The contributions of apoplastic, symplastic and gas phase pathways for water transport outside the bundle sheath in leaves

Water movement from the xylem to stomata is poorly understood. There is still no consensus about whether apoplastic or symplastic pathways are more important, and recent work suggests vapour diffusion may also play a role. The objective of this study was to estimate the proportions of hydraulic conductance outside the bundle sheath contributed by apoplastic, symplastic and gas phase pathways, using a novel analytical framework based on measurable anatomical and biophysical parameters. The calculations presented here suggest that apoplastic pathways provide the majority of conductance outside the bundle sheath under most conditions, whereas symplastic pathways contribute only a small proportion. The contributions of apoplastic and gas phase pathways vary depending on several critical but poorly known or highly variable parameters namely, the effective Poiseuille radius for apoplastic bulk flow, the thickness of cell walls and vertical temperature gradients within the leaf. The gas phase conductance should increase strongly as the leaf centre becomes warmer than the epidermis - providing up to 44% of vertical water transport for a temperature gradient of 0.2K. These results may help to explain how leaf water transport is influenced by light absorption, temperature and differences in leaf anatomy among species. The pathways for water movement outside the xylem are poorly known. This study derived an analytical framework to compute conductances of apoplastic, symplastic and gas phase pathways for horizontal and vertical water transport outside the bundle sheath from measurable anatomical and biophysical parameters. The results suggest most conductance outside the bundle sheath is apoplastic, but that gas phase transport can play a similarly large role in vertical water movement when large temperature gradients exist between the center of the leaf and the epidermis. Commentary: