Phytochrome-regulated Starch Degradation in Germinating Turions of Spirodela polyrhiza

Abstract— Turions of the duckweed Spirodela polyrhiza contain about 70% starch on the dry weight basis. The rate of starch degradation in nondormant turions was highest in continuous red light (cR) followed by continuous blue light (cB), whereas continuous far red light (cFR) is almost ineffective. Continuous R could be substituted by repeated R pulses; this effect was not photoreversible by FR pulses applied after hourly R pulses. This suggested that R-dependent high irradiance response is the mode of phytochrome action in mediating starch degradation. Comparing the mode of phytochrome action with that of phytochrome mutants of other plants it can be assumed that phytochrome B is the photoreceptor involved. Application of the translation inhibitor cycloheximide immediately stopped the phytochrome-dependent starch degradation demonstrating the involvement of de novo synthesis of proteins.     

Light Regulation of Phytochrome Content in Wild-type and Aphototropic Mutants of the Moss Ceratodon purpureus

In filaments of the moss Ceratodon purpureus , phototropism is controlled by the photoreceptor phytochrome. Thirty-three aphototropic mutants with a proposed defect in phytochrome-chromophore biosynthesis were isolated and analyzed. The phototropic response of those mutants was rescued with the precursor of the phytochrome chromophore, biliverdin. Phytochrome spectral activity was measured in 19 arbitrarily chosen mutants. All contained low but still measurable quantities of photoactive phytochrome; the highest level was around 15% of the wild-type. The level of total phytochrome (apophytochrome and holophytochrome) as assayed by immunoblotting was indistinguishable from wild-type. The content of photoactive phytochrome in Ceratodon is light-regulated. Phytochrome of wild-type kept for 24 h in red light was reduced to 50% as compared to dark controls but was unaffected by blue. The red-light-induced decrease was partially reversible by far-red light, indicating that phytochrome itself is the photoreceptor for this response. This regulation was further analyzed with the mutant ptr114 , which contains 15% photoactive phytochrome as compared to the wild-type. In this mutant, continuous red light given for 6 days decreased the level of spectrally active phytochrome down to 25% of dark controls, whereas the amount of phytochrome found on immunoblots was hardly reduced. This indicates that the loss of phytochrome affects only the holoprotein and implies that Ceratodon phytochrome is specifically degraded as a far-red-absorbing phytochrome.     

KINETIC STUDIES TO INTERPRET 'HIGH-ENERGY PHENOMENA' OF PHOTOMORPHOGENESISL ON THE BASIS OF PHYTOCHROME

Abstract— Kinetic studies with the mustard seedling ( Sinapis alba L.) support the hypothesis that the so-called 'high energy reaction' of photomorphogenesis can be understood solely on the basis of phytochrome. Light-induced anthocyanin synthesis (a typical 'positive' photoresponse⁽¹⁾ and light dependent inhibition of hypocotyl lengthening (a typical 'negative' photoresponse⁽¹⁾) have been investigated. In order to explain the experimental data we have to assume that there are two different types of phytochrome 730 which differ greatly as far as their resistance to irreversible destruction is concerned. The existence of these two different types of phytochrome 730 has already been postulated on the basis of spectrophotometric measurements in vivo .⁽²⁾     

THE CONTROL BY PHYTOCHROME OF NITRATE REDUCTASE IN THE CURD OF LIGHT-GROWN CAULIFLOWER

Abstract— The activity of nitrate reductase from the curd of light-grown cauliflower ( Brassica oleracea (L) var botrytis (DC) 'St. Hilary') is modulated by nitrate and by light. Using broad-band sources of equal photosynthetically active radiation but with different proportions of red and far-red light, a linear relationship between nitrate reductase activity and ψ(Estimated phytochrome photoequilibrium) was obtained. This relationship, apparent after 8 h incubation, was maintained and little altered after 48 h incubation. The linearity was apparent between ψE 0.26 and ψE 0.69; ψE 0.26 being no more effective than a dark control. Far-red reversibility confirmed the involvement of phytochrome. Brief pulses of red light were also used to establish a range of phytochrome photoequilibria within the tissue. Again a linear relationship between ψ and nitrate reductase activity was obtained with a threshold for the response at ψ 0.3. With both monochromatic and broad-band sources it was seen that neither photon fluence rate nor duration of exposure affected the final activity of the enzyme and that phytochrome was acting solely through ψ (or [Pfr] since phytochrome is stable in this tissue) to bring about these responses.     

