One Morphotype, Three Physiotypes: Sympatric Species of Clusia with Obligate C3 Photosynthesis, Obligate CAM and C3-CAM Intermediate Behaviour

Abstract: Among about 150 species of the genus of Clusia, morphologically very similar neotropical shrubs and trees, 20 have been studied ecophysiologically and 39 original publications were screened here for key data on the potential performance of the various species. I have collected data on carbon isotope discrimination (8⁷³C), day-night oscilliations of malate (Amal), citrate (Acitr) and titratable protons (AH), maximum rates of net CO₂ exchange in the dark (Dark J_co2) and in the light (Light J_co2) apparent rate of photosynthetic electron transport at PS II (ETR), effective quantum yield of non-cyclic electron transport at PS II (AF/F'_m) at 1000 umol m-² s-¹ PPFD, and potential quantum yield of PS II of leaves adapted to short periods of darkness at midday (F_v/F_m midday) and after relaxation overnight (F_v/F_m predawn). The 20 species can be arranged from predominant C₃ photosynthesis to principally CAM. There may be only a few bona fide obligatory CAM species in the genus, but it is doubtful if there are obligatory C₃ species. The data were then used to provide comparisons of intrinsic capabilities for Clusia species occurring sympatrically at two sites each, in Brazil and Venezuela. No clear advantage of CAM emerges from these comparisons under the stress of high insolation and low water availability at these sites. Different Clusia species are successful in different ways and with different intrinsic ecophysiological capacities. The conventional expectations in CAM as a drought and light stress adaptation are confounded in Clusia to the extent that on some occasions C₃ photosynthesis seems to be the superior strategy. However, it appears, that C₃/CAM plasticity which is so widespread in the genus, with many species and potentially rapid speciation, allows a particularly wide ecological amplitude.