Reproductive compensatory photosynthesis in a semi-arid rangeland bunchgrass

Authors: Hamerlynck, Erik; O'Connor, Rory; Copeland, Stella

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2023
Publication Date: 3/2/2023
Citation: Hamerlynck, E.P., O'Connor, R.C., Copeland, S.M. 2023. Reproductive compensatory photosynthesis in a semi-arid rangeland bunchgrass. Oecologia. 201:625-635. https://doi.org/10.1007/s00442-023-05341-w.
DOI: https://doi.org/10.1007/s00442-023-05341-w

Interpretive Summary: Many grasses increase photosynthesis in the portion of leaves that remain after grazing, and this is thought to help them more rapidly recover and compensate for tissue loss from herbivory. However, while many grasses have seed heads with high photosynthetic capacity, and this is known to contribute to seed production and quality, it is not known if these reproductive structures also increase photosynthesis after tissue loss. We tracked compensatory photosynthesis in crested wheatgrass, a widely planted and successful perennial rangeland bunchgrass, in response to experimentally clipping the lower portion of its seed heads. We found that clipping dramatically increased photosynthesis in the clipped portion, but that it was the unclipped portion above it that grew bigger and developed denser tissue. We think this improved growth was supported by the higher carbon uptake in the damaged portion of the seed head. We concluded this kind of activity contributed to crested wheatgrass’ well-known ability to produce high quality seed under conditions too challenging for most native bunchgrasses, and that by selecting and breeding native grasses that might have this kind of photosynthetic dynamic could improve their successful restoration into degraded sagebrush steppe rangelands.

Technical Abstract: While increased foliar photosynthesis is well documented across many plant species in response to diverse modes of herbivory, the compensatory ability of photosynthetically active reproductive structures is unknown. To address this, we partially defoliated basal forets in seed heads of crested wheatgrass (Agropyron cristatum (L.) Gaertn.), an exotic Eurasian perennial bunchgrass widely distributed across North American sagebrush steppe. We followed direct and indirect responses by tracking post-clipping photosynthesis in clipped basal and unclipped distal forets, respectively, and comparing these to similar forets on unclipped seed heads. Compensatory photosynthesis was apparent 24 h after clipping; over the pre-anthesis period, clipped basal foret photosynthesis was+62%, stomatal conductance was+82%, and PSII photochemical yield was - 39% of unclipped controls. After anthesis, intact forets distal to clipped forets had modestly higher photosynthetic rates compared to controls, while basal foret rates did not difer between treatments. Compensatory photosynthesis reduced intrinsic water use efciency (iWUE; photosynthesis/stomatal conductance) 68–40% below controls over pre- and post-anthesis periods, respectively. Specifc mass (dry mass/area) of clipped forets was - 15% of controls, while forets distal to these had specifc mass 11% greater than distal or basal forets on unclipped seed heads. These results suggest damaged basal forets provided carbon to unafected distal forets. This could explain crested wheatgrass’s ability to produce viable seeds under conditions limiting to native bunchgrasses, and presents a novel mechanism germane to the development of convergent drought- and grazing-tolerance traits important to arid and semi-arid rangeland plant community resilience to climate variability.