PHOTOSYNTHETIC PIGMENT CONCENTRATION, ORGANIZATION AND INTERCONVERSIONS IN A PALE GREEN SYRIAN LANDRACE OF BARLEY (HORDEUM-VULGARE L., TADMOR) ADAPTED TO HARSH CLIMATIC CONDITIONS

Tadmor is a Syrian barley landrace that has adapted to semi-arid envir onments. Its leaves are pale green because of a 30% decrease in the ch lorophyll and the carotenoid content of the chloroplasts (leading to a 7.5% decrease in light absorption) compared with barley genotypes tha t are not adapted to harsh Mediterranean climatic conditions (e.g. Pla isant), This difference in pigment content was attenuated during growt h of the plants in strong light, but was strongly amplified when stron g light was combined with a high growth temperature. The low pigment c ontent of Tadmor leaves was not associated with significant changes in the pigment distribution between the photosystems or between the reac tion centres of the photosystems and their associated chlorophyll ante nnae. No significant difference in the photosynthetic activity (O-2 pr oduction per unit absorbed light) was observed between Tadmor and Plai sant, The conversion of violaxanthin to zeaxanthin in strong light and its reversal in darkness were much faster and operated at a higher ca pacity in Tadmor leaves compared with Plaisant leaves, resulting in an increased photostability of photosystem II in the former leaves. The accelerated xanthophylls interconversion in the Syrian landrace was as sociated with, and possibly related to, an increased fluidity of the t hylakoid membranes. The lipid peroxide level was lower in Tadmor compa red with Plaisant, In contrast, no difference was found in the non-pho tochemical quenching of chlorophyll fluorescence between the two barle y genotypes, The data indicate that the pale green Syrian landrace is equipped to survive excessive irradiance through a passive reduction o f the light absorptance of its leaves, which mitigates the heating eff ects of strong light, and through the active protection of its photoch emical apparatus by a rapid xanthophyll cycling.