Broom snakeweed responses to drought. I. Photosynthesis, conductance, and water-use efficiency.

Abstract

The effects of water deficit on photosynthesis, transpiration, stomatal conductance, canopy development, and water-use efficiency of broom snakeweed (Gutierrezia sarothrae (Pursh) Britt and Rusby) were studied during the spring-summer growing season in pot-grown plants subjected to 5 soil water regimes. Stomatal conductance was proportionately more reduced by a mild water stress (soil water potential = -0.2 MPa) than were canopy development and photosynthesis. However canopy development was most affected by moderate to severe soil water deficit (< -1.1 MPa), followed by photosynthesis; transpiration and leaf conductance were least affected. When subjected to severe water stress, broom snakeweed controlled its water loss mainly through reduced canopy development rather than stomatal closure. Photosynthesis was more limited by mesophyll conductance than by stomatal conductance. Water-use efficiency was not affected by mild water stress. As soil water deficit developed, water-use efficiency declined, which was a response to nonstomatal limitation to photosynthesis and less sensitive stomata to severe water deficit. Broom snakeweed maintained positive net photosynthesis at soil water potential as low as -3.4 MPa and leaf water potential of -8.19 MPa. Waterspending behavior (low water-use efficiency) and high degree of drought tolerance were the main physiological characteristics of broom snakeweed subjected to water stress.