Biphasic effect of red light on the growth of coleoptiles in etiolated barley seedlings

Abstract

InHordeum vulgare cultivar “Kirin-choku No. 1”, the final length of intact coleoptiles of totally etiolated seedlings was approximately twice as long as that of those grown under continuous red light. The fluence response curve of the latter was biphasic; the low-energy effect was saturated by red light of ca. 50 J m⁻² which gave rise to about 40% of the maximum inhibition by continuous irradiation with red light of 1.2 W m⁻², whereas the high-energy effect was induced by irradiation for 1 hr or longer.

Coleoptiles of 3-day-old seedlings were most sensitive to light causing the low-energy effect, which was repeatedly red/far-red reversible. The growth inhibition was correlated to the photometrically measured percentage of Pfr so that the maximum effect was induced by red light of 50 J m⁻² which transformed 70% of phytochrome to Pfr in the coleoptile tip.

Wavelength dependence of the high-energy effect showed that monochromatic light of 400, 600 and 650 nm greatly inhibited the coleoptile growth, whereas light of 700 and 750 nm promoted it instead. The effect was also induced by intermittent irradiation with red light, and the more frequently the intermittent treatment was given, the more the growth was inhibited.