The relative importance of sexual reproduction versus clonal spread in an aridland bunchgrass

Reproductive compensatory photosynthesis in a semi-arid rangeland bunchgrass

While increased foliar photosynthesis is well documented across many plant species in response to diverse modes of herbivory, the compensatory ability of photosynthetically active reproductive structures is unknown. To address this, we partially defoliated basal florets in seed heads of crested wheatgrass (Agropyron cristatum (L.) Gaertn.), an exotic Eurasian perennial bunchgrass widely distributed across North American sagebrush steppe. We followed direct and indirect responses by tracking post-clipping photosynthesis in clipped basal and unclipped distal florets, respectively, and comparing these to similar florets on unclipped seed heads. Compensatory photosynthesis was apparent 24 h after clipping; over the pre-anthesis period, clipped basal floret photosynthesis was + 62%, stomatal conductance was + 82%, and PSII photochemical yield was - 39% of unclipped controls. After anthesis, intact florets distal to clipped florets had modestly higher photosynthetic rates compared to controls, while basal floret rates did not differ between treatments. Compensatory photosynthesis reduced intrinsic water use efficiency (iWUE; photosynthesis/stomatal conductance) 68-40% below controls over pre- and post-anthesis periods, respectively. Specific mass (dry mass/area) of clipped florets was - 15% of controls, while florets distal to these had specific mass 11% greater than distal or basal florets on unclipped seed heads. These results suggest damaged basal florets provided carbon to unaffected distal florets. This could explain crested wheatgrass's ability to produce viable seeds under conditions limiting to native bunchgrasses, and presents a novel mechanism germane to the development of convergent drought- and grazing-tolerance traits important to arid and semi-arid rangeland plant community resilience to climate variability.

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 (ϕ _PSII) and ϕ _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 ϕ _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.

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

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 (A _net) as a function of internal [CO₂] (A/C_i 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 (A _max), mesophyll carboxylation efficiency (CE), and higher intrinsic water-use efficiency (WUE _i ; A _net/stomatal conductance (g _s)), but with similar relative stomatal limitations to photosynthesis (RSL). Post-anthesis seed head A _max, CE, RSL and g _s 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.

Do shrubs improve reproductive chances of neighbors across soil types in drought?

Plant reproduction is highly sensitive to stress from severe weather. While facilitation has been shown to buffer negative impacts along stress gradients, less is known about facilitating plant reproduction in drought periods. Because intensity and frequency of drought are predicted to increase, plant reproductive facilitation has important implications for a species ability to adapt to changes in climate. Our primary study objective is to test if nurse shrubs act as reproductive micro-refugia across soil types, by improving reproductive potential of perennial bunchgrass neighbors subjected to severe drought. To investigate this objective, we designed a fully factored study testing direct interaction between shrub and bunchgrasses in eastern Oregon sagebrush steppe, at two sites with different soil types. The study consisted of six simple effect treatments combining three moisture regimes (moist, ambient, and drought) with two shrub conditions (shrub intact or shrub removed). Our results indicate when facilitation of reproductive potential occurs, it occurs strongly and particularly in drought, consistent with the stress gradient hypothesis (SGH), where several species produced at least 54% more inflorescences in the presence of shrub neighbors. In addition, we found facilitation to be consistent with the SGH at the species level likely reflecting differences in plant strategy and perception of strain, but to follow alternative SGH models more closely at the site level where facilitation declined on the drier soil. Ultimately, our findings highlight the importance of facilitation in improving plant reproductive potential in drought, and support the role of nurse shrubs as micro-refugia in a changing climate.

Pistil Smut Infection Increases Ovary Production, Seed Yield Components, and Pseudosexual Reproductive Allocation in Buffalograss

Sex expression of dioecious buffalograss [Bouteloua dactyloides Columbus (syn. Buchloë dactyloides (Nutt.) Engelm.)] is known to be environmentally stable with approximate 1:1, male to female, sex ratios. Here we show that infection by the pistil smut fungus [Salmacisiabuchloëana Huff & Chandra (syn. Tilletia buchloëana Kellerman and Swingle)] shifts sex ratios of buffalograss to be nearly 100% phenotypically hermaphroditic. In addition, pistil smut infection decreased vegetative reproductive allocation, increased most seed yield components, and increased pseudosexual reproductive allocation in both sex forms compared to uninfected clones. In female sex forms, pistil smut infection resulted in a 26 fold increase in ovary production and a 35 fold increase in potential harvest index. In male sex forms, pistil smut infection resulted in 2.37 fold increase in floret number and over 95% of these florets contained a well-developed pistil. Although all ovaries of infected plants are filled with fungal teliospores and hence reproductively sterile, an average male-female pair of infected plants exhibited an 87 fold increase in potential harvest index compared to their uninfected clones. Acquiring an ability to mimic the effects of pistil smut infection would enhance our understanding of the flowering process in grasses and our efforts to increase seed yield of buffalograss and perhaps other grasses.

Genetic variation among Stipa grandis P. Smirn populations with different durations of fencing in the Inner Mongolian Steppe

The genetic structure of a population should be carefully considered in ecological restoration because it may play a critical role in maintaining the persistence of a restored ecosystem. In the present study, we examined genetic diversity and genetic structure of Stipa grandis P. Smirn populations from fenced and grazed plots using amplified fragment length polymorphism markers. Molecular genetic variation showed that the genetic diversity of the fenced populations was greater compared with the overgrazed population. There was a significant variation among the populations (Fst = 0.3689, P < 0.001) by AMOVA analysis, and the gene flow was 0.4039 among the populations. The results from a comparison of limited morphological characteristics and from an unweighted pair group method with arithmetic mean cluster analysis and non-metric multi-dimensional scaling analysis suggested that genetic differentiation had occurred between the fenced populations and the grazed populations. The largest genetic diversity was in the moderately grazed population, which might be related to higher population density and greater sexual reproduction due to less disturbances in the plots. The genetic diversity of the long-term (24 years) fenced population was similar to that of a short-term fenced population (fenced for 11 years). These results suggested that the genetic diversity in the overgrazed population might be increased to some extent through fencing, but this effect did not occur beyond 11 years.

Genetic differentiation among Hemarthria compressa populations in south China and its genetic relationship with H. japonica

Within and among populations genetic variance of twelve Hemarthria compressa populations and one Hemarthria japonica population from China were analyzed using inter simple sequence repeat (ISSR). Twelve primers amplified a total of 165 genomic DNA fragments across a total of 148 individuals of which 156 were polymorphic (94.55%). 75.76% of the bands were unique to each species, while the average genetic distance (GD) between one population of H. japonica and twelve populations of H. compressa was 0.44, which suggest that there was distinct differentiation between these two species. In H. compressa, twelve primers produced 145 bands across 145 individuals. High genetic diversity was observed at species level. The percentage of polymorphic loci (P) was 86.21% and Shannon's information index of diversity (I) was 0.357. In contrast, there were relatively low levels of genetic diversity within population (P=32.93%, I=0.174). Analysis of molecular variance (AMOVA) showed that a considerable proportion of genetic variation (48.02%) resided among populations. The coefficient of gene differentiation (G(ST)=48.6%) also suggested that there was strong genetic differentiation among H. compressa populations in southern China. An indirect estimate of the number of migrants per generation (N(m)=0.264) indicated that gene flow was low among populations of this species. Relative high clonal diversity was found, and all local genotypes were found.