Phototropism of rice (Oryza sativa L.) coleoptiles: fluence-response relationships, kinetics and photogravitropic equilibrium

Phototropism of rice (Oryza sativa L.) coleoptiles induced by unilateral blue light was characterized using red-light-grown seedlings. Phototropic fluence-response relationships, investigated mainly with submerged coleoptiles, revealed three response types previously identified in oat and maize coleoptiles: two pulse-induced positive phototropisms and a phototropism that depended on stimulation time. The effective ranges of fluences and fluence rates were comparable to those reported for maize. Compared with oats and maize, however, curvature responses in rice were much smaller and coleoptiles straightened faster after establishing the maximal curvature. When stimulated continuously, submerged coleoptiles developed curvature slowly over a period of 6 h, whereas air-grown coleoptiles, which showed smaller phototropic responsiveness, established a photogravitropic equilibrium from about 4 h of stimulation. The plot of the equilibrium angle against log fluence rates yielded a bell-shaped optimum curve that spanned over a relatively wide fluence-rate range; a maximal curvature of 25° occurred at a fluence rate of 1 μmol · m⁻² · s⁻¹. This optimum curve apparently reflects the light sensitivity of the steady-state phototropic response. Received: 28 June 1996 / Accepted: 30 July 1996     

Desensitization by red and blue light of phototropism in maize coleoptiles

The effects of pretreatments with red and blue light (RL, BL) on the fluence-response curve for the phototropism induced by a BL pulse (first positive curvature) were investigated with darkadapted maize (Zea mays L.) coleoptiles. A pulse of RL, giving a fluence sufficient to saturate phytochrome-mediated responses in this material, shifted the bell-shaped phototropic fluence-response curve to higher fluences and increased its peak height. A pulse of high-fluence BL given immediately prior to this RL treatment temporarily suppressed the phototropic fluence-response curve, and shifted the curve to higher fluences than induced by RL alone. The shift by BL progressed rapidly compared to that by RL. The results indicate (1) that first positive curvature is desensitized by both phytochrome and a BL system, (2) that desensitization by BL occurs with respect to both the maximal response and the quantum efficiency, and (3) that the desensitization responses mediated by phytochrome and the BL system can be induced simultaneously but develop following different kinetics. It is suggested that theses desensitization responses contribute to the induction of second positive curvature, a response induced by prolonged irradiation.Abbreviations BL blue light - RL red light CIW-DPB Publication No. 1001     

Phytochrome-mediated phototropism in maize seedling shoots

Unilateral irradiation with red light (R) or blue light (BL) elicits positive curvature of the mesocotyl of maize (Zea mays L.) seedlings raised under R for 2 d from sowing and kept in the dark for 1 d prior to curvature induction. The fluenceresponse curve for R-induced mesocotyl curvature, obtained by measuring curvature 100 min after phototropic induction, shows peaks in two fluence ranges, designated first positive range (from the threshold to the trough), and second positive range (above the trough). The fluence-response curve for BL is similar to that for R but shifted two orders of magnitude to higher fluences. Blue light elicits the classical first positive curvature of the coleoptile, whereas this response is not found with R. Positive mesocotyl curvature induced by either R or BL is eliminated by R given from above just before the unilateral irradiation, whereas BL-induced coleoptile curvature is not eliminated. The above results collectively offer evidence that phototropic curvature of the mesocotyl is induced by R-sensitive photosystem(s). Mesocotyl curvature in the second positive range is reduced by vertical far-red light (FR) applied after phototropic induction with R, but is not affected by FR applied before R. Unilateral irradiation with FR following vertical irradiation with a high R fluence leads to negative curvature of the mesocotyl. It is concluded that mesocotyl curvature in the second positive range results from a gradient in the amount of the FR-absorbing form of phytochrome (Pfr) established across the plant axis. Mesocotyl curvature in the first positive range is inhibited by vertical FR given either before or after phototropic induction with R. Since the FR used here is likely to produce more Pfr than the very low fluences of R eliciting the mesocotyl curvature in the first positive range, it is assumed that FR reduces the response in this case by adding Pfr at both sides of the plant axis. By rotating seedlings on a clinostat with its axis horizontal, the kinetics of mesocotyl curvature can be studied in the absence of a counteracting gravitropic response. On the clinostat, the R-induced mesocotyl curvature develops after a lag, through two successive phases having different curvature rates, the late phase is slower than the early phase. Negative curvature of the coleoptile can be induced by either R or BL; the BL-induced negative curvature is found at fluences higher than those giving positive curvature. The clinostat experiments show that the negative coleoptile curvature induced by either R or BL is a gravitropic compensation for positive mesocotyl curvature.Abbreviations BL blue light - FR far-red light - Pfr phytochrome in the far-red-absorbing form - Pr phytochrome in the red-absorbing form - R red light C.I.W.-D.P.B. Publication No. 824     

