Plants also use temperature as an environmental clue to ensure flowering. Assessing two environmental factors provides added protection. Some plants have a vernalization requirement a chill that promotes or is essential for flowering. The control point regulated by vernalization may be different for different species. Vernalization has been shown to affect the sensitivity of leaves to respond to photoperiod. In some plants, only leaves initiated after the shoot apex has been chilled will induce flowering after the appropriate photoperiod.
Clearly, the competence of the leaves and shoot apex to respond to environmental and internal signals is crucial to the regulation of flowering. Pea mutants have been identified that have shoot apexes with differential sensitivity to floral induction or inhibition. Another pea mutant has an apex that is not competent to initiate flowers and remains perpetually vegetative. The competence of a day-neutral tobacco apex changes with age. In another species, the apexes of shoots cannot respond to vernalization early in development, this period of time is considered to be the juvenile phase.
A juvenile phase of development is most common in woody perennials. During this time, flowering cannot occur even under optimal environmental conditions. Maturation occurs gradually and may be accompanied by changes in leaf morphology and the ability of cuttings to root. The most significant occurrence is that the plant becomes competent to flower.
See also: Temperate Fruits, Photoperiod, Circadian Rhythms
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