Photoreception and photoresponses in the radish hypocotyl

Opening of the hypocotyl hook in seedlings as influenced by light and adjacent tissues

The influence of the cotyledons and apical bud and the root system on the light-induced opening of the hypocotyl hook of etiolated seedlings of Gossypium hirsutum L., Phaseolus vulgaris L., Helianthus annuus L., Ipomoea alla L., Ipomoea sp., Cucumis sativus L., Linum usitatissimum L., Hibiscus esculentus L., and Raphanus sativus L. was studied. Light stimulated the opening of hypocotyl hook in all cases, but the cotyledons and roots had different effects in different plants. Generally, the presence of cotyledons and the remainder of the shoot apical to the hook inhibited light-mediated opening, but in Gossypium the organs stimulated light-mediated opening. Presence of roots either promoted opening, had no effect, or had an effect only when the cotyledons were present. In the dark the adjacent organs had a reduced effect over that shown in the light, but one cultivar of cotton, Acala SJ1, opened the hook in the dark without cotyledons as much as under any condition in the light. The variation between species in hook opening may related to the need of that process for a proper hormonal balance, as affected by light, which must be obtained from adjacent tissues.

Control by light of hypocotyl growth in de-etiolated mustard seedlings : I. Phytochrome as the only photoreceptor pigment

After sowing, mustard (Sinapis alba L.) seedlings were grown for 48 h in white light (25°C). These fully de-etiolated, green seedlings were used as experimental material between 48 and 72 (84) h after sowing. The question researched was to what extent control by light of hypocotyl elongation is due to phytochrome in these seedlings. It was found that the light effect on hypocotyl growth is very probably exerted through phytochrome only. In particular, we found no indication for the involvement of a specific blue light photoreceptor pigment.

Control by light of hypocotyl growth in de-etiolated mustard seedlings : II. Sensitivity for newly-formed phytochrome after a light to dark transtition

After sowing, mustard (Sinapis alba L.) seedlings were grown for 48 h in white light (25°C). These fully de-etiolated, green seedlings were used as experimental material between 48 and 72 h after sowing to study the control by phytochrome (Pfr) of hypocotyl elongation after a white light→dark transition. It was found that the transfer to darkness leads to a complete loss of sensitivity towards newlyformed Pfr for a period of 12 h while the effectiveness of the Pfr, which was established at the end of the 48 h white light pretreatment was maintained. It is concluded that two kinds of Pfr must be kept apart: "old" Pfr which operates during the first 12 h after the light→dark transition and "new" Pfr the appearance of which is totally ignored by the plant. Between 12 and 24 h after the white light→dark transition the sensitivity of the seedling towards "new" Pfr is restored while an effect of the "old" Pfr is no longer detectable. The restoration process is accelerated by light which operates through phytochrome ("positive feedback"). The implications of these findings for plant growth under natural light/dark regimes are discussed.

Action spectra for the inhibition of growth in radish hypocotyls

In etiolated seedlings of Raphanus sativus L. the inhibition of hypocotyl elongation by continuous light showed a major bimodal peak of action in the red and far-red, and two minor peaks in the blue regions of the spectrum. It is argued that, under conditions of prolonged irradiation, phytochrome is the pigment controlling the inhibition of hypocotyl elongation by red and far-red light, but that its mode of action in far-red is different from that in red. A distinct pigment is postulated for blue light.