THE MODE OF INTERACTION BETWEEN BLUE (UV) LIGHT PHOTORECEPTOR AND PHYTOCHROME IN ANTHOCYANIN FORMATION OF THE SORGHUM SEEDLING

Two non-photosynthetic photoreceptors (phytochrome and a blue light photoreceptor) are involved in light-mediated anthocyanin synthesis in the mesocotyl of Sorghum seedlings. The present study was undertaken to investigate the kind of interaction between phytochrome and the blue light photoreceptor. The data show that phytochrome (P_fr) can only act once a blue light effect has occurred. On the other hand, the blue light effect cannot express itself without P_fr. It is concluded that there is an obligatory dependency (or sequential interaction) between the blue light effect and the light effect occurring through phytochrome, although the blue light photoreaction per se is not affected by the presence or absence of phytochrome. The latter statement is based on the results of dichromatic experiments, i.e. simultaneous, high fluence rate irradiation with two kinds of light. Blue light can be replaced by UV light. It is not clarified yet whether the effect of blue and UV light is due to the same photoreceptor.     

PHYTOCHROME-MEDIATED THRESHOLD CONTROL OF HYPOCOTYL GROWTH IN PARTLY DE-ETIOLATED MUSTARD (Sinapis alba L.) SEEDLINGS

Abstract— Hypocotyl elongation in etiolated mustard ( Sinapis alba L.) seedlings is known to be controlled by phytochrome (Pfr) through a threshold mechanism. The Pfr threshold value required to suppress hypocotyl growth was low (3 times 10⁻²% Pfr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%). In the present study the question was addressed whether the threshold control by Pfr of hypocotyl elongation also operates in light-pretreated, partly de-etiolated seedlings after transfer to darkness. The experimental results show that this is the case. Calculation of the threshold level in far-red light pretreated seedlings led to a very low value (3 times 10⁻⁷%) compared to etiolated seedlings (3 times 10⁻²%). In red light pretreated seedlings the threshold level was calculated to be 9 times 10⁻⁷%. Since the light pretreatment affected the rate of degradation of phytochrome strongly (half-life of Ptot in continuous red light was found to be 35 min in far-red pretreated instead of 47 min in etiolated material), the difference in threshold level between far-red and red pretreated material cannot be interpreted unambiguously. However, the conclusion can be drawn that light nretreatment strongly increases the degradation rate of Pfr and decreases the threshold level.     

LIGHT PROMOTION OF SEED GERMINATION IN Datura ferox IS MEDIATED BY A HIGHLY STABLE POOL OF PHYTOCHROME

Abstract— The duration of the far-red light-absorbing form of phytochrome (Pfr) of the photoreceptor pool involved in the control of seed germination was investigated for Datura ferox seeds. These seeds require both Pfr and alternating temperatures (20/30°C) to germinate. After 24 h imbibition (25°C), the seeds received pretreatment-light pulses providing different phytochrome photoequilibria (Pfr/P), followed by a 24 h dark incubation (25°C), and test-light pulses providing different Pfr/P immediately prior to transfer to alternating temperatures. Germination increased with increasing Pfr/P provided by the test-light pulses, but was unaffected by the pretreatment-light pulses. This suggests that phytochrome synthesis, phytochrome degradation and phytochrome-mediated changes in response to phytochrome were negligible. In other experiments, red light-pretreatment pulses were followed by dark incubations (25°C) of different duration before transfer to alternating temperatures. The proportion of Pfr remaining after the 25°C incubation period was estimated by comparing germination rates with those of seeds that received test-light pulses of known calculated Pfr/P immediately prior to the start of the cycles of alternating temperatures. More than 80% of the Pfr established by a Pfr/P= 0.87 light pulse was present and active even after 48 h dark incubation at 25°C. Surprisingly, when a pretreatmentlight pulse providing a Pfr/P= 0.70 was given, the reduction in [Pfr] was significantly faster.
Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light-absorbing form, the times to reach half the original Pfr pool being > 96 h or <14 h after light pulses providing Pfr/P= 0.87 or 0.70, respectively.     

