Coloured Components of Chloroplast Membranes as Intrinsic Membrane Probes for Monitoring the Development of Heat Injury in Intact Tissues

Abstract

Assays for heat injury in intact leaves were investigated by heating leaf tissue at a rate of 1°C per minute and recording changes in chlorophyll fluorescence; chlorophyll absorbance; light-induced absorbance change at 518 nm of the bulk chloroplast pigments (chlorophylls and carotenoids); and photooxidation of chloroplast cytochrome-554 (cytochrome f ). Leaves of the golden passionfruit (Passiflora edulis forma flavicarpa were used as the experimental material.

Heat injury to the chloroplast thylakoid membranes began above 35°C as evidenced by abnormal changes in chlorophyll fluorescence and absorbance and inactivation of chloroplast membrane activities. Chlorophyll fluorescence increased and chlorophyll absorbance decreased above 35°C and the increase and decrease, respectively, were greatest at 52°C.

The measurements made on intact leaf tissue together with assays of photosystem 2 activity in isolated chloroplasts indicated the relative thermostabilities of several chloroplast membrane activities. Inactivation of photosystem 2 activity linked to oxygen evolution appeared to be associated with membrane changes occurring between 35 and 44°C. The 518-nm absorbance changes indicated that energy-dependent proton pumping and associated reactions were inactivated between 44 and 48°C. Photosystem-1-mediated photooxidation of cytochrome-554 became inactivated between 48 and 52°C.

These studies demonstrate that chlorophylls, carotenoids and cytochrome f may be used as intrinsic membrane probes for monitoring the development of heat injury in the chloroplast membrane system. The absorbance and fluorescence changes that these compounds undergo upon heating provide a basis for reliable and rapid methods applicable to intact tissue for determining heat sensitivity in green plants.