Photocontrol of hypocotyl elongation in light-grown Cucumis sativus L. : The end-of-day response to phytochrome.

The control by phytochrome of hypocotyl elongation of light-grown Cucumis sativus L. after a white-light period was examined. The farred-absorbing form of phytochrome inhibits hypocotyl elongation. The response to phytochrome photostationary state (ϕ) is not linear; all values of ϕ from 0.004 to 0.13 promote growth maximally, in the range of values of ϕ from 0.13 to 0.22 there is a linear growth response, between values of ϕ of 0.22 and 0.35 there is again no differential effect, and for ϕ values above 0.35 there is a strong (near linear) effect of ϕ on elongation. A kinetic examination of events following the white-light period shows that the major recovery from the photoperiod requires 8.5 h of darkness. End-of-day far-red treatment produces a very different response pattern, with a minor growth stimulation within 28 min of treatment followed by a major effect after 80 to 90 min. Three hours after far-red treatment there is a transient decline in growth rate which persists for about 2 h. Over the whole time course there is a great stimulation of growth rate compared with the controls. A similar growth-rate pattern also occurs if the end-of-day ϕ is 0.48, although the magnitude of the growth stimulation is less. Two components are affected by end-of-day ϕ, namely the time at which growth recovers and the subsequent growth rate. In the long term, the latter accounts for most of the differences in elongation growth. The dark recovery when only the hypocotyl is irradiated requires 4 h, but end-of-day far-red treatment reduces this to about 1.5 h. The persistence of the far-red-absorbing form of phytochrome for many hours in darkness in these light-grown plants is also demonstrated.