Seed-related traits and their adaptive role in population differentiation in Avena sterilis along an aridity gradient

Four populations of the annual grass Avena sterilis distributed along an aridity gradient in Israel (Mount Hermon, Northern Galilee, Shefela, and the Negev Desert), were studied to (1) reveal a general pattern of seed dormancy and persistence in the soil seed bank in this species; (2) compare seed size and demography of reciprocally introduced seeds and seedlings; and (3) test the adaptive nature of the observed patterns. The steep aridity gradient in Israel represents two parallel clines of environmental productivity (annual rainfall) and predictability (variation in amount and timing of annual rainfall). The four populations examined represented the following environments: (1) desert-low productivity and predictability, drought stress; (2) semi-steppe batha-moderate productivity and predictability; (3) grassland-high productivity and predictability; and (4) mountain-high productivity and predictability but with severe frost stress. The highest proportion of dormant seeds, most sequential germination of the first and the second florets of a spikelet over three years, and highest importance of desiccation tolerance were found at the desert location, consistent with bet-hedging buffering against unpredictability of rainfall and high probability of drought in this environment. Significant population origin by environment interactions were observed for yield and reproductive biomass, but no advantage of local ecotype was detected for these two traits. However, another fitness component, seedling survival, showed not only the interactive effect of origin and locality, but also the superiority of the local ecotype and decreasing fitness rank from indigenous ecotype towards the most environmentally dissimilar ecotype, suggesting local ecotype adaptation of seedlings. There was a genetically determined decrease in seed mass with increase in aridity without concomitant effect of frost. The selective forces that may differentially affect seed size along the aridity gradient are competition, predation intensity, importance of dispersal distance, and bet-hedging against rainfall unpredictability. Further experiments are needed to determine the precise nature of aridity-related evolution of seed size in A. sterilis.