Direct and Indirect Effects of Floral Defoliation on Photochemical and Non-Photochemical Chlorophyll Fluorescence...

Photosynthetic florets support reproductive development and energetic seed provisioning of semi-arid bunchgrasses whose population dynamics rely mainly on sexually produced propagules. Photosynthetic gas exchange studies including crested wheatgrass (Agropyron cristatum) have found compensatory increases in seed-head photosynthesis following floral defoliation are accompanied by reduced light-adapted PSII quantum yield (ϕ _PSII). We undertook a field experiment to ascertain if altered ϕ _PSII and optimal PSII quantum yield (F _v/F _m) were concurrent with higher quantum yield of regulated (ϕ _NPQ) or unregulated (ϕ _NO) non-photochemical PSII absorbed energy dissipation. We quantified these responses in directly affected basal florets, and in unclipped distal florets to establish indirect responses to tissue loss. Clipping basal florets reduced F _v/F _m and increased ϕ _NPQ, indicating effective engagement of regulated non-photochemical photoprotection, but did not reduce ϕ _PSII compared to unclipped controls. Florets distal to clipped basal florets had higher ϕ _PSII but not F _v/F _m, with concurrently lower ϕ _NO compared to those distal to unclipped controls, possibly due to improved electron transport due to carbon supplementation from damaged basal florets to developing distal propagules. These results demonstrate crested wheatgrass possesses a remarkably integrated reproductive photosynthetic apparatus, facilitating its ability to consistently produce viable seed cohorts under conditions that limit native bunchgrasses reproductive success.