Effects of red light on the fluence–response relationship for pulse-induced phototropism of maize coleoptiles

Dark-adapted coleoptiles of maize (Zea mays L.) were treated with red light (3min at 10.5 μmol m^?2S^?1) and were Stimulated, after a dark interval, with a pulse of unilateral blue light to induce phototropism. Phototropic fluence-response curves were obtained in this way for different dark intervals. It was confirmed that the bell-shaped fluence-response curve for the first pulse-induced positive phototropism (FPIPP) shifts to higher fluences following the red-light treatment, the maximal shift being achieved at a dark interval of 2h. We found, however, that the two arms of the Fluence-response curve do not shift synchronously. The shift of the descending arm to higher fluences began at 15 min. The ascending arm showed a slight shift to lower fluences before a greater shift to higher flucnces. the change of the shift direction occurring at 30–40min. Accordingly, the fluence-response curve obtained for a 30 min dark interval was comparatively wide. Although dark-adapted coleoptiles showed only fPIPP, another bell-shaped fluence-response curve, representing the second pulse-induced positive phototropism (sPIPP), appeared gradually after the red-light treatment. These changes of the phototropic fluence– respnse curve following exposure to red light are likely to have adaptive values because they favour phototropism under brighter light.     

Multiple phytochromes are involved in red-light-induced enhancement of first-positive phototropism in Arabidopsis thaliana.

The amplitude of phototropic curvature to blue light is enhanced by a prior exposure of seedlings to red light. This enhancement is mediated by phytochrome. Fluence-response relationships have been constructed for red-light-induced enhancement in the phytochrome A (phyA) null mutant, the phytochrome B- (phyB) deficient mutant, and in two transgenic lines of Rabidopsis thaliana that overexpress either phyA or phyB. These fluence-response relationships demonstrate the existence of two response in enhancement, a response in the very-low-to-low-fluence range, and a response in the high-fluence range. Only the response in the high-fluence range is present in the phyA null mutant. In contrast, the phyB-deficient mutant is indistinguishable from the wild-type parent in red-light responsiveness. These data indiacate that phyA is necessary for the very-low-to-low but not the high-influence response, and that phyB is not necessary for either response range. Based on these results, the high-fluence response, if controlled by a single phytochrome, must be controlled by aphytochorme other than phyA of phyB. Overexpression of phyA has a negative effect and overexpression of phyB has an enhancing effect in the high-fluence range. These results suggest that overexpression of either phytochrome perturbs the function of the endogenous photoreceptor system in an unpredictable fashion.     

