Responses of two CAM species to different irradiances during growth and susceptibility to photoinhibition by high light

Two CAM species, Kalanchoë daigremontiana Hamet et Perrier and Hoya carnosa (L.) R. Br., were grown under a range of five photon flux area densitites (PFD) and then characterized. Significant acclimation to shade was indicated by progressive decreases in leaf thickness, rates of respiratory O2 uptake, light compensation point, maximum rates of photosynthetic O2 evolution, nocturnal acid accumulation, and δ 13C values, and increases in chlorophyll concentration and absolute levels of room temperature (25°C) and 77K fluorescence. Quantum yields (as measured by O2 exchange) and the ratio of variable 77K fluorescence over the maximum yield (Fv/Fm) were relatively constant across the treatments. The only significant deviation from the above characteristics was in H. carnosa grown under full glasshouse PFD, where it apparently experienced photoinhibition. Following a photoinhibitory treatment, K. daigremontiana exhibited increases in the light compensation point and progressively greater reductions in the quantum yield, maximum photosynthetic rate, Fv/Fm, and the variable component of room temperature fluorescence with increasing shade during growth. Thus although Crassulacean acid metabolism plants can adjust to shaded conditions, they are susceptible to photoinhibition when exposed to higher PFD than that experienced during growth.