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.     

PHYTOCHROME ACTION IN ORYZA SATIVA L. III. THE SEPARATION OF PHOTOPERCEPTIVE SITE AND GROWING ZONE IN COLEOPTILES, AND AUXIN TRANSPORT AS EFFECTOR SYSTEM

• 1 In 4-day-old etiolated rice seedlings, 3 mm of the coleoptile tip did mainly perceive the photostimulus to cause the phytochrome-dependent inhibition of coleoptile elongation. At this age, cell elongation occurred most in the middle portion of coleoptiles in the dark, and was reversibly controlled by a brief exposure of the tip to red and far-red light. Thus, the photoperceptive site was evidently separated from the growing zone in intact rice coleoptiles.
• 2 The red-light-induced inhibition of coleoptile elongation was nullified by the removal of tip followed by the exogenous application of IAA. The sensitivity of thus treated coleoptiles to IAA was gradually lost during intervening darkness between the irradiation and the decapitation, and a 50% loss was obtained at ca. 6th hour at 26°C.
• 3 Polar auxin transport from coleoptile tips was remarkably prevented at the period between, at least, 2nd and 4th hour after red irradiation, and it recovered to the level of dark control by the 6th hour. Far-red light given immediately after red irradiation reversed the yield of diffusible auxin up to that of far-red control.

     

Phytochrome Action in Oryza sativa L: IV. Red and Far Red Reversible Effect on the Production of Ethylene in Excised Coleoptiles

Excised apical segments of etiolated rice (Oryza sativa L.) coleoptiles produced ethylene. Increasing the number of cut sites per coleoptile increased the rate of ethylene formation. Ethylene produced by an etiolated-intact seedling in the dark was about a half of that by the excised coleoptile segment. Red light of low energy as well as of continuous irradiation inhibited the production of ethylene. The inhibition by a low energy dose of red light was partly relieved, if the red light was followed immediately by a small dose of far red light. The effect of red and far red light was repeatedly reversible, indicating that ethylene production was regulated by a phytochrome system. If the exposure to far red light was preceded by a period of darkness, this photoreversibility disappeared; 50% of the initial reversibility was lost within 5 hours. Applied ethylene (10 microliters per liter) significantly promoted the growth of intact coleoptiles of either totally etiolated or red light-treated seedlings, but had no effect on the excised apical segment of coleoptile.     

The Effects of Red, Far-red, and Blue Light on the Geotropic Response of Coleoptiles of Zea mays

The effects of red, far-red, and blue light on the geotropicresponse of excised coleoptiles of Zea mays have been investigated.Seedlings were grown in darkness for 5 or 6 days, exposed tovarious light treatments, and then returned to darkness fordetermination of the geotropic response. The rate of response of the coleoptiles is decreased after theyhave been exposed to red light (620–700 mµ, 560ergs cm^–2sec^–1 for the 24 hrs, but not for the 4hrs, preceding stimulation by gravity. Furthermore, their rateof response is greatly reduced if they are exposed to red lightfor 10 min and then returned to darkness for 20 hrs before geotropicstimulation. At 25° C an interval of 6 to 8 hrs elapses between a 10-minexposure to red light and the first detectable decrease in thegeotropic response of the coleoptile. This interval can be lengthenedby exposing the seedlings to low temperatures (0° to 2°C) after the light treatment but cannot be greatly shortenedby increasing the duration of exposure to red light. Using a standard procedure of exposing 5-day-old etiolated seedlingsto light for various times, replacing them in darkness for 20hrs and then determining the response of the coleoptiles to4 hrs geotropic stimulation, it has been found that: (a) Exposureto red light for 15 sec significantly decreases the geotropiccurvature of the coleoptiles and that further reduction occurson increasing the length of the light treatment to 2 and 5 min.(b) Far-red light has no effect on the geotropic response ofthe coleoptiles but it can completely reverse the effect ofred light. After repeated alternate exposure to red and far-redlight the geotropic response of the coleoptile is determinedby the nature of the last exposure, (c) Complete reversal ofthe effect of red light by far-red radiation only occurs whenexposure to far-red follows immediately after exposure to red.The reversing effect of far-red radiation is reduced if a periodof darkness intervenes between the red and far-red light treatments,and is lost after a dark interval of approximately 2 hrs. The effect of red light on the rate of geotropic response ofthe coleoptiles is independent of their age and length at thetime of excision. Blue light acts in a similar way to red light, but the seedlingsare less sensitive to blue than to red light. Coleoptiles grown throughout in a mixture of continuous, weak,red, and far-red light have a lower rate of geotropic responsethan etiolated coleoptiles.     

