Photomorphogenesis of the Gynophore, Pod and Embryo in Peanut, Arachis hypogaea L.

Darkened excized gynophores ceased to elongate after 8–10days in vitro and started to form a pod. Gynophore elongationwas inhibited to a greater extent in total darkness than underlow irradiance, while pod and embryo growth was stimulated indarkness only. Intact gynophores, enclosed in transparent vials containingglass beads, continued to elongate in both light and darkness.In light the elongating gynophores thickened as they penetratedbetween the glass beads, forming a seedless pod at the bottomof the vials. In the dark the elongating gynophores producedsmall pods in which the seeds had started to grow. Excized gynophores elongated in vitro under continuous whitelight at a rate similar to that of intact exposed gynophores.The rate of elongation in vitro, was lower under continuousblue or red-enriched light, than under white light, and wasfurther reduced under continuous far-red irradiation. Pods didnot form during any of the continuous irradiation treatmentsbut only after transfer to darkness, the largest pods formingafter continuous far-red irradiation. As little as 10 min daily exposure to red or far-red irradiancehad the same effect on gynophore elongation as continuous irradiation.Pods formed only when the daily periods of far-red irradiationwere 30 min or less. Reducing the daily exposures to 2 min decreasedthe time to onset of pod formation from 30 to 16 days. Far-redfollowing red irradiation was effective in inhibiting gynophoreelongation stimulated by red irradiation. Pod formation in red/far-redirradiation was only 50 per cent of that observed in far-redirradiation. The involvement of light in continual gynophoreelongation and in the concomitant inhibition of proembryo growthis discussed. Arachis hypogaea L., peanut, gynophore, photomorphogenesis, embryo development, pod development, proembryo     

Apical Growth of Protonemata in Adiantum capillus-veneris IV. Phytochrome-Mediated Induction in Non-Growing Cells

In non-growing two-celled protonemata of Adiantum capillus-veneris,apical growth was induced most effectively by red light irradiation;half of the samples were induced to grow by 660 nm light ofca. 1.5 J m^–2 and the maximum number by ca. 70 J m^–2.The reciprocity law was valid in this photoinduction. The growthresumption became detectable 6 hr after the light irradiationand reached a plateau within 24 hr irrespective of given fluences.When non-growing samples were irradiated with red light of 4.6W m^–2 for 4 sec or shorter, the effect was fully reversedby a subsequent irradiation with far-red light to the far-redlight control level. But, when the red light was given for 16sec or longer, photoreversibility became partial. An interveningdark period of 2 min between red and far-red light did not significantlyinfluence the photoreversibility so that the escape reactionin the dark may not be attributed to the above-mentioned lossof photoreversibility. By means of a local irradiation with a narrow red light beam(10 µm in width), the apical cell was found to be photosensitivefor the growth induction, but basal cell was not. Photoreceptivesite was not localized in any particular region of the apicalcell, but was rather dispersed in the entire apical cell. (Received January 26, 1981; Accepted March 10, 1981)     

Rapid growth responses of dark-grown wheat seedlings to red-light irradiation. II. Kinetic studies on the growth of different coleoptile zones

Continuous recordings of the effect of red light on the over-all and zonal growth responses were made on intact, dark-grown wheat ( Triticum aestivum L., cv. Hatri) seedlings selected 70 or 90 h after sowing. The over-all growth response of intact coleoptiles induced by bilateral continuous red light (660 nm, 17 W m⁻²) was complex and resulted from the overlapping of different zonal growth responses. During a 5 h investigation period, these responses can be divided into two phases. The first phase (short-term response) was a transient growth inhibition. After a lag period of ca 15 min, the rate of extension decelerated to a minimum value at ca 60 min, after which an acceleration was seen. This response was qualitatively the same in all coleoptile zones investigated (tip, subapical zone, base) and independent of coleoptile age. The second phase (delayed response) became measurable between 1.8 and 3 h after onset of red light irradiation and exhibited zonal-specific growth promotion or inhibition, dependent on the coleoptile age. A persistent growth promotion was observed only in the tip region of coleoptiles selected 70 h after sowing and became detectable about 3 h after the onset of red light.     

