Introduction

The responses of higher plants to light have an impact on almost all aspects of growth. This is inevitable given the nature of plants as light-harvesting devices, which satisfy their energy needs, and ultimately those of most of the biosphere, from solar radiation. From germination, through seedling establishment, leaf development, elongation and flowering to seed maturation, information on the light environment steers plant growth. That steering is a combination of positive and negative growth stimuli. Induction of germination is a positive stimulus promoting growth of the embryo, causing first the radicle, then the cotyledons, to break out of the seed coat. Seeds whose germination is light promoted are generally small with few reserves. Following germination, if the seed is under soil the hypocotyl, epicotyl or mesocotyl (depending on the architecture of the seedling) elongates rapidly pushing the shoot apical meristem into the light. However, upon emergence, light simultaneously acts negatively to inhibit this elongation and positively to promote expansion of the cotyledons and to stimulate the apical meristem to produce new leaves. Once the plant is established, light promotes expansion of the new leaves, but also inhibits elongation of the leaf petiole and of inter-

nodes formed between leaves. Overall this combination of responses serves a fitness purpose: to promote photosynthetic development under the best possible environment, or to otherwise invest in elongation growth to reach such an environment (Whitelam and Halliday 2007) (Fig. 1).

The photosynthetic environment can, nevertheless, be a highly varied one, with maximum light intensity (fluence rate or irradiance) levels that individual leaves find themselves under varying naturally over at least two orders of magnitude, up to over 2000 ^molm-2 s-1 at noon under full sunlight in temperate latitudes. As a result growth-related developmental decisions will modulate the rate of leaf production, the surface area occupied by the available leaf biomass, and the internal anatomy and leaf absorption characteristics of those leaves (Walters 2005).

One other aspect of growth is also affected by light: the promotion of flowering is strongly influenced in many species. The transition to flowering involves a dramatic promotion of growth involving the production and expansion of the floral organs, often accompanied by the rapid elongation of a flowering stem. Depending on the species, light can serve to monitor the growth environment or the season, and either act as a promoter or an inhibitor of flowering (Whitelam and Halliday 2007). However, an analysis of this light control of flowering responses is beyond the scope of this chapter.

Fig. 1 Contrasting growth responses of different seedling organs to light. Light promotes (+) root extrusion during seed germination, root elongation (at least in response to high red to far-red ratio light) and leaf initiation and expansion during seedling growth, and leaf palisade division and anticlinal elongation during acclimation to high fluence rate. Light inhibits (-) hypocotyl and internode elongation during seedling growth and during plant acclimation to sun versus shade

Fig. 1 Contrasting growth responses of different seedling organs to light. Light promotes (+) root extrusion during seed germination, root elongation (at least in response to high red to far-red ratio light) and leaf initiation and expansion during seedling growth, and leaf palisade division and anticlinal elongation during acclimation to high fluence rate. Light inhibits (-) hypocotyl and internode elongation during seedling growth and during plant acclimation to sun versus shade

Overall a consideration of photobiology is an indispensable component of a dissection of plant growth responses. Photobiology can also serve as a tool in this respect: light is a relatively easy-to-modulate environmental cue that can place such developmental responses under the tight control of an environmental switch. Although many years and a bewildering number of studies have been devoted to this area of research, interest in it is bound to continue.

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