Photomorphogenesis in Pteris vittata II. Recovery from blue-light-induced inhibition of spore germination

1. 1. In the fern Pteris vittata, low-energy blue-light-inducedinhibition of phytochrome-dependent spore germination and darkrecovery from this inhibition were repeatedly observed severaltimes at intervals of 3 days at 26. The same amount of incidentenergy of blue light was required for inhibition in each successivetreatment.
2. 2. The recovery from blue-light-induced inhibitionof germinationwas markedly accelerated by continuous illuminationwith redlight, and this red light effect was not affected bythe presenceof CMU.
3. 3. The recovery process was not influencedby a single exposureto redlight, but was definitely promotedby brief red irradiationsgiven intermittently, at least 2 times,at equal intervals duringthe first 8 hr after blue light treatment.The effect of intermittentlygiven red light was annulled wheneach red exposure was followedby brief far-red irradiation.These facts suggest that phytochromemay be involved in therestoration of the ability of sporesto germinate (in responseto red light) which had been lostby blue irradiation.

¹Present address: Botany Department, Faculty of Science, Universityof Tokyo, Hongo, Tokyo 113.     

Response of excised Avena coleoptile segments to red and far-red light

Summary In seeking a simple, red light-promoted straight growth test in which phytochrome assays may be conducted without interference by protochlorophyll, the response of excised Avena coleoptile segments to red and far-red light was re-examined. The elongation of apical (non-decapitated) segments is promoted by a brief exposure to red light, and this effect is almost completely nullified by an immediately subsequent exposure to far-red light. Although growth promotion by red light occurs in distilled water alone, the effect is greater on a medium consisting of 0.02 M phosphate buffer, pH 6.2 to 6.4, with 1 to 2% sucrose. Over the pH range 4.5 to 7.4, dark-growth decreases with increasing pH, but the absolute increment brought about by red light is nearly constant. Elongation appears to be entirely the result of increased cell size.Contrary to previous reports, similar results can be obtained with subapical (decapitated) coleoptile segments, although the absolute magnitude of the response is reduced.Research carried out at Brookhaven National Laboratory under the auspices of the U.S. Atomic Energy Commision.     

Ethylene and the Responses to Light of Rice Seedlings

The growth of rice seedlings (Oryza satira L.) in the presence of ethylene caused a change in the response to light of coleoptile elongation. In plants grown in air without added ethylene coleoptile elongation was promoted by red, far-red and yellow-green light only in very young seedlings; in older plants irradiation inhibited the growth of the coleoptile. The effect of growing plants in the presence of ethylene was to prolong the period during which light promoted coleoptile growth. Elongation of the first internode was inhibited by light whether or not the seedlings were grown in the presence of ethylene. A correlation existed between the growth effect of an irradiation and the initial decay rate of phytochrome which was established by the treatment. Regardless of wave length, light sources whose intensities were adjusted to produce a decay rate of about 10% per hour or less induced a moderate rate of coleoptile elongation which persisted for a relatively long period. Irradiation with red or yellow-green light of higher intensity which produced a higher rate of phytochrome decay induced a higher rate of coleoptile elongation, but growth stopped after several hours. Other observations, however, showed that one cannot establish a general simple correlation between the rate of elongation of rice coleoptiles under light and the status of measurable phytochrome in the plant.     

