Carbon dioxide assimilation and stomatal response of afroalpine giant rosette plants

Maximal rates of CO₂ assimilation of 8-11 μmol m^-2 s^-1 at ambient CO₂ concentration were measured for Dendrosenecio keniodendron, D. brassica, Lobelia telekii and L. keniensis during the day in the natural habitat of these plants at 4,200 m elevation on Mt. Kenya. Even at these maximal rates, the CO₂ uptake of all species was found to correspond to the linear portion of the CO₂ response curve, with a calculated stomatal limitation for CO₂ diffusion of 42%. Photosynthesis was strongly reduced at temperatures above 15° C. In contrast to this sensitivity to high temperatures, frozen leaves regained full photosynthetic capacity immediately after thawing. Stomata responded to dry air, but not to low leaf water potentials which occurred in cold leaves and at high transpiration rates. During the day reduced rates of CO₂ uptake were associated with reduced light interception due to the erect posture of the rosette leaves and with high temperatures. Stomata closed at vapour pressure deficits which were comparable in magnitude to those characteristic of many lowland habitats (40 mPa Pa^-1).