PHOTOCONTROL OF HYPOCOTYL ELONGATION IN LIGHT‐GROWNCucumis sativusL. PHOTOSYNTHETIC REQUIREMENT FOR A FLUENCE RATE DEPENDENT PHYTOCHROME RESPONSE

Abstract

We investigated the role of photosynthesis in the photocontrol of extension growth of the hypocotyl of light-grown Cucumb sativw L. Previous work [Gaba and Black (198513) Planr Physiol. 79,10111 demonstrated that the inhibition of cucumber hypocotyl elongation is a fluence rate dependent response in red light. However, the relative contributions of phytochrome and photosynthesis to the photon flux dependent inhibition response were not clear. Here we have shown that photoperception by the foliar cotyledons as well as the hypocotyl itself are responsible for tluence rate dependence in red light. The inhibitor of photosynthesis diuron [3-(3,4-dichlorophenyl)-l, l-dimethylurea] reduced both the magnitude of inhibition and the fluence rate dependency in red light, indicating an involvement of photosynthesis. Furthermore, the growth of non-pigmented seedlings (treated with the herbicide nortlurazon) was less inhibited by red light, with no fluence rate dependency. In particular, inhibition due to cotyledon photoperception was completely lost in non-pigmented (nortlurazon-treated) plants, and much reduced by diuron treatment. Hypocotyl-perceived red light inhibition was only slightly reduced by treatment with nortlurazon and diuron. Photosynthesis was compared directly to photoinhibition of growth: the photon flux response curve of oxygen evolution of green Cucumb cotyledons was distinctly different from that of hypocotyl inhibition. In conclusion, photosynthesis is an essential requirement for the cotyledon-perceived inhibition, but the response itself is not due to photosynthesis.

Photomorphogenesis and phyfohormones

“…The light signal inducing anthocyanin biosynthesis in petals of petunia flowers is perceived by leaves and sepals (Moscovici et al 1996). Similar results have been described concerning the light regulation of white clover stolon elongation (Thompson 1995) and cucumber growth regulation (Gaba et al 1991). Thus mobile factors are involved in the light regulation of plant development, and one can postulate an impottant role for phytohormones in this process.…”

Leaf-Mediated Light Responses in Petunia Flowers

“…the apex, is mostly ineffective (Bernier et al, 1981). In Cucumis sativus green seedlings, RL inhibition of hypocotyl elongation is effected mainly via the cotyledons (Gaba et al, 1991). Photosynthetic activity in the cotyledons has been shown to be required for the inhibition of C. sativus hypocotyl elongation under RL but does not itself cause this response.…”

“…Photosynthetic activity in the cotyledons has been shown to be required for the inhibition of C. sativus hypocotyl elongation under RL but does not itself cause this response. The photosynthesis inhibitor DCMU promoted elongation and significantly reduced the effect of RL (Gaba et al, 1991). In our earlier report we showed that DCMU inhibits pigmentation of detached corolla limbs and proposed that photosynthesis plays a role in the light regulation of corolla pigmentation (Weiss and Halevy, 1991).…”

Green Light Stimulates Early Stem Elongation, Antagonizing Light-Mediated Growth Inhibition

“…Excitation of phytochromes by GL with even extremely dim red light results in a decrease in stem elongation rate (Mandoli and Briggs, 1981;Gaba et al, 1991;Kigel and Cosgrove, 1991). Examination of the action spectra for stem growth elongation in Arabidopsis indicates that GL (525 nm) treatment leads to a slightly longer hypocotyl (Goto et al, 1993) that is unaffected by the hy2 mutation.…”