Phytochrome action in Oryza sativa L. VII. Effects of light and aeration on coleoptile growth under submerged conditions

Coleoptile growth of intact rice seedlings under submerged conditionswas not much affected by the continuous irradiation of red light.But, if aerated, growth was strongly inhibited by a low-energybrief irradiation of red light, and this red light-induced effectwas reversed by a brief exposure to far-red light. The responsesto red and far-red light were repeatedly reversible. The amountof photometrically detectable phytochrome in non-aerated coleoptiletissues was found to be as low as one third that in aeratedones. After the conversion of Pr to Pfr by a brief red irradiation,the total (A) of the aerated tissues decreased to about onefifth that of the original level within 6 hr at 24?C, whilethe Pfr in the non-aerated tissues was significantly less decayed. ¹Present address: The Ocean Research Institute, University ofTokyo, Nakano, Tokyo 164, Japan. (Received April 30, 1974; )     

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.     

Phytochrome action in Oryza sativa L.

Summary In-vivo phytochrome determinations in totally etiolated rice seedlings with a dual-wavelength spectrophotometer showed that on a fresh weight basis phytochrome concentration was highest in the coleoptile apex (0.175 of mean) ( O.D.) g^-1 (fresh weight). The age of the seedlings had little effect on the pattern of phytochrome distribution in the coleoptiles.The extent of growth inhibition observed 2 days after the irradiations was proportional to the logarithm of P _fr amount in the coleoptiles at the time of initial exposure to either red or blue light. Ultraviolet irradiation, however, did not induce either reversible growth inhibition or optically detectable phytochrome changes in vivo.After the conversion of P _r to P _fr bya brief red irradiation, non-photochemical transformation of phytochrome was observed in intact coleoptile tissues. Most of the optically measurable P _fr disappeared within 6 hours at 27°, when the total ( O.D.) decreased to about one fifth of the original level. The optical data did not agree with the fact that 50% of the initial physiological reversibility was still observed 9 hours later. No significant difference in dark transformation rate was seen between intact and excised coleoptile tissues.Abbreviations P _r red light absorbing form of phytochrome - P _fr far-red light absorbing form of phytochrome - ( O.D.) the change in the optical density difference reading at two wavelengths, following irradiation of the sample with actinic sources of red and far-red light - UV ultraviolet light     

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; )     

Rapid Growth Responses of Dark-Grown Wheat Seedlings to Red Light-Irradiation: I. Kinetic Studies on the Short-Term Growth Responses of Intact Coleoptiles at Different Seedling Age

Continuous recordings of the effect of red light on intact darkgrown wheat seedlings (Triticum aestivum L. cv. Hatri) weremade at different times after sowing. When the coleoptile tipregion was irradiated 50, 70 or 90 h after sowing with red lightfrom two opposite fibre bundles a decrease in extension ratewas detectable after a latent period of 10 to 15 min. Growthrate reached a fluence dependent minimum at about 60 min, afterwhich growth acceleration towards the dark control rate wasobserved. When continuous red irradiation was started 50 or70 h after sowing the dark control rate was reached 2.5 h afteronset of irradiation and growth rate was little above this levelduring the next 2 h. With older coleoptiles (90 h after sowing)the growth rate recovery was only up to 50% of the dark controlrate and a second phase in growth inhibition became detectableabout 2.5 h after onset of red exposure, characterized by acontinuous decrease in extension rate. Under R/FR pulse irradiationboth the red-light-induced transient growth inhibition and thesecond phase of growth inhibition exhibit far-red reversibilityup to the level of far-red induced growth rate changes. (Received September 19, 1986; Accepted December 8, 1986)     

Biphasic effect of red light on the growth of coleoptiles in etiolated barley seedlings

InHordeum vulgare cultivar “Kirin-choku No. 1”, the final length of intact coleoptiles of totally etiolated seedlings was approximately twice as long as that of those grown under continuous red light. The fluence response curve of the latter was biphasic; the low-energy effect was saturated by red light of ca. 50 J m⁻² which gave rise to about 40% of the maximum inhibition by continuous irradiation with red light of 1.2 W m⁻², whereas the high-energy effect was induced by irradiation for 1 hr or longer. Coleoptiles of 3-day-old seedlings were most sensitive to light causing the low-energy effect, which was repeatedly red/far-red reversible. The growth inhibition was correlated to the photometrically measured percentage of Pfr so that the maximum effect was induced by red light of 50 J m⁻² which transformed 70% of phytochrome to Pfr in the coleoptile tip. Wavelength dependence of the high-energy effect showed that monochromatic light of 400, 600 and 650 nm greatly inhibited the coleoptile growth, whereas light of 700 and 750 nm promoted it instead. The effect was also induced by intermittent irradiation with red light, and the more frequently the intermittent treatment was given, the more the growth was inhibited.     

