We investigated the coordinated change in leaf structure and function along grass blades. Photosynthesis and stomatal conductance increased acropetally from base to tip, which was correlated with a decreased pathlength for the movement of water from the vascular bundle to stomate. The decreased pathlength for water movement should allow greater stomatal conductance while minimizing the water potential gradient across the leaf mesophyll. These results highlight the tight correlation between changes in leaf structure and function within individual grass blades. © 2011 Blackwell Publishing Ltd.