Phototropic Response Research Articles

Wavelength dependence of suppression of potato sprout growth by light

Abstract Light is used commercially to prevent the sprouts on seed potatoes from growing so long that they are knocked off at planting. We investigated the relationship between wavelength and growth inhibition, using a very long irradiation time of 23 d with a 12‐h photoperiod. The inhibition showed a narrow peak of far‐red activity centered on 707 nm, with a shoulder in the red whose size depended on the degree of inhibition chosen as standard, because the log photon‐fluence rate/response lines were not parallel. There was also inhibitory activity in the blue (< 500 nm). In many respects the wavelength relationship resembled the action spectra for growth inhibition in dark‐grown seedlings, but in the latter the red peak is, or quickly becomes, more pronounced than the far‐red peak: this did not happen with potato sprouts. Blue light caused a positive phototropic response at similar or lower fluence rates. Greening became visible only at the highest fluence rates, between the two spectral regions inhibitory to growth. Broadband sources had much less inhibitory activity in the 650–750‐nm region.

Phototropism in Hypocotyls of Radish

The first positive phototropic curvature induced by a pulse of unilateral white irradiation (0.1 watt per square meter, 30 seconds) of etiolated and de-etiolated Sakurajima radish (Raphanus sativus var hortensis f. gigantissimus Makino) hypocotyls was analyzed in terms of differential growth and growth inhibitor contents of the hypocotyls. In both etiolated and de-etiolated hypocotyls, the growth rates at the lighted sides were suppressed whereas those at the shaded ones showed no change. De-etiolation treatment induced a larger difference between the growth rates at the lighted and shaded sides of the hypocotyls, resulting in a larger curvature of de-etiolated seedlings than of etiolated ones. The contents of growth inhibitors, cis- and trans-raphanusanins, increased in the lighted but not in the shaded halves of the hypocotyls of etiolated seedlings. In de-etiolated seedlings, the two inhibitors increased due to the de-etiolation treatment. When de-etiolated seedlings were exposed to a pulse of unilateral irradiation the level of the two inhibitors remained high along the lighted side for 1 h following the light pulse, whereas at the shaded side the contents of the inhibitors abruptly decreased upon transfer to the dark, the difference between their amounts in the lighted and shaded sides being larger than in etiolated seedlings. Another growth inhibitor, raphanusamide, did not respond to the phototropic stimulus, although its amounts increased by the de-etiolation treatment. These data suggest that cis- and trans-raphanusanins are involved in the first positive phototropic response of radish hypocotyls, and that de-etiolation magnifies the phototropic response through induction of a larger lateral gradient of the raphanusanins in the hypocotyls by the phototropic stimulus.

COMPARATIVE CACTUS ARCHITECTURE AND PAR INTERCEPTION

Because CO2 uptake by cacti can be limited by low levels of photosynthetically active radiation (PAR) and because plant form affects PAR interception, various cactus forms were studied using a computer model, field measurements, and laboratory phototropic studies. Model predictions indicated that CO2 uptake by individual stems at an equinox was greatest when the stems were vertical, but at the summer and the winter solstice CO2 uptake was greatest for stems tilted 30° away from the equator. Stem tilting depended on form and taxonomic group; four barrel cacti in Ferocactus and in Copiapoa and four cylindropuntias in Opuntia tilted toward a horizontal light beam by an average of 18°, 48°, and 52°, respectively, after growth periods of 1 to 4 yr. In contrast, three columnar species showed no significant phototropic response, perhaps because structural stability requires their massive stems to be erect. Field plants of the dense, multiple‐stemmed shrub Opuntia echinocarpa had stems which tended to radiate outward from the plant base, and, although this would not influence the total PAR intercepted, it would result in a more uniform PAR distribution and hence higher CO2 uptake. For O. echinocarpa and the even denser, mound‐forming Echinocereus engelmannii, PAR and chlorophyll decreased approximately exponentially with depth into the canopy. The canopies of O. echinocarpa and other cylindropuntias did not extend to the ground; in certain species, such truncation apparently resulted from a combination of very low PAR levels just below the lower canopy edge and the light‐dependent growth responses of individual stems. In addition, although the canopy surfaces of O. echinocarpa and O. acanthocarpa tilted toward the equator by about 30°, the canopies of other cylindropuntias tilted less or not at all; the computer model predicted that a 30° tilt would decrease interstem shading, increase daily PAR, and increase nocturnal CO2 uptake by as much as 54, 26, and 24%, respectively. Not only can the shape of cacti be affected by PAR, but also shape influences PAR interception and hence CO2 uptake.

