Differences in winter-hardening between phenotypes of Lolium perenne with contrasting water-soluble carbohydrate concentrations

Carbohydrates are essential for the winter-hardening of temperate grasses, affecting both rate of hardening and level of frost hardiness. The rate and extent of winter-hardening of contrasting phenotypes of two Lolium perenne cultivars targeted for differences in water-soluble carbohydrates concentration were quantified under controlled hardening conditions and standardized freeze testing. The phenotype with the lowest water-soluble carbohydrates concentration had a lower rate of hardening and tolerated less frost than the other phenotypes but the effect of water-soluble carbohydrate concentrations on the hardening processes levelled off at increasing water-soluble carbohydrate concentrations, indicating a threshold. Contrasting responses to freezing were found for survival and for potential for tillering and regrowth. Mortality was little affected by the higher test temperatures, while tillering was negatively affected, even at −5°C. Compensatory regrowth of tillers surviving the lower test temperatures partially compensated for the effects of loss of tillers on regrowth. The hardiest phenotypes had a rapid lowering of LT₅₀ during the first 3 d (−1·65°C d⁻¹), acquiring LT₅₀ values at −15°C or below in 10 d. The hypothesis that rate and maximum level of hardening increase with water-soluble carbohydrates concentrations in stubble is supported by the results.