EARLY DEGRADATION OF PHOTOSYNTHETIC MEMBRANES IN CAROB AND SUNFLOWER COTYLEDONS

The assimilatory activity of cotyledons can play an essential role in the survival of seedlings with a slow and delayed development of prima ry leaves. Changes in the photosynthetic activity of the cotyledon, fr om the onset of greening through senescence, were studied in two such plants, carob and sunflower, in order to determine its efficiency and duration, also in connection with the achievement of assimilatory auto nomy by the plantlet. Chlorophyll analyses showed that the cotyledon's chloroplasts reached maximal greening in plantlets with a pair of exp anded leaves. In contrast, the cotyledon's photosynthetic activity, me asured as the rate of oxygen release, started to decrease early, befor e expansion of primary leaves. The decrease was due to the inactivatio n of a number of photosystem II (PSII) units, as revealed by immunodet ection of breackdown products of the reaction centre's D1 and D2 thyla koid proteins. No signals of PSII alteration were noticed in the prima ry leaf chloroplasts that differentiated under the same environmental conditions. The damage to the cotyledon PSII, occurring in a non-photo inhibitory situation, might be due to a slower rate of turnover of D1 polypeptide than in the leaf thylakoids. The differential turnover of this protein in cotyledons and in leaves might represent an organ-spec ific regulation of the photosynthetic activity. The peculiarity of the cotyledon thylakoids make these organs useful objects for studying th e metabolic cycle of both D1 and D2 proteins in vivo, under non-photoi nhibiting conditions.