Changes in photosynthetic capacity and efficiency, and in carotenoid composition have been examined during leaf ontogeny in the pasture legume white clover, Trifolium repens (L.). Leaf chlorophyll, measured as 'g'1 FW, was used as an indicator of leaf maturity, with maximum levels of the pigment denoting the mature-green phase of leaf development, and an observed decrease in chlorophyll content accompanying leaf senescence. For chlorophyll a and b, a constant ratio (a/b) between the two pigments was observed in mature-green leaves, and in the early stages of senescence while the ratio increased during the later stages of leaf senescence. Measurement of the net photosynthetic CO2 assimilation rate (PN) as 'mol m'2 s'1 revealed a decrease in the photosynthetic rate that correlated with the decrease in total chlorophyll content. Measurement of chlorophyll fluorescence in vivo revealed that the optimal quantum efficiency of PSII (Fv/Fm) did not decline significantly, but the effective quantum efficiency of PSII in the light ((Fm' ' Ft)/Fm') did decrease significantly, with a concomitant increase in non-photochemical quenching (NPQ). In terms of changes in the accessory pigments during senescence, the total pool of carotenoids decreased when expressed per unit leaf area, but not as rapidly as total chlorophyll such that the chlorophyll/carotenoid ratio decreased. The relative abundance of the carotenoids comprising the xanthophyll cycle, zeaxanthin (Z), violaxanthin (V) and antheraxanthin (A) altered during leaf ontogeny. In mature-green leaves, violaxanthin was the more abundant pigment, but as leaf senescence progressed, zeaxanthin became the most abundant pigment, and the ratio of (Z + A)/(Z + A + V) increased. These results are discussed in terms of the role of the xanthophylls cycle in the protection of PSII during leaf senescence.