Interactions between water stress, sun‐shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

Abstract

Gas exchange and chlorophyll fluorescence techniques were used to evaluate the acclimation capacity of the schlerophyll shrub Heteromeles arbutifolia M. Roem. to the multiple co-occurring summer stresses of the California chaparral. We examined the influence of water, heat and high light stresses on the carhon gain and survival of sun and shade seedlings via a factorial experiment involving a slow drying cycle applied to plants grown outdoors during the summer. The photochemical efficiency of PSII exhibited a diurnal, transient decrease (AF/F^') and a chronic decrease or photoinhibition (FJFj^) in plants exposed to full sunlight. Water stress enhanced both transient decreases of AFIF^' and photoinhibition. Effects of decreased AF/F^^' and F,^/F^ on carbon gain were observed only in well-watered plants since in waterstressed plants they were overidden by stomatal closure. Reductions in photochemical efficiency and stomatal conductance were observed in all plants exposed to full sunlight, even in those that were well-watered. This suggested that H. arbutifolia sacrificed carbon gain for water conservation and photoprotection (both structurally via sboot architecture and physiologically via down-regulation) and that this response was triggered by a hot and dry atmosphere together with high PFD, before severe water, heat or high PFD stresses occur. We found fast adaptive adjustments of the thermal stability of PSII (diurnal changes) and a superimposed long-term acclimation (days to weeks) to high leaf temperatures. Water stress enhanced resistance of PSII to high temperatures both in the dark and over a wide range of PFD. Low PFD protected photochemical activity against inactivation by heat while high PFD exacerbated damage of PSII by heat. The greater interception of radiation by horizontally restrained leaves relative to the steep leaves of sun-acclimated plants caused photoinhibition and increased leaf temperature. When transpirational cooling was decreased by water stress, leaf temperature surpassed the limits of chloroplast thermostability. The remarkable acclimation of water-stressed Key-words: Heteromeles arbutifolia., heat stress; interaction between stresses; leaf angle; photoinhibition; photosynthesis; sclerophyll; sun-shade acclimation; thermal stability of photosystem II; transpiration cooling; water stress.Abbreviations: A, net CO2 assimilation rate; FJF^, photochemical efficiency of PSII (dark adapted leaves); AF/F^', photochemical efficiency of PSII in the light; g^, stomatal conductance to water vapour; PFD, photosynthetic photon flux density; PSII, photosystem II; T^., critical temperature for heat-induced fluorescence rise; Tp, temperature of heatinduced peak fluorescence; % water potential.

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