ECOPHYSIOLOGICAL INVESTIGATIONS ON PLANTS OF THE GENUS PLECTRANTHUS (LAMIACEAE) - INFLUENCE OF ENVIRONMENT AND LEAF AGE ON CAM, GAS-EXCHANGE AND LEAF WATER RELATIONS IN PLECTRANTHUS MARRUBIOIDES BENTH

Plectranthus marrubioides, a leaf succulent native to the Higher Escar pment of Yemen, shows obligate CAM. Even in the youngest leaves noctur nal accumulation of malic and, to a lower extent, of citri acid (appro ximate to 10% that of malic acid) was measurable. Patterns of gas exch ange were typically that of CAM plants in mature leaves while expandin g leaves exhibited CAM-cycling. Gas exchange patterns were very flexib le. In response to changes in the environmental conditions they could reversibly be switched between all variations of CAM. Decreasing dayti me temperature from 35 to 20 degrees C or varying light intensities fr om 150 to 600 mu mol m(-2) s(-1) did not influence nocturnal CO2 uptak e or malic acid accumulation but enhanced daytime carbon gain. Elimina ting day/night temperature difference induced CAM-cycling, while chang es in nighttime air humidity had no consequences for CAM. Withholding watering suppressed daytime carbon gain within 2 days but neither enha nced nor restricted CAM at moderate environmental conditions. Even at high temperature and low humidity it was reduced only partially with d ecreasing leaf water content; long-term drought led to CAM-idling. Lea f water content was highly correlated with malic acid accumulation und er drought and over the age gradient. The only pronounced effect on no cturnal C-2 gain and malic acid accumulation was exhibited by nighttim e temperature. Temperature response of CAM was influenced by the combi nation of environmental factors during both night and day. Thus, tempe rature optima ranged from 15 to 25 degrees C in many experiments, resu lting in a mean optimum of 20 degrees C. The data show that it is main ly C-3 photosynthesis and leaf conductance that flexibly responded to ecophysiological relevant changes in environmental conditions while CA M is much less affected.