Drought tolerance strategies of grasses: The trade-off between stress tolerance and growth

Background/Question/Methods

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 (K_leaf), stomatal conductance (g_s), photosynthesis (A), and leaf and soil water potential (Y_leaf and Y_soil, respectively).   The point at which plants lost 50% of K_leaf and g_s was determined based on Y_leaf (P50_leaf)_, Y_soil (P50_soil)_. We also monitored the percent of tissue mortality in response to drought to determine when 100% plant tissue mortality occurred.

Results/Conclusions

As Y_soil declined there was a loss of K_leaf that was tightly correlated with decreases in g_s.  P50_leaf was significantly different between species, with drought tolerant species maintaining higher rates of g_s and K_leaf at lower leaf water potentials. Drought tolerant species typically had lower K_leaf values and so operated at lower Y_leaf than drought intolerant species even when soil moisture was readily available.  There was a tight correlation between P50_leaf and maximum K_leaf (K_sat) across species, supporting the idea of a safety vs. efficiency trade-off within the hydraulic pathway of grass leaves.  K_sat did not, however, correlate well with maximum rates of photosynthesis or g_s as has been found in the leaves of woody species.  Leaf-level drought tolerance (p50_leaf) also correlated with whole plant growth rates, suggesting a trade-off between drought tolerance traits and growth in grasses.