Kinetic modelling of phototropism in maize coleoptiles

Blue-light-induced phototropism of maize (Zea mays L.) coleoptiles was studied with a view to kinetic models. Red-light-grown plants were used to eliminate complication arising from the activation by blue light of phytochrome-mediated phototropism. In the first part, mathematical models were developed to explain the phototropic fluence-response data, which were obtained for the responses induced by a single unilateral pulse (30 s) and those induced by a unilateral pulse (30 s) given immediately after a bilateral pulse (30 s, fixed fluences). These data showed bell-shaped fluence-response curves, characteristic of first positive curvature. Modelling began with the assumptions that the light gradient plays a fundamental role in phototropism and that the magnitude of the response is determined by the gradient, or the concentration difference, in a photoproduct between the irradiated and the shaded sides of the tissue. Minimal mathematical models were then derived, by defining chemical kinetics of the photoreaction and introducing the minimum of parameters needed to correlate the incident fluencerate to the functional fluence-rates within the tissue, the functional fluence-rate to the rate constant of the photoreaction, and the photoproduct concentration difference to the curvature response. The models were tested using a curve-fitting computer program. The model obtained by assigning first-order kinetics to the photoreaction failed to explain the fluence-response data, whereas application of second-order kinetics led to a successful fit of the model to the data. In the second part, temporal aspects of the photosystem were examined. Experimental results showed that a high-fluence bilateral pulse eliminated the bell-shaped fluence-response curve for an immediate unilateral pulse, and that the curve gradually reappeared as the time for unilateral stimulation elapsed after the bilateral pulse. The model based on a second-order photoreaction could be extended to explain the results, with assumed changes in two components: the concentration of the reactant for the photoproduct, and the light-sensitivity of the reaction. The reactant concentration, computed with the curvefitting program, showed a gradual increase from zero to a saturation level. This increase was then modelled in terms of regeneration of the reactant from the photoproduct, with an estimated first-order rate constant of about 0.001·s^-1. The computed value for the constant reflecting the light-sensitivity showed a sharp decline after the high-fluence pulse, followed by a gradual return to the initial level. From these analytical results, the appearance of second positive curvature was predicted.Abbreviations FPC first positive curvature - SPC second positive curvature CIW-DPB publication No. 884     

植物光敏素在玉米胚芽鞘不同类型向光性反应中的作用模式

先前的研究考察了红光对玉米（Zea mays L.cv.Royaldent Hit85)胚芽鞘向光性反应的影响,本研究进一步分析了红光对蓝光照射时间依赖型向光性（TDP）影响的光具－反应曲线,发现该反应不像红光对脉冲蓝光诱导的向光性的影响那样属于超低光量反应,而是一种低光量反应,且之后的脉冲远红光可以逆转红光对TDP影响的效果。但远红光预处理延长后,逆转了的TDP反应性可以得到恢复。不仅如此,暗适应胚芽鞘接受不同时间的单独远红光预处理后可同样获得与预处理光量成比例的TDP反应性,表明暗适应胚芽鞘在接受远红光顶处理后亦可建立起长时间向光性蓝光照射的反应能力。远红光对TDP反应性影响的光量－反应曲线分析揭示,该远红光影响依赖于照射时间并要求高光量。鉴于高光量范围内上述远红影响不符合所谓反比定律,远红光对TDP反应性的影响很可能属一高辐照反应,根据上述研究发现探讨了植物光敏素作用模式与不同光性反应信号转导途径之间可能存在的相互关系。     

Modes of Action of Phytochromes in Establishing DifferentPhototropic Responsiveness of Maize Coleoptiles

Previous studies have examined the effects of red light (R) on phototropism of maize ( Zea mays L. cv. Royaldent Hit 85) coleoptiles. The R effect on time-dependent phototropism (TDP) was further studied by characterizing its fluence-response relationship. The results showed the R effect was a low-fluence-response, unlike those on pulse-induced phototropisms that show a very-low-fluence-response mode. A subsequent pulse of far-red light (FR) could reverse the R effect. TDP responsiveness, however, recovered as the following FR was extended. The FR-dependent increase in TDP responsiveness was obtained even coleoptiles were pretreated only with FR. It suggested that TDP responsiveness could also be established in response to a FR signal. The fluence-response relationship for the effect of FR was then investigated. The effect of FR depended on the time of irradiation and required high photon fluences. Because reciprocity was invalid at the higher fluence range, the effect of FR would be a high-irradiance-response mode. Relation between phytochrome action modes and possible multiple pathways for phototropic signal transduction was analyzed based on the experiment results.     