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.     

Inhibitory action of red light on the growth of the maize mesocotyl: evaluation of the auxin hypothesis

Brief irradiation of 3-d-old maize (Zea mays L.) seedlings with red light (R; 180 J m^-2) inhibits elongation of the mesocotyl (70–80% inhibition in 8 h) and reduces its indole-3-acetic acid (IAA) content. The reduction in IAA content, apparent within a few hours, is the result of a reduction in the supply of IAA from the coleoptile unit (which includes the shoot apex and primary leaves). The fluence-response relationship for the inhibition of mesocotyl growth by R and far-red light closely resemble those for the reduction of the IAA supply from the coleoptile. The relationship between the concentration of IAA (1–10 M) supplied to the cut surface of the mesocotyl of seedlings with their coleoptile removed and the growth increment of the mesocotyl, measured after 4 h, is linear. The hypothesis that R inhibits mesocotyl growth mainly by reducing the IAA supply from the coleoptile is supported. However, mesocotyl growth in seedlings from which the coleoptiles have been removed is also inhibited by R (about 25% inhibition in 8 h). This inhibition is not related to changes in the IAA level, and not relieved by applied IAA. In intact seedlings, this effect may also participate in the inhibition of mesocotyl growth by R. Inhibition of cell division by R, whose mechanism is not known, will also result in reduced mesocotyl elongation especially in the long term (e.g. 24 h).Abbreviations FR far-red light - IAA indole-3-acetic acid - P_fr phytochrome in the far-red-absorbing form - P_r phytochrome in the red-absorbing form - R red light     

Phytochrome inhibits the effectiveness of gibberellins to induce cell elongation in rice

Red light controls cell elongation in seedlings of rice (Oryza sativa L.) in a far-red-reversible manner (Nick and Furuya, 1993, Plant Growth Regul. 12, 195–206). The role of gibberellins and microtubules in the transduction of this response was investigated in the rice cultivars Nihon Masari (japonica type) and Kasarath (indica type). The dose dependence of mesocotyl elongation on applied gibberellic acid (GA₃) was shifted by red light, and this shift was reversed by far-red light. In contrast, coleoptile elongation was found to be independent of exogenous GA₃. Nevertheless, it was inhibited by red light, and this inhibition was reversed by far-red light. The content of the active gibberellin species GA₁ and GA₄ was estimated by radio-immunoassay. In the mesocotyl, the gibberellin content per cell was found to increase after irradiation with red light, and this increase was far-red reversible. Conversely, the cellular gibberellin content in japonica-type coleoptiles did not exhibit any significant light response. Microtubules reoriented from transverse to longitudinal arrays in response to red light and this reorientation could be reversed by subsequent far-red light in both the coleoptile and the mesocotyl. This movement was accompanied by changes in cell-wall birefringence, indicating parallel reorientations of cellulose deposition. The data indicate that phytochrome regulates the sensitivity of the tissue towards gibberellins, that gibberellin synthesis is controlled in a negative-feedback loop dependent on gibberellin effectiveness, and that at least two hormone-triggered signal chains are linked to the cytoskeleton in rice.Abbreviations D darkness - FR far-red light - GA₃ gibberellic acid - GC-SIM gas chromatography-selected ion monitoring - R red light This work was supported by a grant of the Human Frontier Science Organization to P.N. Advice and organizational support by Prof. M. Furuya (Hitachi Advanced Research Laboratory, Hatoyama, Japan) and Prof. N. Murofushi (Department of Agricultural Chemistry, University of Tokyo, Japan) is gratefully acknowledged. Seeds of both rice cultivars were kindly provided by Dr. O. Yatou (Institute for Radiation Breeding, Hitachi-Ohmiya, Japan), and the antiGA₁ Me-antiserum for the radio-immunoassays by Dr. I. Yamaguchi (Department of Agricultural Chemistry, University of Tokyo, Japan).     

