Comparisons of photosynthesis and photoinhibition in the CAM vine Hoya australis and several C3 vines growing on the coast of eastern Australia
Abstract. The CAM vine Hoya australis and three C3 vines, Smilax australis, Ipomoea pes-caprae and Kennedta rubicunda, were studied at a site on the coast of northeastern New South Wales, Australia. The level of CAM activity (nocturnal acid accumulation) was comparable in H. australis growing in full sunlight and in deep shade. Acclimation to shade by H. australis was indicated by thinner leaves, increased chlorophyll content, decreased chlorophyll a/b ratios, lower dark respiration rates, and lower light compensation points. When growing in full sunlight H. australis exhibited reductions in photochemical efficiency, as indicated by reduced quantum yields and Fv/Fm fluorescence from PS II as well as low rates of photosynthesis at high light. Sun leaves of H. australis experienced a massive quenching of fluorescence from PS II during normal exposure to midday irradiance which was rapidly reversible under low irradiance conditions in the late afternoon. This quenching indicated a reduction in photochemical efficiency, part of which could be accounted for by an increase in non-radiative energy dissipation, while part of it was due to one or more processes not yet identified. Changes in PS II fluorescence from shade H. australis exposed to full sunlight suggest a decrease in the rate constant for photochemistry indicative of damage to the reaction centre, as well as an increase in non-radiative energy dissipation. The C3 vine S. australis was also shade tolerant, but exhibited little evidence of photoinhibition when growing in full sunlight. Ipomoea pescaprae and K. rubicunda, both of which were apparently shade intolerant (being found only in full sunlight), possessed high quantum yields and much higher photosynthetic capacities than either H. australis or S. australis. From this study, and several others, it appears that plants possessing CAM experience photoinhibition to a greater degree than do C3 species in full sunlight under natural conditions, which is probably exacerbated by some degree of CAM-idling.