AN ANALYSIS OF PHYTOCHROME-MEDIATED THRESHOLD CONTROL OF HYPOCOTYL GROWTH IN MUSTARD (Sinapis alba L.) SEEDLINGS†

Hypocotyl elongation in mustard (Sinapis alba L.) seedlings is known to be controlled by phytochrome (P_fr) through a threshold response. This phytochrome-mediated threshold response was studied in detail with the following results: (i) The P_fr threshold value required to suppress hypocotyl growth is much lower (0.03% P_rr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%) than those threshold valued observed previously in threshold control by hook phytochrome of appearance of 'potential capacity for photophosphorylation' and lipoxygenase appearance in the mustard cotyledons (1.25% P_tr, based on total phytochrome in the hypocotyl at 36 h after sowing = 100%). This probably explains why hypocotyl elongation is so extremely sensitive to light, (ii) The P_fr threshold value controlling hypocotyl growth is a system constant, independent of total phytochrome content, developmental age and actual growth rate, (iii) Threshold control of hypocotyl elongation is unaffected by the removal of the cotyledons and half of the hook. However, removal of the whole hook totally eliminates any light control over the residual hypocotyl growth, (iv) After termination of the threshold control, the hypocotyl growth rate immediately returns to precisely that found in untreated dark control even though the partial growth rates of the different parts of the hypocotyl are quite different, relative to their dark controls. Obviously, the organ grows as an integrated unit.
It is concluded that the all-or-none threshold control over hypocotyl growth is exerted from the plumular hook. It appears that the hook can send off phytochrome all-or-none signals in both directions, to the cotyledons and to the hypocotyl.     

PHYTOCHROME CONTROL OF ITS OWN SYNTHESIS IN Sorghum vulgare AND Avena sativa

The accumulation of phytochrome in the dark was measured for Avena sativa seedlings after a white light pretreatment and for Sorghum vulgare seedlings after continuous red or far-red light treatments, using the herbicide Norflurazon to prevent greening under continuous irradiation. In both cases the accumulation of phytochrome depends on the state of the phytochrome at the light-dark transition: high P_fr levels (red light pulse) led to a slower rate of phytochrome accumulation than lower P_fr levels (long wavelength far-red (RG 9) light pulse). Poly-(A⁺)-RNA was isolated fromA. sativa seedlings grown for 48 h in darkness + 24 h WL + light pulse (5 min) (red, RG 9 light, red followed by RG 9 light or RG 9 followed by red light pulse) + 19 h darkness. The poly-(A⁺)-RNA was translated in a rabbit reticulocyte lysate system and the translation products were immunoprecipitated by specific anti-phytochrome antibodies. It was demonstrated that the activity of mRNA coding for phytochrome was under phytochrome control.     

MODE OF COACTION BETWEEN UV-A AND LIGHT ABSORBED BY PHYTOCHROME IN CONTROL OF APPEARANCE OF RIBULOSE-1.5-BISPHOSPHATE CARBOXYLASE IN THE SHOOT OF MILO (Sorghum vulgare Pers.)

Abstract— In shoots of milo ( Sorghum vulgare Pers.) appearance of ribulosebisphosphate carboxylase (RuBPCase) and of translatable mRNA for its small subunit is stimulated strongly by red light (R, operating through phytochrome) and UV-A light (UV-A). Ultraviolet-A is more effective than R.
The mode of coaction between phytochrome and light absorbed by the blue/UV-A light photoreceptor ('cryptochrome') was analyzed in detail in case of enzyme appearance. Fluence rate dependencies, lagphases and the time course of the response are compatible with the view that UV-A intensifies a process which is occurring in R alone albeit at a lower rate.
With both light qualities the light effect is fully reversible by far-red light up to 1 h. This means that during this period only phytochrome (P_fr) controls the terminal response, i.e. the actual appearance of RuBPCase. During this 1 h period after the onset of light UV-A or R have no effect on the level of translatable mRNA for the small subunit of RuBPCase indicating that it requires more than 1 h for the light signal to affect gene expression.
When R and UV-A are given longer onset of escape from full reversibility is observed at the same time for both light qualities in the case of RuBPCase appearance. The extent of the reversible response is greater after UV-A pretreatment than after a R pretreatment.
It is argued that the data are consistent with the concept that phytochrome (P_fr) controls the terminal photoresponse, in the present case appearance of RuBPCase, while light absorbed via cryptochrome leads to an increase in responsiveness of the RuBPCase producing machinery towards P_fr.     

