Vernalization

Photoperiod is not the only environmental cue that controls flowering time - many biennial and perennial species also need to experience a period of low temperature (roughly 0-5 degrees C) early in their development before they will flower at all, a requirement called vernalization ('spring-like'), a term first used by T. D. Lysenko in 1938. Two competing theories try to explain how the low-temperature signal actually triggers flowering later on. Lysenko's hypothesis of phasic development holds that an annual seed plant's development runs through two sequential phases: first a thermostage (a vegetative phase that specifically needs low temperature and suitable moisture), and then a photostage, which needs high temperature to synthesise florigen, the flowering hormone. Purvis's hypothesis of hormonal involvement (1961) instead proposes a defined biochemical chain (Figure 15.26): a precursor substance is converted by chilling into an unstable intermediate, which at a suitable temperature becomes a stable compound called Vernalin; Vernalin is then converted into Florigen, which induces flowering; but if high temperature intervenes at the intermediate stage, the pathway is instead diverted, undoing the vernalized state - a reversal called devernalization. Practically, vernalization is achieved by first soaking and germinating seeds at a mild 10-12 degrees C, then holding the germinated seeds at a genuinely cold 3-5 degrees C for anywhere from a few days up to 30 days, before drying and sowing them - such treated plants flower noticeably earlier …