Understanding the biophysical limitations of mesophyll and hydraulic conductance in leaves

Abstract

Biophysical limitations to water and carbon dioxide movement within leaves - hydraulic resistance and mesophyll resistance - pose significant restrictions on photosynthetic capacity. Maximising the inverse of these resistances - leaf hydraulic conductance and mesophyll conductance - across a range of environmental conditions is therefore paramount for maximising photosynthetic rates for plants. Although the responses of mesophyll conductance and leaf hydraulic conductance to environmental conditions are well-known, considerably less is known about their coordinated response to variable environmental conditions. Understanding water pathways may also be assisted through the use of leaf water isotopologue models; however the commonly used Péclet effect model has recently come under scrutiny and requires further investigation. The main focus of this thesis was to investigate the coordinated response of mesophyll conductance and leaf hydraulic conductance in several species to different short-term and growth environment conditions. Under short-term changes in environmental conditions, there was a positive correlation between mesophyll and leaf hydraulic conductance for species with parallel venation but not for species with reticulate venation. Variation in growth environmental conditions resulted in a positive correlation between mesophyll and leaf hydraulic conductance in Gossypium hirsutum, which was likely driven by changes in leaf anatomy. Finally, there were inconclusive results regarding the usefulness of an ‘effective path length’ parameter in the Péclet effect models for describing water pathways in leaves. These results demonstrate that more research is needed to understand the limitations of carbon and water movement on maximising photosynthetic capacity and how this is affected by leaf anatomy, such as leaf venation and bundle sheath extensions.