Gas Exchange of Three Cool Semi-Desert Species in Relation to Temperature and Water Stress

Abstract

Arid and semi-arid environments present severe constraints to plant life. Species in arid habitats have evolved different adaptive strategies for avoiding or enduring stress conditions and some involve the processes of gas exchange. Some perennial species may exhibit high photosynthetic and growth rates only during seasons when precipitation and temperature conditions are favourable, becoming largely dormant during harsher periods. Other species appear to be capable of enduring stress conditions through physiological and morphological changes and pose some of the most intriguing and challenging questions from the standpoint of adaptations involving gas exchange. The two shrub species investigated in the present study, Artemisia tridentata Nutt. subsp. tridentata and Gutierrezia sarothrae (Pursh) Britt. & Rusby, are both species possessing the C3 pathway of photosynthesis (Welkie & Caldwell 1970; Williams & Markley 1973). They remain photosynthetically active during periods of water and heat stress, and are frequently found flourishing side by side. These two species none the less exhibit striking differences in phenology and morphology. Artemisia tridentata may attain heights of up to 1-5 m, is deep-rooted, possesses rather dense foliage all the year and has predominantly woody stems. Gutierrezia sarothrae is a much smaller suffrutescent shrub (often c. 0f2 m tall), dies back to a low, woody central stem every year and possesses a shallow, fibrous root system, sparse foliage, and a relatively high proportion of green stem material. In the northern part of its range, this species remains green and vigorous throughout the growing season, even during seasonal drought periods. The general goal of this study was to define differences in the adaptive strategies of gas exchange which allow these species to survive in sirnilar habitats in the same community. A third species present in this community, the perennial bunchgrass Agropyron spicatum (Pursh) Scribn. & Smith var. inerme Heller, which becomes photosynthetically dormant during stress periods, was also studied to present gas exchange characteristics of a species which avoids active gas exchange under extreme physical conditions.t Seasonal changes in the morphological characteristics of each species have been correlated with gas exchange responses. Effects of irradiation were noted, but primary emphasis was placed on gas exchange responses to water stress and temperature.