Grafting experiments with several genotypes provide evidence that vernalization acts through at least two mechanisms. Vernalization of the stock promoted flowering by 26 nodes in genotype If e Sn Hr and 5.5 nodes in genotype If e Sn hr but had no detectable effect in genotype If e sn hr. Cold treatment appears to cause a higher ratio of promoter to inhibitor, at least in part, through low temperature repression of Sn activity. This mechanism is particularly evident in the cotyledons since they form a major area of Sn activity during vernalization. Continuous light was shown previously to prevent Sn forming inhibitor. It seems therefore that both photoperiod and vernalization have an effect through the Sn gene. Vernalization of the shoot promoted flowering by 19 nodes in genotype If e Sn Hr, 3 nodes genotype If e Sn hr, and 1 node in genotype If e sn hr1 grafted to an If e Sn hr stock. The shoot effect may result from one or possibly two mechanisms. Firstly, vernalization may lower the threshold ratio of promoter to inhibitor required at the apex for floral initiation. The same change in threshold could result in changes in the flowering node of quite different magnitude depending on the rate of change in the hormonal levels in the different genotypes. Secondly, vernalization may disturb the ageing process relative to the plastochronic age leading to an earlier (nodewise) decline in inhibitor level.
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Reid et al. (1975) studied this question.