Direct and indirect effects of floral defoliation on photochemical and non-photochemical chlorophyll fluorescence dynamics of a semiarid bunchgrass

Authors: Hamerlynck, Erik; O'Connor, Rory

Submitted to: Plant Environment Interactions II
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2026
Publication Date: 2/19/2026
Citation: Hamerlynck, E.P., O'Connor, R.C. 2026. Direct and indirect effects of floral defoliation on photochemical and non-photochemical chlorophyll fluorescence dynamics of a semiarid bunchgrass. Plant Environment Interactions II. 2026;7:e70119. https://doi.org/10.1002/pei3.70119.
DOI: https://doi.org/10.1002/pei3.70119

Interpretive Summary: Perennial rangeland bunchgrasses fill their seeds by photosynthesis by florets in the seed-head. When insects eat florets, photosynthetic rates in the damaged florets increases, and this is used to provide carbon to the remaining unaffected florets. This compensatory increase in photosynthesis comes at a cost of lower photosynthetic light harvesting efficiency (Fv/Fm and YPSII), which may result from how the damaged florets “scrub off” excess light energy in the form of heat. Scrubbing excess light energy can be regulated in a beneficial physiologically controlled manner (YNPQ), or in a less effective un-regulated manner (YNO). We experimentally clipped florets of crested wheatgrass and found that these clipped florets indeed had higher YNPQ, showing they were able to effectively protect themselves from high light. Unclipped florets located above clipped individuals benefited as well, as they were able to reduce un-regulated energy dissipation (YNO) and have higher YPSII compared to those above intact florets. Being able to regulate exposure to excess light after herbivory very likely contributes to crested wheatgrass’s well-documented ability to readily establish from seed under conditions most native grasses can’t.

Technical Abstract: Photosynthetic activity of seed-head florets are important to reproductive development and energetic seed provisioning of semi-arid rangeland bunchgrasses and is critical as these grasses population dynamics rely exclusively on sexually produced propagules. Photosynthetic gas exchange studies including crested wheatgrass (Agropyron cristatum) have found compensatory increases in seed head photosynthesis following floral defoliation may enhance unaffected floret reproductive effort are accompanied by reduced light-adapted PSII quantum yield (YPSII). We undertook a field experiment to ascertain if reduced YPSII and optimal PSII quantum yield (Fv/Fm) was concurrent with higher quantum yield of regulated photoprotective non-photochemical quenching (YNPQ) or unregulated non-photochemical quenching (YNO). We quantified photochemical and non-photochemical responses in directly affected basal florets, and in unclipped distal florets to establish indirect responses to tissue loss. Clipping basal florets reduced Fv/Fm and increased YNPQ, indicating effective engagement of protective regulated non-photochemical processes, but these did not reduce YPSII compared to unclipped controls. Florets distal to clipped basal florets had significantly higher YPSII but not Fv/Fm, with concurrently lower YNO compared to florets distal to unclipped basal florets. We conjectured this represented improved electron transport possibly due to carbon supplementation from damaged basal florets to developing propagules. These results demonstrate that crested wheatgrass possesses a remarkably integrated reproductive photosynthetic apparatus, explaining why this grass is capable of consistently producing viable seed cohorts under environmental conditions that limit successful seedling establishment and recruitment in other bunchgrasses.