## Abstract

Chlorophyll fluorescence induction (at 20<latex>$^\circ$</latex>C and 77 K) and quenching were analysed in relation to effects of environmental stresses imposed by chilling in high light and by freezing and thawing of spinach (Spinacia oleracea L.) leaves. The data indicate that cold acclimation of spinach plants, which leads to increased frost tolerance of the leaves, results in decreased susceptibility to photoinhibition of photosynthesis at chilling temperatures. When plants acclimated to 18<latex>$^\circ$</latex>C and 260-300 <latex>$\mu$</latex>mol quanta <latex>$m^{-2} s^{-1}$</latex> were exposed to higher light (550 <latex>$\mu$</latex>mol quanta <latex>$m^{-2} s^{-1}$</latex>) at 4<latex>$\circ$</latex>C, they developed strong photoinhibition, as characterized by decreased quantum yield of <latex>$O_2$</latex> evolution and decreased ratio of variable: maximum fluorescence (<latex>$F_v$</latex>/<latex>$F_M$</latex>) of photosystem II. The decrease in <latex>$F_v$</latex>/<latex>$F_M$</latex> resulted from a decline in <latex>$F_v$</latex> and an increase in <latex>$F_0$</latex>. The <latex>$F_v$</latex>/<latex>$F_M$</latex> ratio was lowered to a significantly greater extent when induction was recorded at 20<latex>$\circ$</latex>C, as compared with 77 K. The effects related to photoinhibition were fully reversible at 18<latex>$\circ$</latex>C in dim light. Plants that had been cold-acclimated for 10 days exhibited slightly decreased quantum yield and lowered <latex>$F_v$</latex>/<latex>$F_M$</latex> ratio. However, they did not show further photoinhibition on exposure to 550 <latex>$\mu$</latex>mol quanta <latex>$m{-2} s{-1}$</latex> at 4<latex>$\circ$</latex>C. The reversible photoinhibition is discussed as a protective pathway serving for thermal dissipation of excessive light energy. It is hypothesized that such a mechanism prevents destruction of the photosynthetic apparatus, until other means of protection become effective during long-term acclimation to high light. Inhibition of photosynthetic carbon assimilation caused by freezing and thawing of leaves in the dark was closely correlated with inhibition of photochemical fluorescence quenching (<latex>$q_Q$</latex>). As a sensitive response of the thylakoid membranes to freezing stress, the energy-dependent quenching, <latex>$q_E$</latex>, was inhibited. Only more severe impact of freezing caused a significant decline in the <latex>$F_v$</latex>/<latex>$F_M$</latex> ratio. It is concluded that measurements of fluorescence induction signals (<latex>$F_v$</latex>/<latex>$F_M$</latex> ratios) provide a sensitive tool with which to investigate photoinhibition, whereas freezing damage to the photosynthetic system can be detected more readily by the quenching coefficients <latex>$q_Q$</latex> and <latex>$q_E$</latex> than by <latex>$F_v$</latex>/<latex>$F_M$</latex> ratios.