Phytochrome Regulation of Greening in Pisum: Chlorophyll Accumulation and Abundance of mRNA for the Light-Harvesting Chlorophyll a/b Binding Proteins

A brief pulse of red light eliminates or reduces the lag in chlorophyll accumulation that occurs when dark-grown pea seedlings are transferred to continuous white light. The red light pulse also induces the accumulation of specific mRNAs. We compared time courses, escape from reversal by far-red light, and fluence-response behavior for induction of mRNA for the light-harvesting chlorophyll a/b binding proteins (Cab mRNA) with those for induction of rapid chlorophyll accumulation in seedlings of Pisum sativum cv Alaska. In both cases the time courses of low fluence and very low fluence responses diverged from each other in a similar fashion: the low fluence responses continued to increase for at least 24 hours, while the very low fluence responses reached saturation by 8 to 16 hours. Both responses escaped from reversibility by far-red slowly, approaching the red control level after 16 hours. The fluence-response curve for the Cab mRNA increase, on the other hand, showed threshold and saturation at fluences 10-fold lower than threshold and saturation values for the greening response. Therefore, the level of Cab mRNA, as measured by the presence of sequences hybridizing to a cDNA probe, does not limit the rate of chlorophyll accumulation after transfer of pea seedlings to white light. The Cab mRNA level in the buds of seedlings grown under continuous red light remained high even when the red fluence rate was too low to allow significant greening. In this case also, abundance of Cab mRNA cannot be what limits chlorophyll accumulation.     

Biphasic fluence-response curves for phytochrome-mediated kalanchoë seed germination : sensitization by gibberellic Acid

The fluence-response curves for the effect of two red pulses separated by 24 hours on the germination of Kalanchoe blossfeldiana Poelln. cv Vesuv seeds, incubated on gibberellic acid (GA₃) are biphasic for suboptimal concentrations. The response in the low fluence range corresponds with a classical red/far-red reversible phytochrome mediated reaction. GA₃ induces an additional response in the very low fluence range, which is also phytochrome mediated. The sensitivity to phytochrome-far-red absorbing form (Pfr), however, is increased about 20,000-fold, so that even far-red fluences become saturating. Both in the very low and low fluence response range, the maximal responses induced by saturating fluences are modulated by the GA₃ concentration. GA₃ having no direct influence on the phytochrome phototransformations, alters the Pfr requirement and determines the responding seed population fraction in the very low and low fluence range. The effet of GA₃ appears to be on the transduction chain of the phytochrome signal.     

RED LIGHT,AUXIN RELATIONSHIPS,AND THE PHOTOTROPIC RESPONSES OF CORN AND OAT COLEOPTILES

Briggs , Winslow R. (Stanford U., Stanford, Calif.) Red light, auxin relationships, and the phototropic responses of corn and oat coleoptiles. Amer. Jour. Bot. 50(2): 196–207. Illus. 1963.— Red light decreases the phototropic sensitivity of corn (Zea mays Burpee ‘Golden Cross Bantam’) and oat (Avena saliva ‘Victory’) coleoptiles. The decrease is reflected by a shift of the curve ploting log dosage vs. response to higher dosages, as described in the literature. In the absence of red light treatment, 1,000 meter-candle-seconds (mcs) white light induces first negative curvature in oats and almost no curvature in corn, which appears to lack the mechanism for first negative curvature. Immediately following a 2-hr red light treatment, the same white light dosage induces almost maximum first positive curvature both in corn and in oat coleoptiles. The increase in curvature obtained reflects the decreased phototropic sensitivity of both plants shown by the dosage-response curve shift. After red treatment, the effect of red light remains maximal for an hour, decaying to the level of non-red-treated plants within another 2 hr. Red light suppresses auxin production by corn coleoptiles. The effect decays after the end of red treatment. Both changes follow time courses parallel to those for the phototropic sensitivity changes. The 1,000 mcs light dosage induces lateral transport of auxin both in red-treated and untreated corn coleoptiles, despite the lack of curvature of the latter. Red light does not induce a circadian rhythm for the phototropic sensitivity changes in oats, is not effective if administered after phototropic induction, and its effect is probably mediated by phytochrome. The hypothesis, not original with this paper, that red light induces an increase in the amount of pigment mediating second positive curvature most closely accounts for the results obtained. Pertinent literature is discussed.     

