Light control of seedling morphogenetic pattern.

Plant development is characterized by a high degree of plasticity in response to environmental signals. As s e d e organisms, plants cannot actively move away from sources of stress, nor can they seek out a location with optimal nutrient and light resources. Instead, they must tailor their developmental pattern in a way that maximizes their chances of survival and reproduction. A plant’s “choice” of developmental pattern is based largely on environmental cues, one of the most important of these being light. Given the importance of photosynthesis to plant survival, it comes as no surprise that higher plants respond to light signals by assuming a growth pattern that enhances their access and exposure to light. This control of plant form by ambient light conditions is generally termed photomorphogenesis (Kendrick and Kronenberg, 1994). The light environment in nature is complex. Unobstructed sunlight consists of a wide continuum of photon wavelengths that is conveniently divided into three large spectral domains: UV (<400 nm), visible (400 to 700 nm), and far-red (>700 nm) light (Figure 1A). The spectral quality, or relative photon distribution, at different wavelengths can vary greatly, depending on the location and the time of day. For example, within the canopy, the light available is essentially depleted in the visible and UV regions, and far-red light is highly represented (Figure 1A). Furthermore, twilight normally has a higher farred to red ratio than daylight (Smith, 1994). Although higher plants effectively utilize only visible light for photosynthesis, they have the capability to sense and respond to a much wider range of the spectrum, including UV and far-red light. For example, the effectiveness of different wavelengths of continuous light at inhibiting hypocotyl elongation of dark-grown Sinapis alba seedlings (Beggs et al., 1980) is shown in Figure 1B. It is evident that multiple spectral regions of light, including blue, red, and far-red, all are very effective at inhibiting hypocotyl elongation, suggesting that S. alba seedlings are capable of perceiving all of these light signals and utilizing them to control seedling morphogenesis. Plant responses to light are especially evident in the young seedling, although they occur throughout the life of the plant.