Flower initiation can be stimulated largely by photoperiodic or temperature changes, or a complex interaction between temperature and day length. Cold temperatures experienced during the winter bring about flower initiation (i.e. vernalization) in many biennial species such as Brassica, lettuce, red beet, Lunaria and onion. The period for the response depends on the exact temperature, as with budbreak and seed dormancy (see stratification). The optimum temperature for many of these responses is about 4°C. Hormones are involved in causing the flower apex to be produced. The balance of auxins, gibberellins and cytokinins is important, but some species respond to artificial treatment of one type of chemical; for example, the day length requirement for chrysanthemum plants can be partly replaced by gibberellic acid sprays.
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