Comparative Cactus Architecture and PAR Interception

Because CO2 uptake by cacti can be limited by low levels of photosynthetically active radiation (PAR) and because plant form affects PAR interception, various cactus forms were studied using a computer model, field measurements, and laboratory phototropic studies. Model predictions indicated that CO2 uptake by individual stems at an equinox was greatest when the stems were vertical, but at the summer and the winter solstice CO2 uptake was greatest for stems tilted 30° away from the equator. Stem tilting depended on form and taxonomic group; four barrel cacti in Ferocactus and in Copiapoa and four cylindropuntias in Opuntia tilted toward a horizontal light beam by an average of 18°, 48°, and 52°, respectively, after growth periods of 1 to 4 yr. In contrast, three columnar species showed no significant phototropic response, perhaps because structural stability requires their massive stems to be erect. Field plants of the dense, multiple-stemmed shrub Opuntia echinocarpa had stems which tended to radiate outward from the plant base, and, although this would not influence the total PAR intercepted, it would result in a more uniform PAR distribution and hence higher CO2 uptake. For O. echinocarpa and the even denser, mound-forming Echinocereus engelmannii, PAR and chlorophyll decreased approximately exponentially with depth into the canopy. The canopies of O. echinocarpa and other cylindropuntias did not extend to the ground; in certain species, such truncation apparently resulted from a combination of very low PAR levels just below the lower canopy edge and the light-dependent growth responses of individual stems. In addition, although the canopy surfaces of O. echinocarpa and O. acanthocarpa tilted toward the equator by about 30°, the canopies of other cylindropuntias tilted less or not at all; the computer model predicted that a 30° tilt would decrease interstem shading, increase daily PAR, and increase nocturnal CO2 uptake by as much as 54, 26, and 24%, respectively. Not only can the shape of cacti be affected by PAR, but also shape influences PAR interception and hence CO2 uptake.

Analysis of growth rates during phototropism: modifications by separate light‐growth responses

Abstract The changes in growth rate during phototropism in light‐grown mustard (Sinapis alba L.) seedlings have been investigated to clarify the pattern of differential growth which causes curvature. It is shown that deliberately induced phytochrome‐mediated straight‐growth responses can modify the pattern of differential phototropic growth. A comparison of techniques used to analyse phototropic responses in mustard shows that separate light‐growth responses can explain the contradictory published data. When white light is moved from above to one side, straight‐growth responses not mediated directly by the blue photo‐receptor are induced by an increase in the amount of light within the hypocotyl, and these modify the pattern of differential growth. In the absence of such separate light‐growth responses, phototropism is caused by a redistribution of growth within the plant axis.

Effect of Light on Uredospore Germination and Germ Tube Growth of Soybean Rust (Phakopsora pachyrhizi Syd.)

AbstractThe effect of light on uredospore germination and germ tube growth of Phakopsora pachyrhizi was studied. Frequency of uredospore germination was only partially reduced by high light intensity (> 1,9 * 104 mW * m−2). In uredospores unilaterally irradiated with polychromatic light germ tubes always emerged from the shadowed side. Already developed germ tubes showed a negative phototropic response. Both effects were inducible by low light intensities. Negative phototropism of germ tubes was a blue light effect. Light of 441 nm was more effective than that of 422 nm or 372 nm. Red light (> 600 nm) was ineffective, green light (513 nm) induced medium responses. In half‐side illumination studies longitudinal halves of germ tube tips and spores were irradiated under a microscope. The tips of the germ tubes bent into the illuminating beam. In half‐side illumination studies germ tubes always emerged from the illuminated spore halves. Under unilateral illumination liquid paraffin reversed this light “polarization” of spores and the negative phototropism of germ tubes. These results suggest that during unilateral illumination spores and germ tube tips act as a lens focussing the light on the wall farthest away from the light source., There, in uredospores emergence of germ tubes is stimulated and in germ tubes growth is inhibited. As a consequence, under unilateral illumination germ tubes emerge at the shadowed side of the spores and grow away from the light.

