Photoinhibition at chilling temperatures and effects of freezing stress on cold acclimated spinach leaves in the field. A fluorescence study

1399-3054

Blackwell Publishing Journal Backfiles 1879-2005

Biology

The role of high light stress in a natural environment was studied on spinach plants (Spinacia oleracea L. cv. Wolter) grown in the field during the winter season. Fluorescence induction (at 293 K and 77 K) of leaves was used to characterize the stress effects. Night frost with minimum temperatures between – 1.5°C and –7.5°C (i.e. above the‘frost killing point’at ca. –11.5°C) led to impaired photosynthesis. This was seen as increased initial fluorescence (Fo), decreased ratio of variable to maximum fluorescence (FV/FM) and lowered rates of O2 evolution. The freezing injury was reversible within several frostless days. Exposure to high light (about 900 mol m–2 s–1) at chilling temperatures in the field caused photoinhibition, manifested as decreased variable fluorescence (FV) and FV/FM ratio without changes in FO. The photoinhibitory fluorescence quenching was not stronger after frost than after frostless nights; synergism between light stress and preceding freezing stress was not observed. Fluorescence induction signals at 77 K showed that FV of photosystems I and II decreased to the same extent, indicating increased thermal deactivation of excited chlorophyll. Photoinhibition was fully reversible at +4°C within 1 h in low light, but only partially in moderate light. Preceding night frosts did not affect the recovery. The photoinhibition observed here is regarded as a protective system of thermal dissipation of excess light energy.

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