Ecophysiological investigations on plants of the genus Plectranthus(Lamiaceae). Influence of environment and leaf age on CAM, gasexchange and leaf water relations in Plectranthus marrubioides Benth.

Abstract

Plectranthus marrubioides , a leaf succulent native to the Higher Escarpment of Yemen, shows obligate CAM. Even in the youngest leaves nocturnal accumulation of malic and, to a lower extent, of citric acid (≈ 10% that of malic acid) was measurable. Patterns of gas exchange were typically that of CAM plants in mature leaves while expanding leaves exhibited CAM-cycling. Gas exchange patterns were very flexible. In response to changes in the environmental conditions they could reversibly be switched between all variations of CAM. Decreasing daytime temperature from 35 to 20°C or varying light intensities from 150 to 600 μmol m −2 S −1 did not influence nocturnal CO 2 uptake or malic acid accumulation but enhanced daytime carbon gain. Eliminating day/night temperature difference induced CAM-cycling, while changes in nighttime air humidity had no consequences for CAM. Withholding watering suppressed daytime carbon gain within 2 days but neither enhanced nor restricted CAM at moderate environmental conditions. Even at high temperature and low humidity it was reduced only partially with decreasing leaf water content; long-term drought led to CAM-idling. Leaf water content was highly correlated with malic acid accumulation under drought and over the age gradient. The only pronounced effect on nocturnal CO 2 gain and malic acid accumulation was exhibited by nighttime temperature. Temperature response of CAM was influenced by the combination of environmental factors during both night and day. Thus, temperature optima ranged from 15 to 25°C in many experiments, resulting in a mean optimum of 20°C. The data show that it is mainly C 3 photosynthesis and leaf conductance that flexibly responded to ecophysiological relevant changes in environmental conditions while CAM is much less affected.