Photochemical performance of reproductive structures in Great Basin bunchgrasses in response to soil-water availability

Active restoration, especially seeding, is necessary in sagebrush steppe rangelands degraded by the spread and dominance of exotic invasive annual grasses, in part due to low, episodic seed production of native perennial bunchgrasses. In contrast, the widespread exotic bunchgrass, crested wheatgrass, readily produces viable seed cohorts. How soil-water availability affects the ecophysiology of the reproductive structures that may underlie these differences are unclear. To address this, we measured pre- and post-anthesis chlorophyll fluorescence parameters of optimal (F-v/F-m) and light-adapted PSII quantum yield (phi (PSII)) and phi (PSII)-derived electron transport rate (ETR) response to photosynthetic photon flux density (PPFD) in seed heads and flag leaves of watered and unwatered crested wheatgrass and squirreltail wild rye. Watering increased F-v/F-m in the sampled structures of both species, but phi (PSII) was similar between watering treatments. Pre- to post-anthesis F-v/F-m levels were maintained in crested wheatgrass seed heads but declined in flag leaves, with the opposite pattern apparent in squirreltail. Watering did not affect the ETR-PPFD response, but crested wheatgrass seed heads maintained higher ETR across saturating PPFD than did squirreltail. These findings suggest (i) photochemical efficiency is expressed in structures most closely associated with reproductive effort, and (ii) documented differences in seed head photosynthetic characteristics likely include some degree of allocation to individual floret photosynthetic capacity in addition to structural characteristics. We concluded that these physiological and structural differences may contribute to the differential ability of these species to establish from seed, and may help in effective plant material selection needed to improve restoration and conservation success in sagebrush steppe rangelands. Viable seed production is a critical feature for perennial bunchgrasses needed to restore degraded sagebrush steppe rangelands. Seed filling in bunchgrasses depends on the physiological performance of the seed head and the flag leaf, but how these structures respond to soil-water availability is unknown. We measured photochemical performance of watered and unwatered sagebrush steppe bunchgrasses, crested wheatgrass and squirreltail wild rye, and found watering improved photochemical performance in the reproductive structure most closely associated that species' seed filling. We concluded such information could help develop better bunchgrass cultivars to improve sagebrush steppe restoration success.