Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/1999
Publication Date: N/A
Interpretive Summary: Many plant species grow over a large geographic area and, therefore, experience large variation in rainfall. In general, plants from drier sites use water more efficiently than those from wetter sites. That is, plants from dry areas use less water to grow a given amount than plants of the same species from wetter sites. The amount of plant growth per unit of water used is called water use efficiency. We collected mesquite seed from trees growing between relatively wet Central Texas and very dry Eastern New Mexico to determine if water use efficiency was higher in the plants grown from seed collected in the drier regions. Unexpectedly, we found that plants from the drier regions had lower water use efficiency buy grew faster than those from Central Texas. These results suggest that mesquite plants growing in drier areas have evolved so that germinating seedlings take advantage of relatively rare rainfall events by growing more quickly. This may enable them to extend roots to tap water deep in the soil, and thereby better survive droughts.
Technical Abstract: Carbon isotope composition (delta 13C) is a useful surrogate for integrated plant water-use efficiency (WUE) when measured on plants grown in a common environment. Genetic variation in delta 13C has been linked, in a variety of species, to the distribution of genotypes across gradients in atmospheric and soil water. We examined genetic variation for delta 13C in seedlings of honey mesquite (Prosopis glandulosa), an invasive rangeland shrub which thrives across a wide precipitation gradient in the southwestern US. Sixteen maternal families, representing progeny of 16 adult trees, were studied in three common garden experiments in a glasshouse. Adult trees were located along a 990 km east to west transect situated approximately between Temple in central TX and Las Cruces in south-central NM. Westward along this transect, mean annual rainfall decreases from approximately 95 to 20 cm. We identified substantial genetic variation for delta 13C in mesquite and found that the rank order of half-sib families based on delta 13C was relatively stable across experiments, whereas rankings of families by mean seedling height varied markedly. Mean delta 13C and time of seedling emergence were significantly correlated with rainfall at the site of origin, with seedlings derived from the driest locations emerging more quickly and having more negative delta 13C, indicative of lower WUE. Although delta 13C 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.