Freezing and high temperature thresholds of photosystem 2 compared to ice nucleation, frost and heat damage in evergreen subalpine plants

Abstract

Freezing and high temperature thresholds of photosystem 2 (PS2), ice formation and frost and heat damage were measured in leaves of evergreen subalpine plants under conditions of naturally low (winter) to high (summer) PS2 efficiencies (F V /F M ). The temperature‐dependent change in basic Chl fluorescence (F 0 ) (T‐F 0 ) technique that is usually used to assess the high temperature threshold of PS2 in a new approach was applied to test freezing temperature thresholds of PS2. T‐F 0 curves (+5 °C to −10 °C at 2 K h −1 ) revealed a significant, sudden increase in F 0 on extracellular ice formation (−4.0 or −5.5 °C). The rise in F 0 was recorded 0.3–0.6 K below ice nucleation (10–20 min later) and was produced by freeze dehydration of cells. The rise in F 0 was not caused by frost damage, as during winter LT 50 was lower than −27 °C and not by formation of ice on the leaf surface. Hence, F 0 measurements during freezing are a useful tool to distinguish between surface ice and extracellular ice inside the leaf tissue which cannot be differentiated by other ice‐detecting methods. PS2 efficiency significantly affected the shape of the high temperature T‐F 0 curves (20–65 °C at 1 K min −1 ). Under F V /F M >0.6, two F 0 maxima were recorded. The fast rise phase to the first F 0 maximum corresponded with tissue heat damage (LT 50 : 46.9–54.3 °C). The second F 0 maximum occurred at leaf temperatures between 55 and 60 °C. Under F V /F M <0.2 only, the second F 0 maximum was detectable. Lack of awareness of the missing F 0 maximum would lead to an overestimation of the PS2 high temperature threshold by >10 K; hence, under low F V /F M , it cannot be determined by the T‐F 0 technique.