PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Irreversible changes in photosynthetic rate induced by heating above 40 °C. Radek Kaa^{*,1, 2}, Eva Kotabová¹, Petr Ilík¹, Jan Nau¹, Ondej Práil², ¹ Laboratory of Biophysics, Olomouc, Czech Republic² Institute of Physical Biology, Tebo, Czech Republic

ABSTRACT- We studied heat-induced irreversible decrease in the photosynthetic rate (PR) in 8-day-old barley leaves and its possible connection with the effect of heat stress (HS) on the light photosynthetic reactions (LPR). HS was induced by a plunge of dark-adapted leaves in the water bath (24 - 46 °C) for 5 minutes. Such pretreated leaves were cooled down to room temperature and used for gasometric (detection of changes in dark photosynthetic reactions - DPR) and chl a fluorescence measurements (characterization of changes in LPR). At both methods the same intensity of actinic light was used. HS caused a decline of effective quantum yield of photosystem II (PSII) photochemistry (₂) and an increase in non-photochemical quenching of Chl a fluorescence (qN) that could be observed already after preheating to 38 °C, while decrease in PR could be first observed only after preheating to 40 °C. Simultaneously we observed rapid decrease in the carboxylation efficiency of Rubisco (dA/dC) after preheating to 38 °C. Similarly the time necessary for Rubisco activation by enzyme Rubisco-activase (RA) started to increase after preheating to temperatures around 38 - 40 °C, confirming very high sensitivity of enzyme RA to HS. Finally, the direct effect of damage to Photosystem II (damage to the oxygen evolving complex - OEC, detected as appearance of the K-peak in the very fast chl a fluorescence induction) occurred after preheating to the temperature range 40 - 42 °C when decrease in the maximal efficiency of PSII photochemistry was also observed. In conclusion, the observed decrease in PR is probably caused by concurrent harmful effects of HS on the level of LPR (lower ₂) and on the level of Calvin cycle activity (slower Rubisco activation). Insensitivity of PR to decrease in ₂ (between 38 - 40 °C) is probably caused by heat stress induced amplification of capacity of alternative electron transport pathways (cyclic electron flow, chlororespiration).

KEY WORDS: Rubisco-activase, heat stress, photosystem II, regulation of photosynthesis