Wilh apptopriale ptetteatment of the seeds fluence-t"esponse curves for the itiduction of gertnination of Arahidopsis tttatiana show two phases. A ptoportion of the population tesponds to very low fluetice (VLFR), 10 ' 10 '//tnolm', establishitig 10"*"I0"^% of the total phytochrotne in the far-red absorbing form (Pfr) and a proportion of the population respond to low fluence (LFR), I 1000/nnolm ', establishitig 1 75% Pfr. The VLFR is nol nortnally seen because the pre-existing Pfr level satisfies the Pfr requiretnent or use of green safelight establishes mote Pfr than necessary to saUitate the VLFR. Endogenous Pfr was depleted by a 24 h 35 "C treattnent, presutnably as a result of dark destruction and/or dark reversion to the red absorbing fortn of phytochrotne (Pr), tnaking it possible to visualize the VLFR. A short pulse of 35 T trealtnent in combination with an approptiate tetnpetature tegitne is also able to sensitize a proportion of the seed population. The ptoportion of ihe population showing the VLFR is detertnined by the dutalion of the cold itnbibilion pretteatment as well as the duration of the 35 "C treattnent. Cotnplex lUiencc-respotise curves were observed in which a propot tion of the seeds being protnoted in the VLFR ratige, were inhibited at higher fluenccs before being further promoled in the LFR range. This was particularly clear for seed batches being sensitized by a short 35"C tteatment. The VLFR tnay be of significance in the natural environtnent, enabling seeds buried in the upper layer of the soil to germinate, where the Huence rate falls off sharply and the LFR is not satisfied. A tnodel is ptesented to explain the two phases in the fiuence-response curves.
Photomorphogenetic responses have been studied in a cucumber (Cucumis sativus L.) mutant (lh), which has long hypocotyls in white light (WL). While Photomorphogenesis in higher plants is a complex process regulated by a number of different photoreceptors, including phytochrome, cryptochrome, and UV-B photoreceptor (13,14). In addition, there is evidence for multiple types of a particular photoreceptor (e.g. the light-stable and light-labile types of phytochrome [20, 21], sometimes referred to as green-plant and etiolated-plant phytochrome, respectively). There is interaction between these photoreceptor systems in time and space during differentiation (13,14). The interpretation of physiological experiments is therefore often difficult, if not impossible, particularly in experiments utilizing light which is absorbed by more than one photoreceptor, as is the case for phytochrome and cryptochrome in the BL1 region of the spectrum (8, 13). To understand the roles played by the different photoreceptor systems we have taken a genetic approach in an attempt to resolve the inevitable confusion that has arisen (1,(10)(11)(12) MATERIALS AND METHODS Plant Material. The long-hypocotyl (1h) mutant and isogenic wild type of Cucumis sativus L. used here have been described elsewhere (1).Light Sources. White light was obtained from Philips TL40/33 fluorescent tubes. The broadband light sources for BL, RL, and FR used in long-term growth experiments were qualitatively the same as those described previously (11), and the fluence rates utilized are given in the figure legends. In short-term growth experiments, narrow-band BL (459 nm) and RL (658 nm
The photocontrol of anthocyanin synthesis in dark-grown seedlings of tomato (Lycopersicon esculenfum Mill.) has been studied in an aurea (au) mutant which is deficient in the labile type of phytochrome, a high pigment (hp) mutant which has the wild-type level of phytochrome and the double mutant aulhp, as well as the wild type. The hp mutant demonstrates phytochrome control of anthocyanin synthesis in response to a single red light (RL) pulse, whereas there is no measurable response in the wild type and au mutant. After pretreatment with 12 h blue light (BL) the phytochrome regulation of anthocyanin synthesis is 10-fold higher in the hp mutant than in the wild type, whilst no anthocyanin is detectable in the au mutant, thus suggesting that it is the labile pool of phytochrome which regulates anthocyanin synthesis. The aulhp double mut nt exhibits a small (3% of that in the hp mutant) RLlfar-red light (FR)-reversible regulation of 1 nthocyanin synthesis following a BL pretreatment. It is proposed that the hp mutant is hypersensitive to the FR-absorbing form of phytochrome (Pfr) and that this (hypersensitivity) establishes response to the low level of P, (below detection limits in phytochrome assays) in the auihp double mutant.
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