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-succ ulent 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. W ell-watered plants of these yemenitic species, which are found primari ly with other succulents at altitudes up to 2 600 m, exhibit a nocturn al accumulation of malic acid, and, to a much smaller extent, citric a cid as well as night-time CO2 uptake under controlled-environment cond itions. While the semi-succulent P. cyrpiculoides utilises the CAM-cyc ling 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 temperatu res from 20 to 35 degrees C reduces net daily carbon gain in all, lead ing to CO2 losses in most of the C-3 species. In the CAM species, noct urnal 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 temperat ures. This is the first extensive description of CAM and CAM-cycling i n the Lamiaceae. The presence of CAM in Plectranthus correlated with h abitat aridity in situ.