REARRANGEMENT OF THE CHLOROPLAST THYLAKOID AT CHILLING TEMPERATURE IN THE LIGHT

The leaves of chilling-sensitive pumpkin (Cucurbita pepo L.) showed symptoms reminiscent of photoinhibition when kept for 4 days at 5.degree. C in moderate light. A decrease was observed in the variable part of chlorophyll a fluorescence, apparent quantum yield, and maximum rate of O2 evolution. Chloroplast whole-chain electron transport activity measured from chloroplast thylakoids had decreased to 51% of the control value. Photosystem II (PSII) activity decreased by only 9%, suggesting that photoinhibition was not responsible for the loss of electron transport activity. An increases in the proportion of PSIII.beta. (measured as a .beta.max value) was observed after the chilling treatment. Fractionation of thylakoid membranes showed a 42% increase in PSII activity in the nonappressed region while that in the appressed region decreased slightly. This was accompanied by a decrease in the ratio of the length of appressed to nonappressed thylakoid membranes. Leaf photosynthesis largely recovered within 24 hours of returning to the original growth conditions. We suggest that the increase in the proportion of PSII.beta. during chilling in light plays a role in protecting PSII from photoinhibitory damage.