EXTENSION GROWTH AND ANTHOCYANIN RESPONSES OF PHOTOMORPHOGENIC TOMATO MUTANTS TO CHANGES IN THE PHYTOCHROME: PHOTOEQUILIBRIUM DURING THE DAILY PHOTOPERIOD

Four genotypes of tomato (Lycopersicon esculentum Mill.) in the genetic background ‘Aha Craig’ were used: an aurea (au) mutant, deficient in the bulk light-labile phytochrome pool; a high pigment (hp) mutant, showing exaggerated phytochrome responses at the time of de-etiolation; the au,hp double mutant and the isogenic wild type (WT). A dramatic increase in plant height resulting from an increase in the length of all internodes for each of the genotypes studied was observed upon reduction of the red light: far-red light photon ratio (R:FR) from 6.90 to 0.13 by addition of FR for the whole photoperiod. A concomitant increase in leaf length was also observed. Since au and au,hp mutants, deficient in the bulk light-labile phytochrome pool, respond to this reduction in the R:FR these data demonstrate that the phytochrome pool that mediates this response is present and fully functional. Anthocyanin was detectable in the comparably developed young growing leaves of the WT and hp mutant under the high R:FR, but not in the au and au,hp mutants, suggesting that the potential for anthocyanin synthesis is correlated with the presence of the bulk light-labile phytochrome pool. The kinetics of anthocyanin decrease in the young growing leaves were investigated in the hp mutant and the results suggest a very rapid cessation of flavonoid biosynthesis upon reduction of the R:FR. The functions of different phytochrome types are discussed.     

RELATIVE IMPORTANCE OF BLUE LIGHT AND LIGHT ABSORBED BY PHYTOCHROME IN GROWTH OF MUSTARD (Sinapis alba L.) SEEDLINGS*

Abstract— Hypocotyl straight growth in mustard (Sinapis alba L.) responds very strongly and in precisely the same way to low fluence rate red (RL) and white light (WL). The effect of weak light can be attributed fully to light absorption by phytochrome. Only with increasing fluence rate an effect of blue light (BL) comes into play which cannot be explained by the action of phytochrome. However, this specific action of BL can be demonstrated in hypocotyl growth of mustard seedlings only up to 5 days after sowing (25°C). With older seedlings control of hypocotyl growth seems to be exerted exclusively via phytochrome. Regarding the far-red light dependent “high irradiance reaction” (FR-HIR) it was found that it plays a dominant role in growth of mustard only during a relatively short period. It tends to disappear in favor of a RL-HIR between 3 and 4 days after sowing. It is concluded that the seedling exhibits a largely endogenous temporal pattern of responsiveness to light. Phototropism of the mustard seedling can be elicited by low fluence rates (< 1 mW m^?2) of unilateral BL. This same light has no effect on straight growth. It is concluded that BL-dependent phototropic growth response of a hypocotyl and the effect of BL on longitudinal growth of the hypocotyl are unrelated phenomena.     

PHYTOCHROME REGULATION OF STEM GROWTH AND INDOLE-3-ACETIC ACID LEVELS IN THE lv AND Lv GENOTYPES OF Pisum

Phytochrome influences stem elongation and the mechanism for this is not understood. The levels of indole-3-acetic acid (IAA) were analyzed in an leLv genotype of Pisum sativum L. which responded to end-of-day far-red light by doubling growth rate. The IAA levels in epidermal peels increased 40% after far-red light whereas IAA levels of the entire stem tissue changed insignificantly. This increase was reversible by red light. Under light-grown conditions, the lv mutation increases stem elongation rates by 2–3-fold and is thought to block the transduction of a phytochrome signal. Analysis of the short-term stem elongation kinetics of dark- and light-grown Lv and lv seedlings suggests that lv blocks the action of the light-stable form of phytochrome. The higher growth rate of lv plants was found to be associated with abnormally high epidermal IAA levels typical of far-red treated Lv plants. End-of-day far-red treatments did not substantially increase epidermal IAA levels in lv plants. These observations support the view that phytochrome regulation of stem elongation may occur in part through modulation of epidermal IAA levels. The lv mutation may result in increased internode growth in part by blocking the ability of phytochrome to decrease epidermal IAA levels.     

