Rarely do all factors of the environment remain suitable indefinitely for plant growth. In the temperate latitudes, for example, breaks in the growing season occur when seasons change, bringing reduced temperatures and shorter days in the autumn and winter. In the subtropics and tropics, where temperatures and day lengths remain equitable all year, water availability may fluctuate between a wet and a dry season. Plants have developed mechanisms to survive during such adverse periods.
One effective mechanism, used by annual plants, is to produce a photosynthetic and flowering structure rapidly and then sink the resources derived from photosynthesis into seed production and distribution. The plant body is no longer useful and is abandoned after protected embryos are produced. The seeds withstand the changes of the next unfavorable growth period and germinate when environmental stimuli indicate favorable growth conditions. Perennial flowering plants also use the seed mechanism, but some retain their photosynthetic and root structures, merely dropping the most vulnerable parts (leaves) during the unfavorable growth period. When one or more of the plant organs undergoes a period in which the growth processes are slowed down or suspended, that state is termed dormancy. The growth is reactivated when environmental stimuli are received that, in effect, inform the plant that conditions are again suitable for growth. The signals to break dormancy are extraordinarily precise. External stimuli combine with internal signals to ensure that renewal of growth will occur at the most favorable time. Many plants have internal growth inhibitors that decay slowly over time, such as ABA. Until the inhibitor has dropped to a certain low level, no growth will take place despite external stimuli; both external and internal signals must be correct.
Almost all seeds undergo some period of dormancy—if they did not, they would start to grow in the fruits on the mother plant and defeat their principal purposes: dispersal and survival of the germplasm. The period between the formation of the seed and the time when it will germinate is called the after-ripening period, which may be a few days or months depending on the plant.
Seeds of plants native to regions with cold winters almost all require an after-ripening period of cold temperatures before they will germinate. This requirement can be met in horticultural and crop varieties by refrigerating the moist seed for a period of time. This procedure is called stratification. Dormancy of seeds with hard seed coats often can be broken artificially by scarifying the seed—mechanically thinning the seed coat with a file or nicking it with a knife, allowing water and oxygen to penetrate to the embryo.
Woody and herbaceous perennials produce dormant overwintering buds in habitats with cold winters. The buds are miniature shoots with apical meristems, leaf primordia, and axillary buds, the whole enclosed by several modified leaves called bud scales. The scales protect the embryonic tissues of the bud from mechanical injury and insulate them. In many climates the scales prevent the formation of ice crystals in the young tissues. Bud scales also restrict gas exchange and prevent desiccation. They often accumulate growth inhibitors as well.
Buds start their growth early in the growing season and by midsummer are completely formed. They then undergo a series of physical and physiological changes in preparation for winter. The process is called acclimation and is triggered primarily by the shorter days of late summer. Plants that have acclimated to winter are said to be cold-hardy. Dormancy is broken in the spring in tree buds by the lengthening days. The buds are the photoperiod receptors.