Water stress effects on the seedling growth of Prosopis argentine and Prosopis alpataco

The expansion of the Prosopis genus from the Chaco region towards drier and colder areas would involve several morphological and physiological adaptations that allow these species to occupy different ecological niches. Prosopis argentina and Prosopis alpataco (Fabaceae, Mimosoideae) are good examples of shrubs adapted to highly stressed environments. P. argentine reaches its ecological optimum in sandy soils while P. alpataco attains it in clayish, saline and periodically flooded soils. We analysed the effects of water stress on the early seedling growth of P. argentine and P. alpataco; in addition we discussed the importance of these effects in the ecological success of these species. We used a completely randomized experimental design with two fixed factors: species (P. argentine and P. alpataco), and two levels of water supply (350 and 1400 mm). Water stress produced a detrimental effect in the seedling growth of both species, which was evident from the reduction in leaf area, seedling height, and above-ground and root biomass. However, plant growth in P. alpataco was more strongly affected by water stress than it was in P. argentina, as suggested by the significant species-irrigation interactions (P < 0.05) found in the height and biomass variables. The proportional growth (biomass reach in water stress in relation to control treatment) was lower in P. alpataco (0.14) than in P. argentina (0.32). These results indicate that P. argentina has a greater tolerance to water deficit during plant establishment. Our results, and the morphological features of each species, are consistent with the previous findings that have described P. argentina as a xerophyte able to cope with harsh sand-dune conditions. and P. alpataco as a phreatophytic species with more mesomorphic features. (c) 2005 Elsevier Ltd. All rights reserved.