Possible Role of Hexosamine-Containing Cell Wall Component in Growth Regulation of Rice Coleoptiles

The cell wall of rice coleoptile was found to contain severalhundred microgram hexosamine per gram dry wt with the pectic,hemicellulosic, and -cellulose fractions containing 50%, 40%,and 10%, respectively. The cell wall hexosamine content increasedseveralfold with coleoptile growth and was higher in air-typecoleoptiles (grown on the surface of water) than water-typeones (grown under water). Rice coleoptiles were cultured in glucosamine, NH₄⁺, glutamine,or asparagine solution and growth was inhibited at 10^–4M and above. Coleoptile growth capacity in glucosamine or NH₄⁺solution correlated inversely with the cell wall hexosaminecontent. Both of these solutions also inhibited elongation ofsubmerged air-type coleoptile sections. Azaserine promoted thegrowth of both intact and excised coleoptiles at 10^–6to 10^–5 M and halved the cell wall hexosamine contentof intact ones. 6-Diazo-5-oxo-L-norleucine promoted the elongationof sections. These results suggest that the hexosamine-containingcell wall component is an important growth suppression factorin rice coleoptiles. (Received April 25, 1983; Accepted August 30, 1983)     

Effects of red light on the growth of intact wheat and barley coleoptiles

The final lengths of intact dark-grown coleoptiles vary with species and cultivar. The growth distribution pattern in the apical 25-mm growing zone and the absolute amount of growth in each zone depend on the age and species of the coleoptile. A comparative study of several cultivars of wheat, Triticum vulgare, and barley, Hordeum vulgare, indicates that the growth distribution pattern in 30- to 38-mm coleoptiles varies with the species and cultivar. In barley, there are two patterns of growth distribution among the several cultivars, whereas in wheat, all cultivars exhibit a common zonal growth pattern. The total growth of coleoptiles, initially 30 to 38 mm in length, during a 24-hour dark incubation period is the same in dark-grown coleoptiles as in those irradiated with 3 minutes of red (660 nm) light prior to the incubation period. The growth distribution pattern in the growing zone of this 30- to 38-mm coleoptile is, however, altered by red light. Growth of the apical 5-mm zone is stimulated by red light and the zonal growth 5 to 10 mm below the apex is only slightly affected, whereas growth in the zones 10 to 15 to 20, and 20 to 25 mm below the apex is inhibited. This growth distribution pattern in irradiated coleoptiles changes as the coleoptile increases in length. The response of a zone following exposure to red light is dependent upon the age of the seedlings irradiated. The over-all effect of red light on growth of the intact coleoptile varies with the length of the coleoptile. In young seedling 20 to 29 mm in length, the cells of the coleoptile can compensate for the effects of red light, with the over-all growth of the dark-grown and irradiated coleoptile about the same. As the seedling grows older, the cells of the coleoptile can no longer make up for the effects of red light, and the over-all effect changes from compensation to pronounced inhibition.     

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.     

Involvement of Cell Wall-Bound Diferulic Acid in Light-Induced Decrease in Growth Rate and Cell Wall Extensibility of Oryza Coleoptiles

Irradiation of white fluorescent light (5 W m^–2) inhibitedthe growth of Oryza coleoptiles. Light irradiation increasedstress-relaxation parameters of coleoptile cell walls, minimumstressrelaxationtime and relaxation rate, and decreased cellwall extensibility (strain/load). Under light conditions, thecontents of ferulic and diferulic acids ester-linked to thehemicellulosic arabinose residue in cell walls increased andcorrelated with the modification of the cell wall mechanicalproperties. These results suggest that light irradiation enhancesthe formation of diferulic acid bridges in hemicelluloses, makingcell walls mechanically rigid and thus inhibits cell elongationin rice coleoptiles. Also, irrespective of coleoptile age orthe presence of light, the ratio of diferulic acid to ferulicacid was almost constant, suggesting that the rate limitingstep in the formation of diferulic acid bridges in Oryza cellwalls is in the step of feruloylation. (Received September 24, 1991; Accepted December 3, 1991)