High‐Temperature Responses of North American Cacti

Abstract

High—temperature tolerances of 14 species of North American cacti were investigated. A reduction in the proportion of chlorenchyma cells taking up a vital stain (neutral red) and reduced nocturnal acid accumulation were used as indicators of high—temperature damage. All species tolerated relatively high tissue temperatures, the mean maximum tolerance being 64°C, with an absolute maximum of 69° for two species of Ferocactus. Such tissue tolerances to high temperature may be unsurpassed in vascular plants. Morphological features can affect tissue temperatures. Specifically, thin—stemmed species such as the cylindropuntias attain lower maximum temperatures under identical microclimatic conditions than do more massive species; they also tend to be less tolerant of high—temperature stress. Stem diameter changes of three species of columnar ceroid cacti along a Sonoran Desert latitudinal transect were previously attributed to adaptation to progressively colder temperatures northward. Such changes can also be interpreted as a morphological adaptation to high temperatures, particularly in the southern Sonoran Desert. Interspecific differences in high—temperature tolerance may account for distributional differences among other species, e.g., the coastal—sage—inhabiting Ferocactus viridescens was less tolerant of high temperature than were the three Ferocactus species that inhabit desert regions. Acclimation of high—temperature tolerances in response to increasing day/night air temperatures was observed in all 14 species, especially at higher growth temperatures. From 40° day/30° night to 50°/40°, the tolerable tissue temperatures increased an average of 6°. Half—times for the acclimation shifts were 1—3 d. Although cacti attain extremely high tissue temperatures in desert habitats, tolerance of high temperatures and pronounced acclimation potential allow them to occur in some of the hottest habitats in North America.