Phytochrome-Mediated Photoorientation of Chloroplasts in Protonemal Cells of the Fern Adiantum Can be Induced by Brief Irradiation with Red Light

Measuring the ratio of the number of photooriented chloroplaststo the total number of chloroplasts, we found that photoorientationof chloroplasts in protonemata of the fern Adiantum capillus-veneriscould be induced by brief irradiation with polarized red light.After irradiation with red light (R) of 3 or 10 min, orientationalmovement was detected as early as 10 min after the irradiation;it continued during the subsequent dark period for 30–60min, after which chloroplasts gradually dispersed again. WhenR-treated protonemata were irradiated briefly with a second10-min pulse of R, 60 min after the onset of the first irradiation,the orientational response of chloroplasts was again observed.Typical red/far-red photoreversibility was apparent in the response,indicating the involvement of phytochrome. By contrast, irradiationwith polarized blue light for 10 min was ineffective, whileirradiation with blue light (B) at the same fluence for a longerperiod of time clearly induced the photoorientation of chloroplasts.It is likely that longterm irradiation is necessary for theresponse mediated by a blue-light receptor. When protonemata were irradiated with far-red light (FR) immediatelyafter R or after a subsequent dark period of 10 min, the magnitudeof the orientational response was smaller and chloroplasts dispersedmore quickly than those exposed to R alone. When FR was appliedat 50 min, when the response to R had reached the maximum level,chloroplasts again dispersed rapidly to their dark positions.These results indicate that P_FR not only induces the photoorientationmovement of chloroplasts but also fixes the chloroplasts atthe sites to which they have moved as a result of photoorientation. (Received June 2, 1993; Accepted January 11, 1994)     

Growth and Dormancy in Lunularia cruciata (L.) Dum: VI. GROWTH REGULATION BY DAYLENGTH, BY RED, FAR-RED, AND BLUE LIGHT, AND BY APPLIED GROWTH REGULATORS AND CHELATING AGENTS

Experiments with photoperiods ranging from 2 to 24 h confirmthat 8 h light per day is optimal for Lunularia: there is nogrowth in the dark or in continuous light, which causes therapid onset of dormancy. Short-day cycles intercalated amonga series of continuous light cycles promote growth; in cycleslonger than 24 h very long dark periods are detrimental. Withvery short photoperiods (5 min) red light promotes growth moreeffectively than white light at higher intensity; far-red actsas dark. The growth effects of red and far-red light breaks(3 min) depended on the time of application; red light inhibitedin the middle but promoted at the beginning of the 16-h darkperiod of a short day; far-red light had the opposite effect;in each case red and far-red effects were reversible by theother wavelength. Blue light gave the same response as red includingthe reversibility of far-red effects and vice versa. Surprisingly,significant effects of 5 min red, blue, and far-red irradiationwere also found in the middle of the main high-intensity white-lightperiod, red and blue promoting growth, far-red reducing it;again there was ready reversibility of the effects. Growth promoters of higher plants are generally inhibitory toLunularia or have little effect; among growth retardants TIBA,Phosphon D, and CCC gave a slight promotion of growth. EDTApromoted growth (cell numbers) very significantly while 8-hydroxyquinolinewas initially inhibitory, but had a marked latent promotingeffect when subsequently washed from the thalli.     

Rapid suppression of extension growth in dark-grown wheat seedlings by red light

Continuous recordings were made using a linear displacement transducer to investigate short-term growth responses of intact dark-grown wheat (Triticum aestivum L. cv Maris Huntsman) seedlings to red light. To eliminate any effect of light prior to the experimental treatments, the seedlings were grown and mounted on the transducer apparatus in total darkness. The growth kinetics after irradiation were complex and appeared to consist of three successive phases of growth deceleration. When the tip of the intact coleoptile was irradiated with red light from two opposite fiber bundles (fluence rate: 2 × 64 micromoles per square meter per second) for varying periods of time (10 seconds, 1 minute, 5 minutes, continuous), a decrease in extension rate was detectable after a latent period of 8 to 10 minutes. Up to 30 minutes after the start of the irradiation treatment, there was no difference in the kinetics of inhibition (about 20 to 25% inhibition) between the different lengths of irradiation. Extension rate reached a minimum (65% inhibition) at about 85 minutes, after which growth acceleration toward the dark control rate was observed. Far-red reversibility of the rapid effect of red light on growth was not observed, even when far-red light was given only 4 seconds after the end of 10 seconds red light. Short (15 seconds) far-red light did not induce a response.     

