Should we delay leaf water potential measurements after excision? Dehydration or equilibration?

Background: Accurate leaf water potential (Ψ) determination is crucial in studying plant responses to water deficit. After excision, water potential decreases, even under low evaporative demand conditions, which has been recently attributed to the equilibration of pre-excision Ψ gradients across the leaf. We assessed the influence of potential re-equilibration on water potential determination by monitoring leaf Ψ and relative water content decline after excision using different storage methods.

Results: Even though leaf Ψ declined during storage under low evaporative demand conditions, this was strongly reduced when covering the leaf with a hydrophobic layer (vaseline) and explained by changes in relative water content. However, residual water loss was variable between species, possibly related to morpho-physiological leaf traits. Provided water loss was minimized during storage, pre-excision leaf transpiration rate did not affect to the magnitude of leaf Ψ decline after excision, confirming that transpiration-driven Ψ gradients have no effect on leaf Ψ determination.

Conclusions: Disequilibrium in water potentials across a transpiring leaf upon excision is dissipated very quickly, well within the elapsed time between excision and pressurization, therefore, not resulting in overestimation of leaf Ψ measured immediately after excision. When leaf storage is required, the effectiveness of a storage under low evaporative demand varied among species. Covering with a hydrophobic layer is an acceptable alternative.