8.4.3  Phytochrome operation and light quantity

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The quantity of light required to initiate phytochrome responses varies enormously. At one end of the range, very low fluence responses (VLFR) are amazingly sensitive, requiring around 10–8 moles of quanta m–2, equivalent to 2min of moonlight or a mere 5µs of full sunlight (Smith and Whitelam 1990). The VLFR class is mediated by phyA and is not reversible by far-red light because, at the light intensities involved, far-red-induced reversion of Pfr to Pr is insignificant compared with other mechanisms of Pfr degradation. The high concentration of phyA in dark-grown tissues is probably an adaptation for maximised sensitivity to minuscule amounts of light. The VLFR mode operates exclusively in tissues in darkness, especially deep-buried seeds that may germinate in response to light penetrating through the soil, or a seedling shoot detecting its first few photons, allowing early warning of arrival at the soil surface and initiating conversion to de-etiolated development.

Low fluence responses (LFR) also operate with very little light and saturate after the equivalent of 1s of full sunlight. Unlike VLFR, this class operates via Type II phytochromes and is typified by the classic R–FR photoreversible response, and by perception of spectral quality (R:FR ratio) involved in growth adjustments under leaf canopies. It is interesting to note that the latter is manifested as an increase in shoot extension rate, whereas at VLFR intensities, the same wavelengths can cause decreased elongation.

High-irradiance response (HIR) is a slightly misleading term because, although requiring more sustained light than LFR, these responses still operate at only a few per cent of full sunlight. HIR covers several different types of response, but sometimes is rather unhelpfully used to include blue-light responses (see below) that do not involve phytochrome at all. Both red and far-red can initiate HIR through Type II and Type I phytochromes respectively. The latter is probably part of the daylength perception system in LDPs. Many other far-red-induced HIR disappear soon after plants are exposed to light, presumably because most of the phyA has been degraded.