Phytochrome Activation of K+ Channels and Chloroplast Rotation in Mougeotia: the Escape Times

Red light mediates chloroplast movement and increased activityof calcium-activated potassium channels on the plasma membraneof the alga Mougeotia sp. (UTEX LB 734). When activation ismediated by phytochrome, a far-red light irradiation given sometime after the red light irradiation will reverse the effectof the red light, due to phytochrome photoreversibility. Wecharacterized the escape times (time required for loss of photoreversibility)for these two processes to compare the transduction pathwaysinvolved in chloroplast rotation and channel activation. Theescape time for chloroplast rotation was 2.5 min after red lightirradiation (red and far-red light irradiations were 30 s).For channel activation, shorter red and far-red light irradiations(10 s) had to be used to obtain an escape time of 20 s. Thedifference in the escape times suggests that there is relativelyrapid divergence in the transduction pathways leading from phytochromeactivation (only one molecular species of phytochrome is foundin Mougeotia) to each of the two responses in the same cellularsystem. Because channel activation occurs 2–4 min afterirradiation while the escape time is 20 s, it is unlikely thatphytochrome acts directly on the channel. (Received September 26, 1995; Accepted December 28, 1995)     

Responses of the rice shoot apex to irradiation with red and far-red light

Summary Determinations of cell-doubling times using the technique of colchicine-induced metaphase accumulation showed that after 40 h exposure to red or far-red light the rates of cell division in young rice (Oryza sativa L. cv. Ballila) shoot apices were faster than in dark controls. In red light, the increase was already taking place after 16 h of irradiation but in far-red the rates at this time were slower than in the dark controls. Seedlings became more responsive to far-red light as they continued to grow in darkness between 2 1/2 and 4 1/2 days. Mitotic activity at the apex increased in the leaf primordium and decreased in the sub-summit corpus between the 4th and 5th days of growth in darkness at 30° C.     

Time course of phytochrome-mediated changes in growth gradients on coleoptiles of Triticum aestivum L.

The length of parenchyma cells along the axis of dark-grown coleoptiles of Triticum aestivum L. and the pattern of competence for red-light-(R-) induced stimulation or inhibition of cell elongation in the course of coleoptile development were determined by microscopic measurements in a file of 240 cells from the tip to the base. On the basis of these measurements distinct zones (responding in different ways to R) were selected for studying the early time course of phytochrome-mediated growth-rate changes in intact coleoptiles by use of a sensitive transducer system. Between 2 d and 4 d after sowing dark-grown coleoptiles showed a graded incline in cell growth activity from the apex to the base (growth gradient). Whereas cell elongation in the coleoptile base ceased 4 d after sowing, cell elongation speeded up in the tip and middle region at that time. Those cells that grew slowly in darkness (tip and middle region between 2d and 3 d after sowing) were stimulated in growth by R-pulse irradiation (1 min R, 660 nm, 1000 J · m^–2). In contrast, the growth of fast-growing cells (base between 2 d and 4 d after sowing, tip and middle region between 4 d and 5 d after sowing) was inhibited by R. However, the starting time for R-induced growth changes was different for different coleoptile zones. The respective data point to the storage of a phytochrome-mediated signal in the cells of the middle region, until these cells become competent to respond to it; alternatively, Pfr, the far-red-light-absorbing form of phytochrome, may be stored in a stable form. Continuous recordings on the effect of R, far-red (FR) and R/FR on the zonal growth responses were made on intact coleoptiles, selected 3 d after sowing. During a 5-h investigation period the R-induced changes in growth rate could be divided into two phases: (i) A transient growth inhibition which started approx. 15 min after R. This response was qualitatively the same in all coleoptile zones investigated (tip, middle region, base). (ii) Zonal-specific growth responses which became measurable approx. 2.5 h after R, i.e. growth promotion in the tip, growth inhibition in the base and an adaptation of growth rate to the dark control level in the middle region. The R-induced growth rate changes were reversible by FR for both phases. Additional growth experiments on excised coleoptile segments under R and auxin application indicated that the zonal-specific growth promotion or inhibition may be not mediated by an influence of R on the auxin level.Abbreviations FR far-red light - Pfr far-red-light-absorbing form of phytochrome - R red light The technical assistance of Mrs. B. Liebe is gratefully acknowledged.     

