CONTROL OF HYPOCOTYL GROWTH IN MUSTARD SEEDLINGS AFTER LIGHT-DARK TRANSITIONS

Persistent effects of changes in phytochrome status on internode growth in light-grown mustard: Occurrence, kinetics and locus of perception

Extension growth of the first internode in fully de-etiolated mustard (Sinapis alba L.) seedlings (11-12.5 d old) is under the control of both the current phytochrome photoequilibrium (Pfr/P, ratio of the far-red-absorbing form of phytochrome to total phytochrome) and that established by short (<12 h) pretreatments. Plants were pretreated with either light pulses providing different calculated Pfr/P followed by dark incubations of different durations (a), or with a 12-h period of white light establishing different Pfr/P (b). After the pretreatments, the plants received either light pulses providing different Pfr/P, followed by dark incubations (c), or continuous white light with or without addtional far-red light (d). Thus, four experimental approaches were followed: (a)→(c); (a)→(d); (b)→(c) and (b)→(d). Extension growth during the second period (c or d) was not only affected by the current phytochrome status, but also by that established during the pretreatment period (a or b). The results show the existence of a long-term promotion of stem growth which persists after the end of the low Pfr/P pretreatment. This effect is different from the previously reported rapid effect of far-red light added to background white light as follows: (i) the duration of low Pfr/P required to effect a full response is longer (2.5 h); (ii) the duration of the promotion after returning to high Pfr/P is longer (approx. 24 h) and (iii) the locus of perception is mainly in the leaves, rather than the growing internode.

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.