Blue-light-induced shift of the phototropic fluence-response curve in Pilobolus sporangiophores

The effects of preirradiation with blue light on the shift of the fluence-response curve for the first and the second positive curvatures were examined in Pilobolus crystallinus (Wiggers) Tode sporangiophores. A 1-min preirradiation with blue light at 47 or 960 nmol·m^-2 lowered the fluence-response curve for the first positive curvature and shifted the peak to a higher fluence. The fluence-response curve was shifted back to a lower fluence when a dark period was inserted between the preirradiation and the curvature-inducing light. This shift back to lower fluence was biphasic when the fluence was high (960 nmol · m^-2), indicating the participation of two components in the phototropic reaction for the first positive curvature.The fluence-response curve for the second positive curvature did not seem to be shifted to a higher fluence region when fluence was varied by varying exposure time. However, the fluence-response curve obtained by varying the fluence rate of a 20-min irradiation period indicated that the second positive curvature was also shifted to a higher-fluence region by preirradiation with blue light. A small shoulder appeared on the fluence-response curve when preirradiation at a high fluence rate was given.Abbreviations BL blue light - CIL curvature-inducing light     

Evidence for a phytochrome-mediated phototropism in etiolated pea seedlings

Entirely etiolated pea seedlings (Pisum sativum, L. cv Alaska) were tested for a phototropic response to short pulses of unilateral blue light. They responded with small curvatures resembling in fluence-dependence and kinetics of development a phytochrome-mediated phototropic response previously described in maize mesocotyls. Irradiations from above with saturating red or far-red light, either immediately before or after the unilateral phototropic stimulus, strongly reduced or eliminated subsequent positive phototropic curvature. Only blue light from above, however, entirely eliminated curvature at all fluences of stimulus. It is concluded that the phototropism is primarily a result of phytochrome action.     

Interaction of gravi- and phototropic stimulation in the response of maize (Zea mays L.) coleoptiles

The influence of gravitropic stimulation upon blue-light-induced first positive phototropism for stimulations in the same (light source and center of gravity opposite to each other) and in opposing directions was investigated in maize cole-optiles by measuring fluence-response patterns. As a result of gravitropic counterstimulation, phototropic bending was transient with maximum curvature occurring 100 min after stimulation. On a horizontal clinostat, however, the seedlings curved for 20 h. Gravistimulation in the opposite direction acted additively upon blue-light curvature. Gravistimulation in the same direction as phototropic stimulation produced a complex behaviour deviating from simple additivity. This pattern can be explained by a gravitropically mediated sensitization of the phototropic reaction, an optimal dependence of differential growth on the sum of photo-and gravistimulation, and blue-light-induced inhibition of gravitropic curvature at high fluences. These findings indicate that several steps of photo-and gravitransduction are separate. Preirradiation with red light desensitized the system independently of applied gravity-treatment, indicating that the site of red-light interaction is common to both transduction chains.Abbreviations BL blue light - G⁺ stimulation by light and gravity in the same direction (i.e. light source and center of gravity opposite to each other) - G^- stimulation by light and gravity in opposing directions     

Asymmetric, blue light-dependent phosphorylation of a 116-kilodalton plasma membrane protein can be correlated with the first- and second-positive phototropic curvature of oat coleoptiles

The possible correlation between blue light-dependent phosphorylation of a 116-kD protein and phototropic responses of etiolated oat (Avena sativa L.) seedlings was tested by a micromethod for protein phosphorylation. Quantitation of the basipetal distribution of this protein showed that the in vitro 32p phosphorylation values declined exponentially from tip to node, with more than 50% of the total label being found in the uppermost 5 mm. Nonsaturating preirradiation of the coleoptiles in vivo resulted in partial phosphorylation with endogenous ATP. Subsequent in vitro phosphorylation under saturating irradiation allowed the determination of the degree of in vivo phosphorylation. Unilateral preirradiation resulted in higher in vivo phosphorylation on the irradiated than on the shaded side of the coleoptile. The fluence-response curve for the difference in phosphorylation between both sides of the coleoptile resembles the fluence-response curve for first-positive phototropic curvature, although it is shifted by two orders of magnitude to higher fluences. Possible reasons for this shift are discussed. In the coleoptile base the phosphorylation gradient across the coleoptile becomes larger with increasing time of irradiation at a constant fluence. Thus, phosphorylation of the 116-kD protein, in accordance with second-positive phototropic curvature, does not obey the Bunsen-Roscoe reciprocity law.     