THE EFFECT OF NORFLURAZON ON PHOTOTROPIC REACTION IN Phaseolus aureus Roxb. SEEDLINGS

Abstract. The phototropic response of norflurazon‐treated mung bean seedlings has been studied to evaluate the possible role of carotenoids, carotenoid‐derived growth substances, or other factors in the perception/reaction system. Phototropism was slowed significantly in plants grown in white and blue light in the presence of norflurazon. This effect was evident in norflurazon‐bleached seedlings, as well as in those whose pigment system was not affected, due to a shorter period of herbicide action. The possible modes of norflurazon action are discussed.

Inhibition by carbon dioxide of the phototropic response of the Avena coleoptile to blue light

When intact oat coleoptiles (Avena sativa var. Harmon) were submerged in water, saturation of the water with CO2 promoted their elongation but eliminated their phototropic response to blue light. Increasing the pH of the CO2-saturated water prevented the promotion of coleoptile elongation but did not prevent the elimination of the phototropic response. In air, phototropic curvature was significantly reduced by 10% CO2 and was eliminated by 30%, without any reduction in growth. It is postulated that the increase in CO2 concentration may eliminate the phototropic curvature of the coleoptile by preventing the light-induced inhibition of growth on the illuminated side of the organ. Possible mechanisms are briefly discussed.

Enhancement of phototropic response to a range of light doses in Triticum aestivum coleoptiles in clinostat-simulated microgravity

The phototropic dose-response relationship has been determined for Triticum aestivum cv. Broom coleoptiles growing on a purpose-built clinostat apparatus providing gravity compensation by rotation about a horizontal axis at 2 rev min-1. These data are compared with data sets obtained with the clinostat axis vertical and stationary, as a 1 g control, and rotating vertically to examine clinostat effects other than gravity compensation. Triticum at 1 g follows the well-established pattern of other cereal coleoptiles with a first positive curvature at low doses, followed by an indifferent response region, and a second positive response at progressively increasing doses. However, these response regions lie at higher dose levels than reported for Avena. There is no significant difference between the responses observed with the clinostat axis vertical in the rotating and stationary modes, but gravity compensation by horizontal rotation increases the magnitude of first and second positive curvatures some threefold at 100 min after stimulation. The indifferent response is replaced by a significant curvature towards the light source, but remains apparent as a reduced curvature response at these dose levels.

Time-Lapse Photography of Phototropism in Pellia epiphylla

Growth patterns leading to positive phototropism of Pellia epiphylla sporophytes were analyzed by time-lapse photography. Curvature toward 6 W/m2 of unilateral blue light occurred over the entire length of sporophyte setae, and was detected within 10-15 min of continuous illu- mination. Growth rates increased significantly on shaded sides of sporophytes (from 0.50 to 0.96 mm/hr) and decreased on lighted sides (to 0.26 mm/hr). Current interest in the phototropic responses of bryophytes has centered largely on the role played by phytochrome in tip-growing moss filaments (Hartmann & Jenkins 1984). Little is known about phototropism in multicellular organs where, in con- trast to filaments, differences in growth rates on op- posite sides of the structures are responsible for tropic curvature (Garjeanne 1932; Banbury 1959; Newton 1985), and either phytochrome (Fredericq & De Greef 1968; Cove et al. 1978) or some as yet un- identified blue light receptor (Newton 1985) appears to participate in the response. Physiological mech- anisms involved in translation of the light stimulus into growth rate changes have not been elucidated. By analogy to higher plants, Banbury (1959) sug- gests that a growth hormone (probably auxin) is involved.

