Germination sensitivities to water potential among co-existing C3 and C4 grasses of cool semi-arid prairie grasslands

Abstract

An untested theory states that C4 grass seeds could germinate under lower water potentials (Ψ) than C3 seeds. We used hydrotime modelling to study seed water relations of C4 and C3 Canadian prairies grasses to address both Ψ divergent sensitivities and germination strategies along a risk-spreading continuum of responses to limited water. C4 grasses were Bouteloua gracilis, Calamovilfa longifolia and Schizachyrium scoparium; C3 grasses were Bromus carinatus, Elymus trachycaulus, Festuca hallii and Koeleria macrantha. Hydrotime parameters were obtained after incubation of non-dormant seeds under different Ψ PEG 6000 solutions. A t-test between C3 and C4 grasses did not find statistical differences in population mean base Ψ (Ψb(50)). We found idiosyncratic responses of C4 grasses along the risk-spreading continuum. B. gracilis showed a risk-takers strategy of a species able to quickly germinate in a dry soil due to its low Ψb(50) and hydrotime (θH). The high Ψb(50) of S. scoparium indicates it follows the risk-averse strategy so it can only germinate in wet soils. C. longifolia showed an intermediate strategy: the lowest Ψb(50) yet the highest θH. K. macrantha which thrives in the driest habitats showed the highest Ψb(50) suggesting a risk-averse strategy for a C3 species. Other C3 species showed intermediate germination patterns in response to Ψ relative to C4 species. Our results indicate that grasses display germination sensitivities to Ψ across the risk-spreading continuum of responses. Then seed water relations may be poor predictors to explain C3-C4 grasses differential recruitment and distribution in the Canadian prairies.