Short‐Term Changes in Heat Tolerance in the Alpine Cushion Plant Silene acaulis ssp. excapa [All.] J. Braun at Different Altitudes

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Abstract
The habit of cushion growth positively affects plant temperature but at the same may increase the risk of occasional overheating. In order to determine the adaptive response to short‐term heat stress, we exposed S. acaulis cushions at field sites to controlled heat treatments using infrared lamps. Natural diurnal changes in heat tolerance were monitored at alpine sites and at a site distinctly below the natural distribution boundary, where higher temperatures were expected. The range of heat tolerance limits in summer, 45.5 ‐ 54.5 °C (9 K), exceeded that reported for other alpine species (0.1 ‐ 5 K) and even that for total seasonal changes (5 ‐ 8 K). Heat tolerance either increased or decreased on most days (80 %). The maximum diurnal increase was + 4.7 K. Under the experimental conditions heat hardening started at leaf temperatures around 30 °C and proceeded at mean rates of 1.0 ± 0.5 K/h. The onset of functional disturbances in photosystem II also occurred at 30 °C. Heating rates exceeding those naturally found above 30 °C (> 10 K/h) appeared to retard heat hardening. During summer average leaf temperature maxima were 12.4 K (600 m) and 13.0 K (1945 m) higher than air temperature which corroborates the heat trapping nature of cushion plants. At 600 m, as compared to 1945 m, cushions experienced significantly higher leaf temperature maxima (+ 8.8 K) and exceeded 30 °C on most days (80 %). This resulted in a significantly higher heat tolerance (LT50) at 600 m (51.7 ± 0.2 °C) than at 1945 m (49.8 ± 0.2 °C). The fast short‐term changes of heat tolerance in summer help S. acaulis to cope with the occasional diurnal short‐term heat stress associated with cushion growth.