Two contrasting cultivars of Lolium perenne were exposed to a range of daily radiation integrals during hardening at 2°C for 15 d. The maximum induced freezing tolerance measured as LT50 (temperature for 50 % kill) differed markedly between the cultivars. The observed LT50 values were unaffected by changes in the radiation integral above 10 mol m−2 d−1, whereas accumulation of water-soluble carbohydrate showed a strong positive correlation with the radiation integral over the entire range of the experiment. The correlation between LT50 and soluble carbohydrate content at the end of the hardening period was poor and showed no obvious connection with genotype. Fructan polymers and sucrose were the major components of the soluble carbohydrates in both cultivars. The depression of freezing point attributable to the accumulation of soluble, osmotically active carbohydrate was not sufficient to account for the observed changes in LT50 in the hardy genotype. These results are discussed in relation to the interactions between growth, photosynthesis and assimilate partitioning during hardening.
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Pollock et al. (1988) studied this question.