A conformational study of the topographical requirements of a phytotropin recognition site on the naphthylphthalamic acid receptor

Phytotropins are a group of chemicals which have the ability to abolish the gravitropic response in plants by inhibiting the polar movement of auxin in the plant. They have other physiological properties such as inhibiting the phototropic response of stems. They bind to the naphthylphthalamic acid receptor and may elicit their physiological responses by this means. Most phytotropins consist of a benzoic acid moiety substituted at the ortho position by a bridging group connected to a second aryl group. Conformational energy calculations were performed on a subset of phytotropins. The calculations yielded a single, low energy conformation common to each molecule and thus identified three dimensional requirements for binding to the receptor. Electrostatic potential calculations, in the vicinity of the benzoic acid moiety, identified recognition and binding requirements for this group. Similar calculations for the second aryl group indicated that some similarity exists between the electrostatic potentials of molecules which bind most tightly to the receptor. The revised binding model was assessed by consideration of a second series of molecules showing phytotropic activity. The model was consistent with the biological activities of these molecules.

Phototropism of Whole Trees: Effects of Habitat and Growth Form

The hypothesis that phototropism applies to entire trees as well as to apical meristems was tested. The hypothesis was correct but with important modifications: (1) canopy emergent species exhibit very little phototropic response; (2) streamside species exhibit significantly more response than upland species. Costs and benefits of learning are shown to provide an evolutionary explanation for these differences. Extrapolation of the cost/benefit model yields predictions about what habitats are likely to select for phototropic response.

A single positive phototropic response induced with pulsed light in hypocotyls of Arabidopsis thaliana seedlings

The fluence-response curves were measured for phototropic curvature in response to unilateral 450-nm light in hypocotyls of Arabidopsis thaliana (L.) Heynh. These show the classical "first positive" (peak curvature of 9-10°), "indifferent" and "second positive" phototropic response. Reciprocity is valid only for the "first positive" response; the fluence requirements for its induction are similar to those for induction of the "first positive" phototropic response of coleoptiles. Large angles of curvature also may be induced by multiple pulses if the individual pulses are separated by an optimum dark period of about 15 min. The curvature induced by a given fluence, whether applied in continuous irradiation or a sequence of pulses, is a linear function of the duration of continuous irradiation or the duration between first and last pulse, respectively. For a given fluence applied in a sequence of pulses, reciprocity remains valid provided the duration between first and last exposure is kept constant. When the duration between first and last pulse is sufficiently long, the fluence required for high phototropic curvature falls in the "first positive" fluence range. These results are interpreted to indicate the existence of a kinetic limitation in the transduction sequence, and a relatively short lifetime of an initial physiologically active photoproduct. The apparent existence of more than one positive response may have resulted from these characteristics of the transduction sequence.

The Phototropic Responses ofAvenaColeoptiles

Macleod, K., Firn, R. D. and Digby, J. 1986. The phototropic responses of Avena coleoptiles.—J. exp. Bot. 37: 542–548. A number of studies of the elongation rate changes causing phototropism have been made but the findings of different groups have not been entirely Consistent. Studies of oat coleoptile phototropism in response to first-positive and second-positive doses indicate that no single pattern of elongation rate changes causes phototropism even in a single species. The relative effect of phototropic stimulation on the elongation rate at the shaded or the illuminated side of coleoptiles subject to unilateral illumination depends on physiological state of the cell-for instance its position in the elongation zone or whether it has been given red light recently. Models of phototropism will have to account for such a diversity of phototropic responses. The importance of making full elongation rate measurements has once again been demonstrated.