Redistribution of Phosphorus, Sulfur, Potassium and Calcium in Relation to Light-Induced Gravitropic Curvature in Zea Roots

Phosphorus, sulfur, potassium and calcium were measured by particle-inducedX-ray emission (PIXE) in horizontally oriented, light-irradiatedor non-irradiated primary roots of Zea mays L., cv. Golden CrossBantam 70 which exhibit gravitropic response only after exposureto light. The content of the four elements increased in thelower half of horizontally oriented roots which had been brieflyexposed to white or red light, while there were no marked differencesin distribution between the upper and lower halves of non-irradiatedroots. The increase of each element in the lower half was observed15–30 min after irradiation in root caps and 30–60min after irradiation in the elongation zones. The effect ofred light was not reversed by far-red light given immediatelyafter the red irradiation. Ethylene glycol-bis(ß-aminoethylether)-N, N, N',N'-tetraacetic acid (EGTA) treatment of roottips inhibited the gravitropic curvature of roots, and the additionof Ca reduced this inhibition. The meaning of Ca redistributionin root caps and elongation zones during light-induced gravitropiccurvature of maize roots is discussed. (Received December 4, 1985; Accepted March 22, 1986)     

Phytochrome action inOryza sativa L. VI. Red far-red reversible effect on early development of coleoptiles

When 3–4 mm long coleoptiles of etiolated rice seedlings (cv. Koshijiwase) were irradiated with continuous red light their growth was seriously inhibited. If a brief exposure of red light (4×10³ ergs cm⁻²) was given to the short coleoptiles, the growth rate dropped immediately after the irradiation, but the growth did not stop till the coleoptile reached some calculated length. If another brief red irradaition of the same order was given 24 hr after the first, the growth rate and the final length dropped further. The effect of red light was reversed by successively given far-red light, and this response was repeatedly red and far-red reversible. The escape reaction was rather slow so that photoreversibility was not lost at all by 8th hr, and 50% of the initial reversibility was lost within ca. 16 hr at 25±0.5 C. Blue light also induced the inhibition of coleoptile elongation, the effect was reversed by subsequent far-red irradiation, and this could be obtained repeatedly. Thus, the photoinhibition of the young coleoptile can be concluded to be under the control of phytochrome, and the mode of action appeared quite different from the previously reported results with longer coleoptiles.     

Effects of long-term storage on phytochrome-mediated germination in lettuce seeds

The effects of long-term seed storage on the physiological properties of phytochrome-mediated germination including water uptake, the temperature and light flunnce dependencies of germination and dark germination were studied. The fluenceresponse relationships of the brief irradiation with monochromatic red (660 nm, 7.5 W m⁻²) and far-red (750 nm, 6.6 W m⁻²) light at various times after sowing were also studied. The samples used consisted of three lots of seeds ofLactuca sativa L. cv. MSU-16, which had been harvested in 1976, 1979 and 1985 and stored dry for 9, 6 and 0 years, respectively, in darkness at 23±2 C until the experiments were carried out in July–August, 1985. Seeds with the longer storage periods showed the higher ability to germinate in both continuous darkness and continuous white fluorescent light at 20–30 C. In the seeds stored for 6 or 9 years, red light irradiation for 20 sec given at 15 min or more after sowing at 25 C induced as high a percent germination (85–95%) as those under continuous white fluorescent light. In the freshly harvested seeds, however, germination under continuous white fluorescent light (46%) was considerably lower than the germination induced by the red pulse (97%). Germination of the seeds decreased when the intervals between sowing and a far-red irradiation for 20 sec increased up to 100 min (or 30 min in the freshly harvested seeds). The far-red pulse given later than 100 min (or 6 hr in the freshly harvested seeds) after sowing resulted in an increased germination up to the dark-germination levels with increasing intervals between sowing and the pulse irradiation. Before or at 3 min after sowing, the seeds stored for 6 or 9 years were responsive to the far-red pulse although they were not or hardly responsive to the red pulse, while the freshly harvested seeds were responsive to both the far-red and the red pulses. These data indicate that normal functions of phytochrome completely survived in the dry seeds during storage at 25 C for as long as 6 or 9 years and that these functions are restored into full operation by means of imbibition. The differences in the dependence of germination on the time and fluence of a single pulse of red or far-red light seems to be related to the smaller water content throughout the imbibition in the seeds with the longer storage periods. The greater ability to germinate in the dark indicates the greater amounts of P_FR or the greater responsivity to P_FR, in the seeds with the longer storage periods.     

