In nature, plants are exposed to broad-band irradiation for long periods of time. This radiation is periodic on a daily cycle, and superimposed on the daily cycle is an annual seasonal cycle. Daylight covers the whole spectrum over which photochemical reactions are possible (i.e. ca. 350–1200 nm) but peaks in photon units around 600 nm. Solar radiation is filtered in the atmosphere by various elements and molecules which absorb narrow bands to a lesser or greater effect. The result is a radiation spectrum for photochemical action that effectively ranges from ca. 400 nm to ca. 1200 nm, with a number of apparent peaks and hollows.To relate this spectrum to phytochrome action we can estimate the relative amounts of radiation incident upon a plant at the absorption maxima of Pr and Pfr (i.e., the R:FR ratio); in open daylight this value is close to 1.15, and hardly changes with cloud cover or weather conditions (see Smith, 1983). Solar radiation with such a R:FR ratio will establish, at photoequilibrium, about 55% of the total phytochrome as Pfr; that is, Pfr/P = 0.55. Such a concentration of Pfr is sufficient to saturate most LFRs, such as seed germination, the inhibition of hypocotyl extension and the stimulation of leaf development during seedling establishment. Also, of course, all VLFRs will be saturated at such Pfr concentrations. In effect, therefore, daylight may be regarded as red light as far as most photomorphogenic phenomena are concerned.
When daylight interacts with vegetation the spectrum of the radiation transmitted or reflected from that vegetation is massively altered. The photosynthetic pigments absorb most of the blue and red photons and also much of the green, but radiation beyond ca. 700 nm is unaffected. Thus, transmitted or reflected radiation is low in R and relatively high in FR. Indeed, it is possible to use a suitable leaf, such as a bean leaf, as a FR filter in simple laboratory experiments. When plants grow in dense stands, as in vigorous herbaceous communities, forests or, indeed, in crop plantations, the radiation transmitted and reflected from leaves represents a major component of radiation present within the stands. Thus, the R:FR ratio above a crop may be 1.15, whereas within the crop it can be as low as 0.1. At such a R:FR ratio the phytochrome photoequilibrium will be less than 0.2, that is, less than 20% of the total phytochrome will be present in the active Pfr form.
Many LFR responses would still be saturated by such Pfr concentrations, but in mature plants, as opposed to seeds or seedlings, the low Pfr level causes a number of responses, known collectively as the shade avoidance syndrome, which has a marked effect on growth, architecture, and reproduction. These R:FR ratio responses, briefly mentioned above, allow plants to respond sensitively to the competitive threat posed by neighbours. The most striking response is a strong stimulation of stem elongation. When this is successful it elevates the leaves to positions within the canopy where unfiltered radiation is available; it is essentially a strategy to enhance the capture of radiation for photosynthesis. In natural communities this response, mediated by phyB principally but with subsidiary action by phyD and phyE, is a potent competitive strategy. In very dense canopies, such as forests, competitive strategies are obviously useless for relatively small herbs, and in these conditions another component of shade avoidance improves the chances of survival of the germ line. Deep shade, via the associated low R:FR ratio, accelerates flowering, sometimes in a quite spectacular fashion. In some ecotypes of Arabidopsis, for example, flowering may take 50–60 days in white light but in a low R:FR simulating deep shade it happens within 10 days (Botto and Smith, 2002).
What does shade avoidance mean for crop plants? Most crops are grown in dense plantations, for example, in the UK it is common practice for farmers to drill up to 400 seeds per m
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