Recent studies on chilling tolerance mechanisms in maize revealed a significant positive correlation between genotypic chilling tolerance and chilling‐induced accumulation of the stress hormone abscisic acid (ABA) under controlled growth chamber conditions. Based on this and other results, the hypothesis was developed that chilling tolerance in maize is related to the ability to accumulate large amounts of ABA rapidly, as a protection against chilling injury. The objective of the present study was to test this hypothesis under field conditions during natural cold weather periods in spring, which often cause severe chilling injury in maize fields. In two experiments at two locations in Europe with contrasting climates, eight maize genotypes with different genotypic chilling tolerance were cultivated in spring in the field according to agronomic practice for maize. Before and at the end of cold weather periods, ABA levels and water contents were determined in the third leaves. It was found that chilling‐tolerant genotypes accumulated higher amounts of ABA during the chilling period than chilling‐sensitive genotypes. A significant positive correlation between chilling tolerance and the levels of ABA in the leaves was found. These results support the above‐mentioned hypothesis. In contrast to earlier studies performed under growth chamber conditions, the water content in chilled leaves was mostly higher than in non‐chilled leaves. The increase in ABA is therefore not attributable to chilling‐induced water deficit, but probably to the low temperature itself.

In previous studies in vitro-selection of proline overaccumulating lines of winter wheat (Triticum sativum L. cv. Jo 3063) with increased frost tolerance was reported. These traits were found to be genetically stable. In the present study the improvement of frost tolerance (winter hardiness) under field conditions is confirmed for F 7 progenies of the mutants. Moreover, the mutants accumulated higher levels of glucose and fructose, soluble protein and abscisic acid (ABA) in addition to proline than the wild type under cold hardening conditions in a growth chamber as well as under cold hardening field conditions. ABA and proline levels peaked when the temperature decreased, whereas carbohydrate levels increased more slowly at decreasing temperature. Soluble protein levels also increased during cold hardening, but in addition showed sharp declines during frost periods. Increased carbohydrate levels of the mutants were associated with lower osmotic potential values. The differences in carbohydrate, protein and ABA levels between the mutants and the wild type are probably due to pleiotropic effects of the mutation.

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