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

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 (F(o) ), decreased ratio of variable to maximum fluorescence (F(V) /F(M) ) and lowered rates of O(2) 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 (F(V) ) and F(V) /F(M) ratio without changes in F(O) . 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 F(V) 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.