Stable carbon isotope ratios represent a newly available technique for assessing integrated plant responses to environmental stress. These isotope ratios offer the unique advantage of yielding time-averaged data about long term conditions and adaptations. The objectives of this study are to delineate the environmental parameters, such as vapor pressure deficit, temperature, respiratory fraction and soil nitrogen which exert control over stable carbon isotope ratios and to assess how different strategies of adaptation to water stress are expressed on a physiological level. Environmental factors which influence carbon isotope discrimination will be investigated in a series of controlled laboratory experiments and field studies. Two study species which utilize contrasting ecophysiologic strategies for adaptation to environmental stress will be studied. One of these is the creosote bush (Larrea tridentata), a dominate evergreen shrub throughout the southwestern deserts of the United States. The second species is the ohia (Metrosideros polymorpha), an ecologically wide ranging forest tree in the Hawaiian Islands. A model will be developed to predict water stress responses from 13C values, and tested by sampling tissues for 13C, and comparing predicted with measured parameters in plants growing under controlled conditions in the laboratory and in the field along a gradient of soil water availability. The information gathered in this study is expected to provide the basis for wide-ranging application of this isotopic technique and provide new insights into the nature of plant adaptation to environmental stress.
