Application of heat stress in situ demonstrates a protective role of irradiation on photosynthetic performance in alpine plants

Abstract

The impact of sublethal heat on photosynthetic performance, photosynthetic pigments and free radical scavenging activity was examined in three high mountain species, Rhododendron ferrugineum, Senecio incanus and Ranunculus glacialis using controlled in situ applications of heat stress, both in darkness and under natural solar irradiation. Heat treatments applied in the dark reversibly reduced photosynthetic performance and the maximum quantum efficiency of photosystem II (Fv/Fm), which remained impeded for several days when plants were exposed to natural light conditions subsequently to the heat treatment. In contrast, plants exposed to heat stress under natural irradiation were able to tolerate and recover from heat stress more readily. The critical temperature threshold for chlorophyll fluorescence was higher under illumination (Tc′) than in the dark (Tc). Heat stress caused a significant de-epoxidation of the xanthophyll cycle pigments both in the light and in the dark conditions. Total free radical scavenging activity was highest when heat stress was applied in the dark. This study demonstrates that, in the European Alps, heat waves can temporarily have a negative impact on photosynthesis and, importantly, that results obtained from experiments performed in darkness and/or on detached plant material may not reliably predict the impact of heat stress under field conditions. Heat stress will become an increasingly significant factor for alpine plants. We simulated sub-lethal heat spells by a novel Heat Tolerance Testing System (HTTS) that allowed us to expose whole plants to defined temperatures in situ, both in darkness and under natural solar irradiation. The impact of the heat-treatments on xanthophyll cycle and free radical scavenging activity was determined and recovery of photosynthetic functions was studied. The results illustrate that under current climatic conditions, photosynthesis of individual alpine plant species can already be temporarily reduced by naturally occurring heat waves. Further, the alpine plant species studied tolerated heat stress much better in the light, at levels that corresponded to natural solar irradiation, than in darkness.