Photoinhibition of the CAM succulent Opuntia basilaris growing in Death Valley: evidence from 77K fluorescence and quantum yield

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Abstract
Diurnal measurements of low temperature (77K) fluorescence at 690 nm (PS II) from north, south, east, and west facing cladode surfaces of Opuntia basilaris in Death Valley, California were made on six occasions during 1985. The absolute levels of Fo(instantaneous fluorescence) and Fm(maximum fluorescence), as well as the ratio Fv/Fm(variable fluorescence, Fm-Fo, over maximum fluorescence), were greater in the north face relative to the other faces. Diurnal decreases in Fo, Fmand Fv/Fmwere found concomitant with increases in incident photon flux area density (PFD). Fv/Fmwas fairly low throughout the year, indicative of photoinhibition, but became somewhat elevated after a spring rain. In early fall the quantum yield of the south face was considerably depressed relative to that of the north face, and corresponding differences were observed in Fv/Fm. A decrease in PFD during growth of glasshouse plants led to an increase in chlorophyll concentration, Foand Fm, but not Fv/Fm. Although there was some variability in the quantum yield of well watered glasshouse cladodes, a correlation was found between quantum yield and the light and CO2 saturated rate of photosynthesis. When O. basilaris was water stressed under glasshouse conditions, reductions in quantum yield, Fm, and Fv/Fmwere observed. Reductions in Fv/Fmalways indicated a reduced quantum yield, although the converse was not necessarily so in well watered glasshouse plants. The results of this study indicate that O. basilaris is likely to experience photoinhibition throughout much of its life in Death Valley.