Studies on the Photoreceptors for the Promotion and Inhibition of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy

Three-day-old etiolated seedlings of Pharbitis nil were exposedto red light for 10 min and sprayed with N⁶-benzyladenine beforetransfer to a 48-h inductive dark period, after which they weregrown under continuous white light. A second red irradiationpromoted flowering when given at the 5 and 24th hour of theinductive dark period but inhibited flowering at the 10 and15th hour. Far-red light inhibited flowering when given at anytime during the first 24 h of the dark period. Red/far-red reversibilitywas clearly observed at the 0, 5, 10 and 24th hour, but notat the 15th hour when both red and far-red lights completelyinhibited flowering. The action spectrum for the inhibition of flowering at the 15thhour of the inductive dark period had a sharply defined peakat 660 nm and closely resembled the absorption spectrum of theP_R form of phytochrome. The photoreceptors involved in thesephotoreactions are discussed. (Received June 10, 1983; Accepted July 6, 1983)     

PHOTOMORPHOGENESIS IN PTERIS VITTATA: I. PHYTOCHROME-MEDIATED SPORE GERMINATION AND BLUE LIGHT INTERACTION

1. Spores of the fern Pteris vittata did not germinate under totaldark conditions, while an exposure of the spores to continuouswhite light brought about germination. The germination was mosteffectively induced by red light and somewhat by green and far-red,but not at all by blue light. The sensitivity of spores to redlight increased and leveled off about 4 days after sowing at27–28. The promoting effect of red light could be broughtabout by a single exposure of low intensity. Far-red light givenimmediately after red light almost completely reversed the redlight effect, and the photoresponse to red and far-red lightwas repeatedly reversible. The photoreversibility was lost duringan intervening darkness between red and far-red irradiations,and 50% of the initial reversibility was lost after about 6hr of darkness at 27–28. These observations suggest thatthe phytochrome system controls the germination of the fernspore.
2. When the imbibed spores were briefly exposed to a low-energyblue light immediately before or after red irradiation, theirgermination was completely inhibited. The blue light-inducedinhibition was never reversed by brief red irradiation givenimmediately after the blue light. The escape reaction of redlight-induced germination as indicated by blue light given aftervarious periods of intervening darkness was also observed, andits rate was very similar to that determined by using far-redlight. Spores exposed to blue light required 3 days' incubationin darkness at 27–28 to recover their sensitivity tored light. The recovery in darkness of this red sensitivitywas temperature-dependent. It is thus suggested that an unknownbluelight absorbing pigment may be involved in the inhibitionof phytochrome-mediated spore germination.

(Received August 21, 1967; )     

Photobiology of phytochrome-mediated growth responses in sections of stem tissue from etiolated oats and corn

The far-red reversibility of the phytochrome-controlled stimulation of elongation of coleoptile sections by low fluence red light has been characterized in subapical coleoptile sections from dark-grown Avena sativa L., cv Lodi seedlings. The fluence dependence of the far-red reversal was the same whether or not the very low fluence response is also expressed. The capacity of far-red light to reverse the red light-induced response began to decline if the far-red light was given more than 90 minutes after the red irradiation. Escape was complete if the far red irradiation was given more than 240 minutes after the red irradiation. Sections consisting of both mesocotyl and coleoptile tissue from dark-grown Avena seedlings were found to have physiological regulation of the very low fluence response by indole 3-acetic acid and low external pH similar to that seen for sections consisting entirely of coleoptile tissue. The fluence-dependence of the red light-induced inhibition of mesocotyl elongation was studied in mesocotyl sections from dark grown Zea mays L. hybrid T-929 seedlings. Ten micromolar indole 3-acetic acid stimulates the control elongation of the sections, while at the same time increasing the sensitivity of the tissue for the light-induced inhibition of growth by a factor of 100.     

