PARENT SESSION
Posters P3B Photosynthetic acclimation: Ecophysiology, diverse environments. Abstracts (479-522)

Impact of drought and heat stress on the photosynthetic apparatus of Quercus pubescens: A field study. Pierre Haldimann^*,1, Alexander Gallé¹, Urs Feller¹, ¹ Institute of Plant Sciences, Bern, Bern, Switzerland

ABSTRACT- The response of the photosynthetic apparatus of oak (Quercus pubescens) leaves to prolonged drought and heat stress was examined in the field. Gas exchange and chlorophyll a fluorescence parameters were measured over the course of the day for different stress situations. When the shrubs were neither water (predawn leaf water potential (PLWP) = -0.34 MPa) nor temperature stressed (leaf temperature (LT) < 30°C), net photosynthesis (P_n) increased in the morning with increasing solar irradiance. Light-saturated P_n reached a maximum (P_nmax) before noon and thereafter rapidly decreased by 30%. P_n then remained constant until photosynthesis became light-limited in the late afternoon. Similar changes were observed for the stomatal conductance (g_s). At moderate drought stress (PLWP = -1.90 MPa), the P_nmax achieved in the morning was reduced by 18%. In the afternoon the combination of water stress and high LT (40 °C) resulted in a strong inhibition of P_n. After prolonged drought stress (PLWP = -2.70 MPa), the P_nmax was reduced by more than 80% and P_n got virtually completely inhibited in the afternoon, when LT occasionally increased above 41.5 °C. Reduction of P_n by drought stress correlated with reduced g_s values, but the intercellular CO₂ concentration (C_i) did not decrease. At high LT the C_i showed an increase, indicating metabolic limitations. Chlorophyll fluorescence measurements revealed that inhibition of P_n was accompanied by a down-regulation of photosystem (PS) II activity, which was associated with the accumulation of large amounts of zeaxanthin. The maximum quantum yield of the PSII primary photochemistry showed a reversible reduction over the day. The results suggest that the capability of Q. pubescens to preserve the functional potential of its photosynthetic apparatus during drought and heat stress periods is an important factor for the remarkable ability of this tree species to grow and survive in hot and dry habitats.

KEY WORDS: heat, drought, photosynthesis, Quercus pubescens