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Native sagebrush-steppe bunchgrass populations are threatened by the spread and dominance of exotic invasive annual grasses, in part due to low, episodic seed production. In contrast, the widespread exotic bunchgrass, crested wheatgrass, readily produces viable seed cohorts. The mechanisms underlying these differences are unclear. To address this, we measured seed head specific mass (g m−2) and net photosynthetic assimilation (Anet) as a function of internal [CO2] (A/Ci curves) in pre- and post-anthesis seed heads and flag leaves of crested wheatgrass and four native bunchgrasses to determine if differences in allocation and photosynthetic characteristics of seed heads was consistent with differential reproductive success. Crested wheatgrass seed heads had 2-fold greater specific mass compared to the native grasses, concurrent with greater CO2-saturated photosynthesis (Amax), mesophyll carboxylation efficiency (CE), and higher intrinsic water-use efficiency (WUEi; Anet/stomatal conductance (gs)), but with similar relative stomatal limitations to photosynthesis (RSL). Post-anthesis seed head Amax, CE, RSL and gs decreased in native grasses, while crested wheatgrass RSL decreased and CE increased dramatically, likely due to tighter coordination between seed head structural changes with stomatal and biochemical dynamics. Our results suggest native sagebrush-steppe bunchgrasses have greater stomatal and structural constraints to reproductive photosynthesis, while the exotic grass has evolved seed heads functionally similar to leaves. This study shows elucidating reproduction-related ecophysiological mechanisms provide understanding of plant attributes that underlie restoration success and could help guide the development of native plant materials with functional attributes needed to overcome demographic bottlenecks that limit their restoration into degraded sagebrush-steppe.

Photosynthetic regulation in seed heads and flag leaves of sagebrush-steppe bunchgrasses