Grasses dominate a large portion of the earth’s land surface and occupy habitats that receive a wide range of precipitation amounts, from desert grasslands (<200 mm of precipitation) to savannas (~ 1700 mm of precipitation). Investigating the mechanisms that allow this growth form to succeed across such a wide precipitation gradient is important to understand as many of these systems are experiencing changes in the amount and timing of precipitation. We investigated leaf-level drought tolerance traits of grass species from a range of habitats. Grasses were exposed to an extreme drought treatment where water was completely withheld, and leaf level physiological traits were measured in response to decreasing soil moisture. As soil moisture declined we measured leaf hydraulic conductance (Kleaf), stomatal conductance (gs), photosynthesis (A), and leaf and soil water potential (Yleaf and Ysoil, respectively). The point at which plants lost 50% of Kleaf and gs was determined based on Yleaf (P50leaf), Ysoil (P50soil). We also monitored the percent of tissue mortality in response to drought to determine when 100% plant tissue mortality occurred.
Results/Conclusions
As Ysoil declined there was a loss of Kleaf that was tightly correlated with decreases in gs. P50leaf was significantly different between species, with drought tolerant species maintaining higher rates of gs and Kleaf at lower leaf water potentials. Drought tolerant species typically had lower Kleaf values and so operated at lower Yleaf than drought intolerant species even when soil moisture was readily available. There was a tight correlation between P50leaf and maximum Kleaf (Ksat) across species, supporting the idea of a safety vs. efficiency trade-off within the hydraulic pathway of grass leaves. Ksat did not, however, correlate well with maximum rates of photosynthesis or gs as has been found in the leaves of woody species. Leaf-level drought tolerance (p50leaf) also correlated with whole plant growth rates, suggesting a trade-off between drought tolerance traits and growth in grasses.