Photochemical and non-photochemical chlorophyll fluorescence dynamics in reproductive structures of semi-arid rangeland perennial bunchgrasses

Photosynthesis by flag leaves and florets supporting reproductive development and seed provisioning is critical to North American sagebrush steppe bunchgrass populations that rely exclusively on sexual reproduction. How reproductive structures vary between species in ability to dissipate excess light energy has not been established. To address this, we determined optimal (Fv/Fm) and light-adapted (φPSII) PSII quantum yield and regulated (φNPQ) and non-regulated (φNO) PSII non-photochemical quantum yields of flag leaves, and basal and distal florets of the exotic crested wheatgrass (Agropyron cristatum) and the native bluebunch wheatgrass (Psudoroegnaria spicata) in the field over reproductive period. The species differed in φNPQ primarily in basal and distal florets, with higher φNPQ in bluebunch wheatgrass concurrent with lower Fv/Fm and φPSII compared to crested wheatgrass. Fv/Fm was also lower in bluebunch wheatgrass flag leaves, but with concurrent higher φNO, not φNPQ. Species divergence in floret photochemical and non-photochemical parameters resulted from changes in bluebunch wheatgrass over the post-anthesis grain filling period, while crested wheatgrass performance was consistent over the reproductive period. These results demonstrate that crested wheatgrass, which is better capable of consistently producing viable seed cohorts, possesses physiological mechanisms that more effectively protect PSII functional integrity.