Inversion of gravitropism by symmetric blue light on the clinostat

Gravitropic stimulation of maize (Zea mays L.) seedlings resulted in a continuous curvature of the coleoptiles in a direction opposing the vector of gravity when the seedlings were rotated on a horizontal clinostat. The orientation of this response, however, was reversed when the gravitropic stimulation was preceeded by symmetric preirradiation with blue light (12.7 mol photons·m^–2). The fluence-response curve of this blue light exhibited a lower threshold at 0.5 mol·m^–2, and could be separated into two parts: fluences exceeding 5 mol·m^–2 reversed the direction of the gravitropic response, whereas for a range between the threshold and 4 mol·m^–2 a split population was obtained. In all cases a very strong curvature resulted either in the direction of gravity or in the opposite orientation. A minor fraction of seedlings, however, curved towards the caryopsis. Furthermore, the capacity of blue light to reverse the direction of the gravitropic response disappeared with the duration of gravitropic stimulation and it depended on the delay time between both stimulations. Thistonic blue-light influence appears to be transient, which is in contrast to the stability observed fortropistic blue-light effects.This work was supported by the Deutsche Forschungsgemeinschaft.     

Phototropism of maize coleoptiles Influences of light gradients

The lateral fluence-rate gradients in unilaterally irradiated maize (Zea mays L.) coleoptiles were calculated on the basis of the proportions of P _fr (far-red-absorbing form of phytochrome) measured spectroscopically in transverse slices of the coleoptiles (top 1 cm). The results showed the occurrence of significant gradients that are wavelength-dependent. The gradient at 449 nm was steeper than those measured at 516, 534 and 551 nm, which were steeper than that measured at 665 nm. The ratios between the sides proximal and distal to the light source were, for example, 1:0.12 (449 nm), 1:0.23 (534 nm), and 1:0.28 (665 nm). Fluence-response curves for coleoptile phototropism (first positive curvature produced by less than 100 s unilateral irradiation) were measured at 449, 516, 534 and 551 nm. Comparison of the threshold fluences indicated that the responsiveness to 551 nm is about 10^4.8 less than that to 449 nm. Increasing wavelengths led to a decrease in maximal curvature, which correlated with the decrease of the fluence-rate ratios between the proximal and distal sides. Phototropic fluence-response curves were also measured using bilateral irradiation (449 nm). In one set of experiments, the fluence ratio was kept constant (either 1:1/2, 1:1/4 or 1:1/16) and the total fluence was varied, and in the other set the fluence applied to one side was kept constant and the fluence ratio was varied. A simple model based on the assumption that only one photoreaction occurs, and that the response is a function of the difference between the proximal and distal sides in the local photoreceptor action was tested. A fluence-response curve for this local photoreceptor action was calculated based on the fluence-rate ratio and the phototropic fluence-response curve measured for 449 nm. This curve was used, in conjunction with the measured fluence-rate ratios, as a basis for calculating phototropic fluence-response curves for other wavelengths and those for 449 nm obtained with bilateral irradiation. The calculated fluence-response curves showed excellent agreement with the experimental data. It is concluded that the threshold for maize coleoptile phototropism reflects the apparent photoconversion cross-section of the blue-light receptor whereas the maximal curvature depends on the steepness of the light gradient across the coleoptile.Abbreviations and symbols I(x) fluence rate at the depth x - P _fr phytochrome (far-red absorbing) - P _r phytochrome (red absorbing) - P _tot total phytochrome (P _r+P _fr) - photoconversion cross-section     

Comparison of fluence-response relationships of phototropism in light- and dark-grown buckwheat