The 'end-of-day' phytochrome control of internode elongation in mustard: kinetics, interaction with the previous fluence rate, and ecological implications

Abstract The ‘end-of-day’ phytochrome control of internode growth was characterized in Sinapis alba, seedlings previously grown under continuous white light for 13 d. The transition from white light to darkness caused a reduction in internode extension rate with a lag of less than 10 min. Following this, extension rate remained almost constant for at least 48 h. i.e. ‘re-etiolation’ was not noticed. The phytochorme controlling the growth processes was stable in the Pfr form. The growth rate of plants receiving a red light pulse, and the growth promotion caused by a far-red light pulse, increased with increasing fluence rate of the previous white light treatment. In far-red treated plants a first growth rate acceleration peaked at 20–30 min after the end of white light, followed by a transient deceleration which led to a growth rate minimum at 40–60 min, followed by a final growth rate recovery yielding a more-or-less steady elevated rate. Pulses establishing different Pfr/P modified the extent, but not the early kinetics, of the growth response. The relative promotion of growth caused by low Pfr/P was limited by darkness as follows: (a), The growth promotion caused by far-red directed to the internode alone was transient. (b), The promotion caused by a reduction of Pfr/P in the whole shoot persisted in darkness for at least 48 h and also persisted if, after a 3–9 h dark period, the plants were returned to continuous white light. In darkness, however, the magnitude of this growth rate promotion decreased with time, particularly when the previous white light fluence rate was low, or the pulse preceding darkness provided the lowest Pfr/P. (c), When compared over the same period in darkness, growth rate was higher in those seedlings in which Pfr/P was reduced during the continuous white light pretreatment than in those ones in which the Pfr/P was only reduced immediately before darkness. It is proposed that in the natural environment, red/far-red signals could be more effective when provided during daytime than at the end of the photoperiod, as both the background growth rate and the relative promotion caused by low Pfr/P are reduced by darkness.     

The Effect of Light Quality on Shoot Extension Growth in Three Species of Grasses

The effect of light quality on the extension growth of vegetativeshoots and on the final size of their leaves was investigatedin plants of Lolium multiflorum, Sporobolus indicus and Paspalumdilatatum. Three experimental approaches were used, (a) redor far-red end-of-day irradiations of sunlight-grown plants,(b) different red/far-red ratios of white light in a growthroom and (c) sunlight enrichment with radiation of differentred/far-red ratios or with different amounts of far-red lightduring the photoperiod. Plants treated with end-of-day far-redor low red/far-red ratios throughout the photoperiod developedlonger leaves and, as a result, longer shoots. This effect wasmore marked in leaf sheaths than in blades. Tiller extensionand leaf sheath length increased with the amount of far-redadded to sunlight in a simple hyperbolic relationship. Theseresults show that vegetative grass shoots respond to light qualityin a way similar to internodes of dicotyledonous plants. Lolium multiflorum Lam., Sporobolus indicus (L.), Paspalum dilatatum (Poir.), leaf growth, tiller growth, photomorphogenesis     

Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

Control of the Hypocotyl Hook Angle in Phaseolus mungo L.: the Role of Parts of the Seedling

Phaseolus mungo L. seedlings were grown in the dark for 5 d.They were either kept in the dark or exposed to 5 min red light.The location of the light-sensitive region (the hook) and theanatomy of light-induced opening are described. The responsewas phytochrome-mediated. The time course of light-induced openingand its far-red reversal are described and the implicationsfor the mechanism of light-induced opening discussed. The lagphase between irradiation and the start of opening was >8h, for 3 h of which the response remained completely photoreversible.This implies that some critical level of a factor is requiredbefore hook opening can be initiated. The terminal parts (leavesand cotyledons) kept the hooks shut in the dark, but did notdirectly affect the light response. The necessity for distinguishingbetween treatment effects on hook angle and on light-inducedopening is noted. We hypothesize that the terminal parts produceda hook-maintaining factor whose level declined after their removal.This factor was dispersed or inactivated following irradiation,or the hook cells were rendered less sensitive to it. Differencesbetween the responses of P. mungo and those reported for otherspecies are discussed.