Desiccation and Freezing Phenomena for Plants with Large Water Capacitance — Cacti and Espeletias

Abstract

Cacti and espeletias are native to dissimilar habitats, although the two groups must cope with several similar environmental constraints. Cacti, which are succulent plants that utilize crassulacean acid metabolism (CAM), grow in habitats characterized by long periods of drought (Ting 1985; Nobel 1988a). Over 80% of the aboveground plant biomass in parts of the Sonoran Desert can be cacti (Ting and Jennings 1976), and epiphytic CAM species occupying arid microhabitats are common in many lowland tropical forests (Medina et al. 1989). As for cacti, espeletias also have conspicuous stem water storage reservoirs, but they utilize the C3 pathway and grow in tropical high-altitude (alpine) habitats. These habitats are characterized by diurnal rather than seasonal temperature variations that frequently involve nocturnal freezing (Troll 1968; Meiner and Goldstein 1986). In the American tropics the giant rosette species belonging to the genus Espeletia (Compositae) experience air temperatures that even during the daytime tend to be suboptimal for many physiological processes (Goldstein et al. 1989). Studies on plants from cold temperate zones, particularly conifers (Kaufmann 1975, 1977; Running and Reid 1980), as well as on cacti (Lopez and Nobel 1991), indicate that water uptake by roots may be severely impaired by low soil temperatures (0 to 5 °C). In the tropical alpine habitats, such physiological drought is particularly likely during the early morning when soil temperatures in the root zone of espeletias are near freezing and potential transpiration is high due to high solar irradiation leading to relatively high leaf temperatures. Thus, both cacti and espeletias are exposed to drought episodes, the former on a seasonal basis and the latter predominantly on a daily basis.