Fluence-response relationships of phototropism in light- and dark-grown buckwheat (Fagopyrum esculentum Moench.) were compared using systematically varied fluence rates and irradiation times of unilateral monochromatic blue light. Etiolated seedlings respond to most fluence rates in a tri-phasic manner. Phase one differs from classic first positive in that reciprocity is not observed and the peak occurs at a wide variety of fluences, often orders of magnitude less than those characteristic of first positive. Light-grown plants display this pattern only when stimulated by low fluence rates. Phase three is an ascending arm directly related to irradiance time and is comparable to classic second positive. Phase two is a nearly indifferent zone separating phases one and three. At the lowest fluence rates, the maximal observed curvature is greater for dark-grown than for light-grown plants and the former curve more in response to short (2-second) exposures than do the latter. At the highest fluence rates, the maximal observed curvature is much greater for light-grown than for dark-grown seedlings, particularly at irradiation times of 2 to 3 minutes or more. Tropic curvatures correlate positively with increasing fluence rate up to some inflection range, above which the relationship becomes negative. This inflection range is approximately two orders of magnitude higher for light-grown plants.     

Two distinct blue-light responses regulate epicotyl elongation in pea

Blue light induces a long-term suppression of epicotyl elongation in red-light-grown pea (Pisum sativum L.) seedlings. The fluence-response characteristics are bell-shaped, indicating the possibility of two different blue-light responses: a lower fluence response causing suppression and a higher fluence response alleviating the suppression. To determine if two responses are in effect, we have grown pea seedlings under dark conditions hoping to eliminate one or the other response. Under these growth conditions, only the lower fluence portion of the response (suppression of elongation) is apparent. The kinetics of suppression are similar to those observed for the lower fluence response of red-light-grown seedlings. The response to blue light in the dark-grown seedlings is not due to the excitation of phytochrome because a pulse of far-red light large enough to negate phytochrome-induced suppression has no effect on the blue-light-induced suppression. Furthermore, treatment of the dark-grown seedlings with red light immediately prior to treatment with high fluence blue light does not elicit the higher fluence response, indicating that the role of red light in the blue high fluence response is to allow the plant to achieve a specific developmental state in which it is competent to respond to the higher fluences of blue light.     

Phototropism: mechanisms and ecological implications

Abstract. Phototropism in seed plants, either etiolated or de-etiolated, is mediated by unidentified photoreceptor(s) sensitive to blue and near-UV regions of the light spectrum. Green plants may have an additional phototropic system sensitive to red light. Fluence-response studies of the blue light-sensitive phototropism, initially made on oat coleoptiles, have indicated the occurrence of multiple response types. Of those, two are found to be general: the first pulse-induced positive phototropism (fPIPP), or the so-called first positive curvature, and the time-dependent phototropism (TDP) or the second positive curvature. The fPIPP, elicited by a pulse stimulus shorter than a few minutes, is characterized by a bell-shaped fluence-response curve and the validity of reciprocity. The TDP, elicited by prolonged irradiation, is characterized by its dependence on the exposure time and the invalidity of reciprocity. Studies made on these two response types have revealed the following: (1) plants acquire directional light information for phototropism by sensing internal light gradients created by light scattering and absorption; (2) phototropism results from redistribution of growth, i.e. inhibition on the irradiated side and compensating stimulation on the shaded side; (3) lateral movement of growth regulators, the principle of the Cholodny-Went theory, can account for the growth redistribution, and auxin is clearly the mediating regulator in maize coleoptiles. This review further describes some mechanistic implications of fPIPP. Experimental results indicate that (1) fPIPP is mediated by a single step of photoreaction, (2) the responsiveness, reflected in the height of the fluenceresponse curve, is reduced by pre-irradiation with blue light and recovers gradually afterward, and (3) the light sensitivity, reflected in the position of the fluence-response curve along the log fluence axis, is also reduced by the pre-irradiation and recovers gradually. Analyses of these results, based on kinetic models, suggest that the bell-shaped fluence-response curve is caused by the difference in the amounts of a photoproduct between irradiated and shaded sides, and that fPIPP represents a mechanism of TDP. It is also indicated that phytochrome in the red-absorbing form exerts two separate effects on phototropism: reduction of the light sensitivity and enhancement of the responsiveness. Along with the discussion of the mechanisms of phototropism, their ecological implications are considered.