A GENERAL HYPOTHESIS TO INTERPRET 'HIGH ENERGY PHENOMENA' OF PHOTOMORPHOGENESIS ON THE BASIS OF PHYTOCHROME

Abstract— It is demonstrated that the high energy reaction of photomorphogenesis (blue-far-red reaction) in the case of seed germination and hypocotyl lengthening of Lactuca sativa (L.) 'Grand Rapids' is a consequence solely of phytochrome. A hypothesis is presented in which all high energy phenomena can be explained on the basis of phytochrome.     

PHOTOCONTROL OF ANTHOCYANIN SYNTHESIS IN TOMATO SEEDLINGS: A GENETIC APPROACH*

The photocontrol of anthocyanin synthesis in dark-grown seedlings of tomato (Lycopersicon esculentum 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 au/hp , 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 au/hp double mutant exhibits a small (3% of that in the hp mutant) RL/far-red light (FR)-reversible regulation of anthocyanin synthesis following a BL pretreatment. It is proposed that the hp mutant is hypersensitive to the FR-absorbing form of phytochrome (P_fr) and that this (hypersensitivity) establishes response to the low level of P_fl. (below detection limits in phytochrome assays) in the au/hp double mutant.     

PHEOMELANIN PHOTOCHEMISTRY. PHOTOLYSIS OF MODEL COMPOUNDS

Abstract— Phytochrome control of nitrate reductase activity has been studied in cotyledons and hypocotyls of light-grown Sinapis alba. Under polychromatic irradiation, an increase in the fluence rate of far-red light added to a constant source of photosynthetically active radiation causes a decrease in the phytochrome photoequilibrium and, in the hypocotyl, this results in decreased nitrate reductase activity. However, in the cotyledons this difference is only observed transiently. In both organs, enzyme activity is correlated with the level of the far-red light absorbing form of phytochrome, P_fr. These correlations are not altered when the fluence rate (with respect to phytochrome) is increased, suggesting that the responses are not fluence rate dependent. The results obtained are consistent with the notion that in fully de-etiolated seedlings, P_ft alone controls nitrate reductase activity.     

PHYSIOLOGICAL CHARACTERIZATION OF A HIGH-PIGMENT MUTANT OF TOMATO

Abstract— A high-pigment (hp) mutant, which shows exaggerated phytochrome responses and three other genotypes of Lycopersicon esculenrum Mill. cv. Ailsa Craig: the aurea (au) mutant deficient in the bulk light-labile phytochrome (PI) pool, the au, hp double mutant, and their isogenic wild type, were used in this study. Measurements of phytochrome destruction in red light (R) revealed that the exaggerated responses of the hp mutant are not caused by a higher absolute phytochrome level or a reduced rate of phytochrome destruction. Fluence-response relationships for anthocyanin synthesis after a blue-light pretreatment were studied to test if the hp mutant conveys hypersensitivity to the far-red light (FR)-absorbing form of phytochrome (Pfr), i.e. the threshold of Pfr required to initiate the response is lower. The response range for the hp mutant and wild type was identical, although the former exhibited a 6-fold larger response. Moreover, the kinetics of anthocyanin accumulation in continuous R were similar in the wild-type and hp-mutant seedlings, despite the latter accumulating 9-fold more anthocyanin. Since the properties of phytochrome are the same, the hp mutation appears to affect the state of responsiveness amplification, i.e. the same amount of Pfr leads to a higher response in the hp mutant. We therefore propose that the hp mutation is associated with an amplification step in the phytochrome transduction chain. Escape experiments showed that the anthocyanin synthesis after different light pretreatments terminated with a R pulse was still 50% FR reversible after 4–6 h darkness, indicating that the Pfr pool regulating this response must be relatively stable. However, fluence-rate response relationships for anthocyanin synthesis and hypocotyl growth induced by a 24-h irradiation with 451, 539, 649, 693, 704 and 729 nm light showed no or a severely reduced response in the au and au, hp mutants, suggesting the importance of PI in these responses. We therefore propose that the capacity for anthocyanin synthesis (state of responsiveness amplification) could be established by PI, while the anthocyanin synthesis is actually photoregulated via a stable Pfr pool. The Hp gene product is proposed to be an inhibitor of the state of responsiveness amplification for responses controlled by this relatively stable Pfr species.     

