Anticipation of Seasonal Changes in Environmental Conditions

A higher plant passes through distinct developmental phases during its life cycle (Fig. 2.30). The formation of a zygote after pollination and fertilisation triggers embryo development within a ripening seed. The seed can enter a state of dormancy and germinates when favourable environmental conditions are perceived. Vegetative growth is divided into a juvenile phase and an adult phase. Only in the latter is a plant competent to enter the reproductive phase, which is characterised by the formation of flowers.

Many plant species, most prominently trees, go through an extended juvenile period. It takes several years of development before they are competent to respond to environmental signals that trigger flowering. Within the flowers, the male and female gametophytes arise from spores and produce the male and female gametes.

Fig. 2.30. Plant developmental phases (Huijser and Schmid 2011)

Obviously, in seasonal environments the timing of major life history transitions such as germination and flowering is crucial for establishment, survival and successful reproduction. Thus, most plants in such environments have evolved the ability to escape from stressful conditions such as cold or water scarcity by restricting the major phases of development to windows of rather advantageous conditions.

What is arguably best understood is the control of the flowering time via anticipation of seasonal changes (Fig. 2.31). Thus, flowering under favourable environmental conditions not only is a direct escape mechanism but also can serve as an example for the molecular mechanisms underlying the anticipation of recurring stress. A reliable and therefore important environmental cue is the day length. Processes governed by it are summarised under the term photoperiodism.

Fig. 2.31. Control of flowering time. In Arabidopsis thaliana (note that the flower morphology and petal colour in the figure are not representative of A. thaliana), four different pathways are known to control flowering time. Two of them are described in this chapter and are highlighted here as examples of the anticipation of seasonal changes via photoperiodism and stress memory (vernalisation). SOC1 Suppressor of Overexpression of Constans1, for other abbreviations see text

Many plant species have been classified, for example, as short-day plants or long-day plants. A short-day plant flowers strictly only when the day length falls below a specific threshold. Conversely, a long-day plant requires a day length exceeding a specific threshold in order to flower. Plant species not influenced in their flowering behaviour are called day neutral. Variations in day length requirements exist not only between plant species but also within plant species. There are, for example, short-day as well as day-neutral and long-day Nicotiana tabacum genotypes.

Instead of—or in addition to—a particular day length, many winter annual and perennial plants require prolonged exposure to cold (i.e. to winter conditions in temperate regions) before they flower (vernalisation). This ensures flowering at a time when conditions are likely to be favourable for reproduction. Spring temperatures enable resource allocation to the development of reproductive organs and allow the activity of pollinators, for example.