ABSTRACT
The permeability of leaf tissue to water has been reported to increase under illumination, a response reputed to involve aquaporins. We studied this 'light response' in red oak (Quercus rubra L.), the species in which the phenomenon was first detected during measurements of leaf hydraulic conductance with the high-pressure flow meter (HPFM). In our HPFM measurements, we found that pre-conditioning leaves in darkness was not sufficient to bring them to their minimum conductance, which was attained only after an hour of submersion and pressurization. However, pre-conditioning leaves under anoxic conditions resulted in an immediate reduction in conductance. Leaves light- and dark-acclimated while on the tree showed no differences in the time course of HPFM measurement under illumination. We also studied the effect of light level and anoxia on rehydration kinetics, finding that anoxia slowed rehydration, but light had no effect either in the lab (rehydration under low light, high humidity) or on the tree (acclimation under high light, 10 min of dark prior to rehydration). We conclude that the declines in conductance observed in the HPFM must involve a resistance downstream of the extracellular air space, and that in red oak the hydraulic conductivity of leaf tissue is insensitive to light.
Subject(s)
Light , Plant Transpiration/physiology , Quercus/physiology , Water/metabolism , Aquaporins/metabolism , Biological Transport , Cryoelectron Microscopy , Gene Expression Regulation, Plant/radiation effects , Permeability/radiation effects , Plant Leaves/anatomy & histology , Plant Leaves/physiology , Plant Leaves/radiation effects , Plant Stomata/anatomy & histology , Plant Transpiration/radiation effects , Quercus/anatomy & histology , Quercus/radiation effects , Time FactorsABSTRACT
This study investigates the functional significance of heterophylly in Ginkgo biloba, where leaves borne on short shoots are ontogenetically distinct from those on long shoots. Short shoots are compact, with minimal internodal elongation; their leaves are supplied with water through mature branches. Long shoots extend the canopy and have significant internodal elongation; their expanding leaves receive water from a shoot that is itself maturing. Morphology, stomatal traits, hydraulic architecture, Huber values, water transport efficiency, in situ gas exchange and laboratory-based steady-state hydraulic conductance were examined for each leaf type. Both structure and physiology differed markedly between the two leaf types. Short-shoot leaves were thinner and had higher vein density, lower stomatal pore index, smaller bundle sheath extensions and lower hydraulic conductance than long-shoot leaves. Long shoots had lower xylem area:leaf area ratios than short shoots during leaf expansion, but this ratio was reversed at shoot maturity. Long-shoot leaves had higher rates of photosynthesis, stomatal conductance and transpiration than short-shoot leaves. We propose that structural differences between the two G. biloba leaf types reflect greater hydraulic limitation of long-shoot leaves during expansion. In turn, differences in physiological performance of short- and long-shoot leaves correspond to their distinct ontogeny and architecture.
