POSSIBLE WATER MOVEMENT FROM OLDER TO YOUNGER LEAVES AND PHOTOSYNTHESIS DURING DROUGHT STRESS IN TWO SUCCULENT SPECIES FROM SOUTH AFRICA, DELOSPERMA TRADESCANTIOIDES BGR AND PRENIA SLADENIANA L BOL (MESEMBRYANTHEMACEAE)

摘要

Water loss and chlorophylla fluorescence kinetics in younger leaves of Prenia sladeniana and Delosperma tradescantioides were measured during a period of desiccation in shoots with older leaves attached and those with older leaves removed. Water loss of the younger leaves in both species was significantly lower when older leaves remained on the shoot, although this effect was much more pronounced in P. sladeniana. A large decline in the fluorescence parameter F-V/F-M, quantifying the quantum yield of photochemistry (Butler and Kitajima, 1975; Bolhar-Nordenkampf et al., 1989), during drought was observed in P. sladeniana leaves. This decline was significantly smaller when older leaves remained on the shoot. In D. tradescantioides, only a slight decline in F-V/F-M was observed during the drought treatment, and the presence of older leaves did not influence this decline. Photochemical quenching (qP) decreased during drought treatment in both species, while non-photochemical quenching (qN) increased. No significant difference in quenching between shoots with and without older leaves was found. These results are discussed in relation to potential drought avoidance mechanisms of these species. We conclude from these results that for P. sladeniana, unlike for D. tradescantioides, a water shift from older to younger leaves is most likely a major feature of drought adaptation.