MORPHOLOGICAL AND ISOTOPIC INDICATORS OF GROWTH ENVIRONMENT - VARIABILITY IN DELTA-C-13 IN SIMMONDSIA-CHINENSIS, A DIOECIOUS DESERT SHRUB

Carbon isotope discrimination is used increasingly in ecological resea rch as an indicator of plant water use efficiency, although field samp ling methods may not always consider the genetic and environmental var iability, both spatial and temporal, which contribute to the foliar is otopic signature. In greenhouse manipulations of water availability to cloned female plants of Simmondsia chinensis, a dioecious Sonoran Des ert shrub, foliar delta(13)C increases 2.3 parts per thousand with inc reased water deficit, while leaf area is reduced 4-fold. Long-term mor phological adjustment may be more important than modification of water -use efficiency in response to water limitation in S. chinensis. In co ntrast with this experimental data, we found a 5.1 parts per thousand range in delta(13)C in a natural population of S. chinensis in an appa rently homogeneous habitat, despite consistent within-plant values. de lta(13)C values were correlated with leaf size, but not consistently w ith plant size, leaf specific mass, or foliage area density. delta(13) C was also correlated with the seasonal amplitude in predawn water pot ential. We found no other correlation of delta(13)C with field measure s of plant water status, nor with instantaneous measures of water-use efficiency or gas exchange, most likely due to leaf age. There were no differences in carbon isotope ratios between female and male plants. We attribute this wide variability in isotopic composition in the fiel d at least in part to genotypic diversity within the population of thi s dioecious species. Sampling technique for stable carbon isotope anal ysis in field studies of natural populations should consider the impli cations of potential genetic variability.