Leaf Resistance in Succulent Plants

Abstract

REPORTS in the literature indicate or suggest that the stomata of succulent plants open in the dark and close in the light. This conclusion is based on direct observations¹, measurements of diffusion with a porometer², and inference³. Nishida² reported nocturnal stomatal opening in several succulent species, but diurnal opening in others. Thick leaves, few stomata, and small stomatal openings make measurements of the stomatal aperture and subsequent determinations of leaf resistance difficult. The innovation of a resistance hygrometer by Wallihan⁴ and van Bavel et al.⁵ allowed the precise determination of leaf resistance to water vapour transfer in succulent plants without the complications frequently encountered with other methods. Measurements of leaf resistance are based on changes in the resistance of a lithium chloride relative humidity sensor as water vapour diffuses from the leaf to the sensor. The sensor is calibrated with standard length tubes placed over a free water surface; thus the leaf resistance can be estimated in meaningful physical units (sec/cm)⁵. In this communication, we present data indicating the magnitude of leaf resistance to water vapour transfer in Kalanchoe blossfeldiana v. Poelln. in the light and dark. K. blossfeldiana was chosen because many investigators have used it as an experimental plant for dark carbon dioxide fixation studies^6,7.