Internal Gradients of Chlorophyll and Carotenoid Pigments in Relation to Photoprotection in Thick Leaves of Plants With Crassulacean Acid Metabolism

Abstract

Studies of two species of Crassulacean acid metabolism (CAM) succulent, Cotyledon orbiculata and Cotyledon paniculata reveal external and internal mechanisms of protection against photoinhibitory damage. Both species have thick leaves (up to 8 mm) in which chlorophyll extends throughout the leaf, being somewhat more concentrated towards the outer layers. Epidermal wax on C. orbiculata, which is produced in response to bright light during growth, reflects up to 60% of the incident light and creates a shaded environment within the leaf. These leaves have a low carotenoid/chlorophyll ratio and relatively low levels of the xanthophyll cycle pigments, two of which, antheraxanthin and zeaxanthin, are thought to be involved in controlled dissipation of excess energy and therefore in photoprotection. Under growth irradiance (1300 μmol m^-2 s^-1) little antheraxanthin or zeaxanthin is found in the epidermal layers and there is none in the interior. However, when the wax is removed by brushing, antheraxanthin and zeaxanthin are found throughout the leaf. In contrast C. paniculata has low reflectivity and is a typical sun plant with a much higher carotenoid/chlorophyll ratio concentrated towards the upper epidermis. In both species the xanthophyll cycle pigments are concentrated towards the exposed surfaces of the leaf, whereas, lutein, β-carotene and neoxanthin are more evenly distributed. Changes in the relative concentrations of the xanthophyll cycle pigments throughout the leaf, measured at various light intensities, show that antheraxanthin and zeaxanthin are always greater at the exposed surfaces and decline towards the centre of the tissue. These results show that variations in the capacity for photoprotection correspond closely with gradients in the light environment within the leaf.