Genetic differentiation of intrinsic water-use efficiency in the Hawaiian native Acacia koa

We examined genetic differentiation of intrinsic water-use efficiency (WUE) and the relationship between WUE and phyllode morphological and nutritiona l traits in the native Hawaiian tree koa (Acacia koa Gray, Fabaceae). We ob tained both integrated (stable isotope C-13 composition of phyllode tissues ; delta C-13) and instantaneous (assimilation/stomatal conductance to water vapor; A/g) estimates of WUE on koa accessions grown in a progeny trial at Maunawili on the island of O'ahu. The accessions were from sites along an elevation (900-1300 m) and rainfall (ca. 2200-1300 mm) gradient on the sout hwestern slopes of Mauna Loa on the island of Hawai'i. The delta C-13 value s of koa phyllodes at Maunawili ranged between -28 parts per thousand and - 31 parts per thousand and were positively related to elevation of the seed source. Among seed sources grown at the common site, phyllode delta C-13 va ried with similar magnitude and direction to that associated with the field populations, indicating genetic differentiation of WUE in koa. Instantaneo us measurements of WUE showed similar trends of increasing WUE with elevati on of the seed source. Phyllode delta C-13 was not correlated with foliar c oncentrations of nitrogen (N), indicating that differences in delta C-13 we re not caused by nutrient-related changes in photosynthetic capacity but ra ther by stomatal limitation. Phyllode morphological traits such as specific leaf mass (SLM), length, and area were unrelated to delta C-13. The simila rity between the slopes of SLM and phyllode N content at Maunawili and Hona unau supported the contention that there is global interdependence between these two traits. The finding of genetic differentiation of WUE in koa is i mportant for native forest restoration and seed selection.