Ecophysiological investigations on plants of the genus Plectranthus (fam Lamiaceae) native to Yemen and southern Africa

In this first survey of the ecophysiology of 8 succulent and semi-succulent species from the genus Plectranthus (Lamiaceae) native to Yemen and southern Africa it has been shown that three species, Plectranthus arabicus, It hadiensis and II marrubioides are obligate CAM plants. Well-watered plants of these yemenitic species, which are found primarily with other succulents at altitudes up to 2 600 m, exhibit a nocturnal accumulation of malic acid, and, to a much smaller extent, citric acid as well as night-time CO2 uptake under controlled-environment conditions. While the semi-succulent P. cyrpiculoides utilises the CAM-cycling mode, the semi-succulent species, II barbatus, P. purpuratus, P. zatarhendi, and P. zatarhendi var. tomentosus, are typical C-3 plants. Thus, the ability to perform CAM was well correlated with succulence. Neither CAM nor succulence was, however, directly related to maximum desiccation resistance in these species. Increasing day-time temperatures from 20 to 35 degrees C reduces net daily carbon gain in all, leading to CO2 losses in most of the C-3 species. In the CAM species, nocturnal CO2 uptake at least partially alleviated the effect of the high temperature conditions. On the other hand, the C-3 species P. barbatus, growing in succulent plant communities in high mountain habitats (up to 3000 m), was also relatively insensitive to high day-time temperatures. This is the first extensive description of CAM and CAM-cycling in the Lamiaceae. The presence of CAM in Plectranthus correlated with habitat aridity in situ.