Genetic variation for carbon isotope composition in honey mesquite (Prosopis glandulosa)

Carbon isotope composition (delta(13)C) is a useful surrogate for integrated, plant water-use efficiency (WUE) when measured on plants grown in a common environment. In a variety of species, genetic variation in delta(13)C has been linked to the distribution of genotypes across gradients in atmospheric and soil water. We examined genetic variation for delta(13)C in seedlings of honey mesquite (Prosopis glandulosa Torr.), an invasive grassland shrub that thrives in the southwestern USA. Fifteen maternal families, representing progeny of 15 adult trees, were studied in three common garden experiments in a greenhouse. The 990-km east-west transect along which the adult trees were located encompasses a wide precipitation gradient, and includes mesic grassland, semiarid grassland, and Chihuahuan desert ecosystems. Genetic variation for delta(13)C in mesquite was substantial, with the rank order of half-sib families based on delta(13)C relatively stable across experiments, which were conducted under different environmental conditions. Conversely, rankings of families by mean seedling height (an index of growth rate) varied markedly among experiments. Seedlings derived from Chihuahuan desert adults emerged more quickly and had more negative delta(13)C (indicative of lower WUE) than seedlings derived from the other regions. Although delta(13)C and seedling height were not correlated, these results suggest that mesquite genotypes at the drier, western extreme of the species' range are adapted for quicker emergence and possibly faster growth than genotypes from mesic areas. Together, these traits may facilitate exploitation of infrequent precipitation events.