Analysis of the dual role of phytochrome in the photoinhibition of seed germination

Abstract

Light can both promote and inhibit germination, even of seeds of the same species^1–3. Many seeds considered to be positively photoblastic are inhibited by prolonged irradiation with light of a wide variety of spectral qualities, including those, such as sunlight, that promote germination when given as a short exposure⁴. The promoting effect of light is mediated by the red/far-red reversible receptor pigment phytochrome operating in its P_fr form. The nature of photoinhibition is much less clear, although it is possible that continuous excitation of phytochrome is involved⁵. It has been suggested that light could control photomorphogenesis in seedlings not only through the establishment of a particular amount of phytochrome in its active P_fr form but also by determining the rate of ‘cycling’ between P_r and P_fr forms^6,7. Such effects of prolonged irradiation are both wavelength and fluence rate dependent. We show here that the photocontrol of seed germination in Sinapis arvensis L. involves a promoting reaction dependent on the proportion of phytochrome in the P_fr form at photoquilibrium (φ) and an inhibiting reaction dependent on the rate of phytochrome interconversion or ‘cycling’ (H). Thus percentage germination can be quantitatively predicted from the spectral quality and the fluence rate of the seed's light environment.