Processes contributing to photoprotection of grapevine leaves illuminated at low temperature Luke Hendrickson a , Britta Fo¨rster b , Robert T. Furbank c and Wah Soon Chow a, * a Research School of Biological Sciences, b School of Biochemistry and Molecular Biology, Australian National University, Canberra, ACT 0200, Australia c CSIRO Plant Industry, GPO Box 1600 Canberra ACT 2601, Australia *Corresponding author, e-mail: chow@rsbs.anu.edu.au Received 12 November 2003; revised 12 January 2004 Photoinactivation of photosystem II (PSII) and energy dis- sipation at low leaf temperatures were investigated in leaves of glasshouse-grown grapevine (Vitis vinifera L. cv. Riesling). At low temperatures (, 15  C), photosynthetic rates of CO 2 assimilation were reduced. However, despite a significant increase in the amount of light excessive to that required by photosynthesis, grapevine leaves maintained high intrinsic quantum efficiencies of PSII (F v /F m ) and were highly resist- ant to photoinactivation compared to other species. Non- photochemical energy dissipation involving xanthophylls and fast D1 repair were the main protective processes redu- cing the ‘gross’ rate of photoinactivation and the ‘net’ rate of photoinactivation, respectively. We developed an improved method of energy dissipation analysis that revealed up to 75% of absorbed light is dissipated thermally via pH- and xanthophyll-mediated non-photochemical quenching at low temperatures (5–15  C) and moderate (800 mmol quanta m 2 s 1 ) light. Up to 20% of the energy flux contributing to electron transport was dissipated via photorespiration when taking into account temperature-dependent mesophyll conductance; however, this flux used in photorespiration was only a relatively small amount of the total absorbed light energy. Photoreduction of O 2 at photosystem I (PSI) and subsequent superoxide detoxification (water-water cycle) was more sensitive to inhibition by low temperature than photorespiration. Therefore the water-water cycle represents a negligibly small energy sink below 15  C, irrespective of mesophyll conductance. Introduction Low temperatures can limit light- and CO 2 -saturated photosynthetic capacity of leaves in many plant species because the proportion of absorbed light that is excessive to photochemistry is increased (Allen and Ort 2001). This can drastically reduce growth and yield of crop species grown in many different climates. The photo- systems are the primary targets for chilling-induced photoinactivation. In some chilling-sensitive plant species inhibition of photosynthetic electron transport can occur, despite relatively minimal reductions in F v /F m due to net photoinactivation of PSI rather than PSII (Terashima et al. 1994; Tjus et al. 1998; Sonoike 1999) when leaf temperature drops to around 10  C. Inhibition of PSI commonly occurs in species that have evolved in tropical or subtropical climates and appears to indicate a high degree of chilling sensitivity. Under these circum- stances the F v /F m assessment could underestimate the impact of chilling in the presence of light on leaf photo- synthesis. Low temperature-induced stress has been shown to limit growth of grapevine (Buttrose 1969), which is an economically important C 3 crop in many parts of the PHYSIOLOGIA PLANTARUM 121: 272–281. 2004 DOI: 10.1111/j.1399-3054.2004.00324.x Printed in Denmark – all rights reserved Copyright # Physiologia Plantarum 2004 Abbreviations – DpH, trans-thylakoid pH gradient; F f , quantum yield of fluorescence; F D , quantum yield of light-independent thermal energy dissipation; QY I , quantum yield of PSII photoinactivation; F NPQ , quantum yield of light-dependent and DpH- and xanthophyll-mediated thermal energy dissipation; F PSII , quantum yield of PSII reaction centre photochemistry; C c and C i , CO 2 concentration in the chloroplast and intercellular airspace, respectively; F v /F m , intrinsic quantum efficiency of PSII photochemistry; I A , PAR absorbed by the leaf; J PSII ,J NPQ and J f,D , rate of energy dissipation via linear photosynthetic electron transfer, light-dependent thermal processes, and light-independent thermal processes and fluorescence, respectively; NPQ, non-photochemical quenching; PAR, photosynthetically active radiation; T leaf , leaf temperature. 272 Physiol. Plant. 121, 2004