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Free keywords:
advection-diffusion
deuterium
delta H-2
delta O-18
drought
non-steady state
oxygen
Peclet
transpiration
water isotopes
ORGANIC-MATTER
STOMATAL CONDUCTANCE
STABLE-ISOTOPES
COTTON LEAVES
HYDRAULIC CONDUCTANCE
TRANSPIRATION RATE
FOREST ECOSYSTEMS
RICINUS-COMMUNIS
ATMOSPHERIC CO2
GAS-EXCHANGE
Abstract:
Current models of leaf water enrichment predict that the differences between isotopic enrichment of water at the site of evaporation (Delta(e)) and mean lamina leaf water enrichment (Delta(L)) depend on transpiration rates (E), modulated by the scaled effective length (L) of water isotope movement in the leaf. However, variations in leaf parameters in response to changing environmental conditions might cause changes in the water path and thus L. We measured the diel course of Delta(L) for O-18 and H-2 in beech seedlings under well-watered and water-limited conditions. We applied evaporative enrichment models of increasing complexity to predict Delta(e) and Delta(L), and estimated L from model fits. Water-limited plants showed moderate drought stress, with lower stomatal conductance, E and stem water potential than the control. Despite having double E, the divergence between Delta(e) and Delta(L) was lower in well-watered than in water-limited plants, and thus, L should have changed to counteract differences in E. Indeed, L was about threefold higher in water-limited plants, regardless of the models used. We conclude that L changes with plant water status far beyond the variations explained by water content and other measured variables, thus limiting the use of current evaporative models under changing environmental conditions.
