Effects of leaf temperature on initial stomatal opening and their roles in overall and biochemical photosynthetic induction

Abstract

The more stomata affect photosynthetic induction courses, the stronger the influence of leaf temperature on the induction parameters becomes. Initial stomatal conductances relative to species-specific stomatal thresholds explain seemingly contradictory response patterns. We analyzed the effects of leaf temperature, T leaf (at simultaneously altered leaf-to-air vapor concentration difference, Δw) on “overall” and “biochemical” photosynthetic induction as modified by stomata and in response to single, rectangular steps from darkness to saturating light. Studies were performed in sun leaves of shade-intolerant Betula pubescens Ehrh. and shade-tolerant Fagus sylvatica L. In general, induction proceeded faster with higher T leaf. This pattern was clearly visible when limitations to induction were dominated by stomata, which was the case at low initial stomatal conductance (g ini), as well as when there was a long stomatal lag time in response to the light-step (t lag) and slow stomatal opening (long time to reach 90% of full stomatal conductance, t 90%g). t lag and t 90%g became shorter with rising T leaf despite strongly increasing Δw (temperature-driven), while g ini was not affected. Species-specific thresholds, namely g ini(crit), above which halftimes of induction are no longer related to g ini, were higher in Betula (≈35 mmol m−2 s−1) and most likely not temperature related, while they decreased with T leaf (from 15 to 35 °C) in Fagus from about 30 to 10 mmol m−2 s−1. In Betula, g ini values were typically above and in Fagus typically below g ini(crit). Induction states 60 s after the light-step (IS60) rose with temperature while becoming more sensitive to g ini with higher T leaf. Induction courses which began either above or below g ini(crit) resulted in entirely different dependencies of induction halftimes on T leaf, confirming the importance of g ini(crit). This must be kept in mind when comparing different species. Implications for modeling are discussed.