Abstract: Identifying functional responses of grasses to drought will be increasingly important under future climate change scenarios. The distribution of C4 grass subtypes and lineages have been observed to be associated with precipitation gradients, however the mechanistic basis of this distribution is not yet well understood. Stomatal strategy likely plays an important role when water is limiting, with past studies demonstrating that C4 grass species from drier areas have more responsive stomata that close in response to soil dry down or have higher water use efficiencies. Both strategies act to conserve water and maintain physiological functioning. We conducted a greenhouse experiment with 24 C4 grass species of diverse lineages and subtypes in order to assess physiological strategies in response to drought. Genetic duplicates of all species were assigned to either a drought or well-watered treatment from July-October 2022. Well-watered plants were watered as needed while drought treatment plants had water withheld until VWC reached 10% (m3 m-3) after which physiological measurements (net photosynthesis (Anet), stomatal conductance (gsw), transpiration (E), instantaneous (WUEi = Anet/E) and intrinsic (WUEg = Anet/gsw) water use efficiency were made on treatment pairs and repeated on 3-5 species replicates each month within a weeklong timeframe. Plants were rewatered and drought resumed, with drought plants watered once a week to prevent mortality between monthly measurement regimes. We expected Panicoideae species to have higher Anet under well-watered conditions and to be more negatively impacted by drought than other lineages. Chloridoideae and Aristidoideae species were expected to have more conservative Anet under well-watered conditions and to maintain these rates under drought via responsive stomata and higher water use efficiencies.
Species differed in their response to drought and in their photosynthetic C gain under well-watered conditions (p< 0.01 and p< 0.001). Panicoideae species generally performed better than Chloridoideae and Aristidoideae species under well-watered conditions, with Anet averaging 20.0 and 18.4-18.5 µmol CO₂ m-2 s-1, respectively. Panicoideae species also generally had greater declines in Anet, gsw, E, and WUEi under drought, with the Chloridoideae having only a slight photosynthetic advantage under drought.Under drought, gsw declined by twice as much for Panicoideae species as compared to Chloridoideae. However, Panicoideae species typically had the highest WUEi and WUEg under both drought and well-watered conditions. Generally, stomatal conductance was less responsive to drought for PCK subtype species. Our results suggest that species-level coordination of traits, lineage, and subtype all determine gas exchange response to drought.