PHYTOCHROME ACTION AT HIGH PHOTON FLUENCE RATES: RAPID EXTENSION RATE RESPONSES OF LIGHT-GROWN MUSTARD TO VARIATIONS INFLUENCE RATE and RED: FAR-RED RATIO*

Abstract— Extension growth rate of light-grown mustard (Sinapis alba L.) seedlings was monitored continuously using a sensitive linear displacement transducer system. When high fluence rates (ca 2 mmol m^?2 s^_1) of mixed red and far-red light were presented to the growing internodes from fibre optic probes, fluctuations in extension rate occurred during the first 30 min. High red: far-red ratios (R: FR) caused growth deceleration, whilst low R: FR caused transitory growth acceleration. These changes in extension rate were not exactly as predicted from the proportions of Pr (the red-absorbing form of phytochrome) and Pfr (the far-red absorbing form of phytochrome) calculated to be established by the light sources. Nevertheless, the data demonstrate that phytochrome is able to control extension growth at fiuence rates approaching those of summer sunlight, thereby providing the capacity to sense the presence of neighbouring vegetation before shading seriously compromises photosynthesis. Varying fiuence rate over two orders of magnitude whilst maintaining R: FR constant evoked transient fluctuations in extension rate. At high R: FR, a 100-fold step down in fiuence rate led, after a lag of ca 10 min, to a transient (i.e. 20 min) deceleration of extension that was followed by a marked transient (i.e. 20 min) acceleration. After a 100-fold step up in fiuence rate, a transient (i.e. 20 min) acceleration only was observed, beginning after a lag of ca 10 min. When R: FR was low, neither a step-down nor a step-up in fluence rate resulted in appreciable fluctuations in extension rate. The data are discussed in relation to the possible role played by the accumulation of photoconversion intermediates using a simple computer model for simulating active phytochrome concentrations at high fluence rates. The possibility that the mechanism for the photoperception of light quality by phytochrome may be capable of rapid adaptation to fluence rate fluctuations is proposed.     

PHOTOCONTROL OF PHYTOCHROME DESTRUCTION AND BINDING IN DICOTYLEDONOUS vs MONOCOTYLE-DONOUS SEEDLINGS. THE INFLUENCE OF WAVELENGTH AND IRRADIANCE

Abstract— The irradiance and wavelength dependence of phytochrome destruction in vivo was analysed in etiolated cotyledons of Cucurbita pepo L. and etiolated seedlings of Amaranthus caudatus L. In contrast to grass seedlings, the rate of P _tot destruction could only be saturated by light sources that establish relatively high P _fr levels (about 50% of total phytochrome, corresponding to the photostationary state established by 693 nm light). To explain the irradiance dependence of P _tot destruction in dicots at irradiances above 0.1 Wm^-2, where the light reaction is at least one order of magnitude faster than P _fr destruction, we suggest there is a fast intercalary dark reaction between photoreaction and destruction. This dark reaction is probably—as in grass seedlings—the binding of P _fr to a receptor site. We conclude that the differences between dicots and grass seedlings with respect to the phytochrome system are of a quantitative rather than a qualitative nature.     

RESPONSE OF LIGHT-GROWN WILD-TYPE and LONG HYPOCOTYL MUTANT CUCUMBER PLANTS TO END-OF-DAY FAR-RED LIGHT

Abstract— A long-hypocotyl mutant ( lh ) of cucumber ( Cucumis sativus L.) has been studied which has previously been shown to lack phytochrome control of growth in de-etiolated seedlings and thought to be modified with respect to the light-stable type of phytochrome. We have analyzed the response of lh mutant and isogenic wild-type (WT) plants to daily treatment with end-of-day far-red light (EODFR). Only the WT responded to this treatment resulting in a large increase in internode length; an increase in petiole length; changes in leaf development (increased area, decreased thickness and reduction in indentation); redistribution of dry matter from leaf blades to stem; increased apical dominance and promotion of tendril formation. There were only small or no significant effects on chlorophyll and total carotenoid content, chlorophyll alb ratio, soluble protein levels and net photosyn-thetic rates. The lh mutant failed to respond to EODFR treatment, and had the appearance of a shade-avoiding plant growing in extreme shade. The lh mutant appears to completely lack the phytochrome responses attributable to the type of phytochrome that is active in shade detection. A discussion of the possible roles of the stable and labile types of phytochrome in light grown plants follows.