COS 244-1 - Raise it to save it: The role of compensatory photosynthesis in bunchgrass seed-heads in response to simulated floral herbivory

Abstract: While increased photosynthesis is well-documented across many plant species in response to a wide variety of foliar herbivory, the compensatory ability of photosynthetically-active reproductive structures is currently unknown. To address this, we experimentally defoliated basal florets in seed-heads of crested wheatgrass, a widely distributed bunchgrass across North American grassland and rangeland steppe ecosystems. We followed direct and indirect photosynthetic responses by measuring post-clipping photosynthetic rates in clipped basal and unclipped distal florets, respectively, and compared these to rates in florets at similar positions on unclipped seed-heads. Photosynthetic rates increased in clipped basal florets 24 h after clipping, and pooled over the pre-anthesis period, had photosynthetic rates +62% and stomatal conductance to water vapor +82% greater than in unclipped controls; distal floret photosynthesis and stomatal conductance did not differ between treatments over this period. After anthesis, intact florets distal to clipped basal florets had slightly, but not significantly higher photosynthetic rates compared to control counterparts, while basal floret rates did not differ between clipping treatments. Specific mass (dry mass/area) of clipped basal florets was -15% of controls, while florets distal to the clipping treatment had specific mass 11% greater than either basal or distal florets on unclipped seed-heads. These results suggest damaged basal florets provided carbon to unaffected distal florets, and such a response could contribute to crested wheatgrass's ability to produce viable seeds under conditions limiting to native bunchgrasses. Moreover, this presents a novel mechanism that likely relates to the development of convergent drought- and grazing-tolerance traits important to arid and semi-arid rangeland plant community resilience to climate variability.