Midday Stomatal Closure in Arbutus Unedo Leaves: Measurements with a Steady-State Porometer in the Portuguese Evergreen Scrub

Abstract

General types of diurnal leaf stomatal behavior were described by Stocker (1956) who suggested an increasing degree of midday stomatal closure depending on the degree of water stress experienced by a plant. Initially the closure may restrict water use in such a manner that a single peaked curve is obtained for transpiration rate plotted against time, the changes in conductance at midday only nearly compensating for changes in evaporative demand and resulting in an approximately constant transpiration rate. The daily pattern of CO₂ exchange which is often directly affected by conductance changes may, under these conditions, reveal the stomatal closure response, and one observes two peaks in net photosynthesis rate. With further increase in plant water stress, Stocker suggested that extreme midday stomatal closure may affect the daily time course of water loss to such an extent that a two-peaked curve of transpiration rate versus time results, with a depression at midday.