Studies on the growth and development in Spirogyra

The phototropic response of Spirogyra sp. filaments and its relation with the different phases of their diurnal movements were studied. The filaments rapidly responded to unilateral irradiation; curvature towards the light source began in their tip region but shifted down to more basal regions. However, this typical and steady phototropic curvature could be observed only in the GnSt phase of the diurnal movement. In the other phases competitive states between the phototropic movement and other kinds of movement were evident. Thus, from the results of our previous and present studies it is proposed that the diurnal movement of Spirogyra filaments is composed of various individual movements, including a phototropic one; among these movements there exists a certain balance determined by the culture conditions and the time of day, and the phototropic movement is relatively inferior to the other movements.

PHOTOTROPISM AND AXIS EXTENSION IN LIGHT‐GROWN MUSTARD (Sinapis alba L.) SEEDLINGS‡

Abstract— The relationship between phototropism and axis extension was examined in light‐grown mustard (Sinapis alba L.) seedlings using the low pressure sodium lamp (SOX)† technique to eliminate growth responses due to phytochrome. Addition of blue light caused no net inhibition of hypocotyl elongation, but plants showed a phototropic response. Curvature was caused by a simultaneous inhibition of growth on the illuminated side of the hypocotyl and an acceleration on the shaded side. Phototropism thus occurs independently of axis elongation and suggests that they are two separate processes. The results are inconsistent with the Blaauw theory of phototropism.

DIURNAL MODULATION OF PHOTOTROPIC RESPONSE BY TEMPERATURE AND LIGHT IN Chenopodium rubrum L. AS RELATED TO STEM EXTENSION RATE ANDARGININE DECARBOXYLASE ACTIVITY*

Abstract— Seedlings of Chenopodium rubrum were grown under 12: 12 h light‐dark cycles with the light period at 32.5°C and darkness at 10°C (“normal” conditions) or with light at 10°C and darkness at 32.5CC (“inverse” conditions). When grown under unilateral white light with the photo‐thermoperiodic conditions described above, inverse seedlings did not show phototropic curvature while normal seedlings did. Regardless of temperature regime used, plants irradiated with unilateral blue light (456 nm) at 10°C failed to curve on subsequent incubation at 32.5°C. If the phototropic stimulus was given at 32.5°C followed again by 32.5°C in darkness, phototropic curvature was observed. The results with monochromatic blue light thus explain the lack of phototropic response of “inverse” seedlings with the unilateral light period given at 10°C. “Inverse “seedlings showed a sharp rhythm in their capacity to respond to phototropic stimulation at 32.5°C. The greatest capacity to respond came when the stem extension rate was actually zero. The differential growth accompanying phototropic curvature thus seems to be different from the growth responsible for uniform stem elongation. The curvature may result from differential turgor changes as in the case of sun tracking of leaves. “Inverse seedlings” showed clear signs of stress and displayed a rhythm in arginine decarboxylase, an enzyme thought to be stress‐related, with maxima during the low temperature light periods. It is concluded that the phototropic perception transduction chain is modified by the inverse conditions somewhere before the final growth‐turgor steps, since they exhibit geotropic sensitivity.

Control of growth by auxin and its specific neutral inhibitor

Evidence for the existence of a neutral inhibitor specific for auxin has been in the literature for 45 years. The inhibitor is demonstrable by its effect in causing positive curvature of theAvena coleoptile. The growth of the mesocotyl of oat and corn seedlings in darkness and its inhibition by light are determined by the neutral inhibitor as is the phototropic response of the sunflower stem. Production of the inhibitor is promoted by red and fluorescent light. Irradiance at 730 nm promotes auxin production while irradiance at 660 nm promotes production of the inhibitor. The positive curvature induced by 2,3,5-triiodobenzoic acid can be used to quantify the neutral inhibitor. Since the benzoic acid is more effective than iodoacetate in reacting with the sulfhydryl of cysteine, a sulfhydryl group is indicated to bo one reaction site for auxin and to be the basis of polar transport.

PHOTOTROPISM OF SELAGINELLA: THE DIFFERENTIAL RESPONSE TO LIGHT

Selaginella kraussiana (Kunze) A. Braun has a plagiotropic dorsiventral stem with two rows of small leaves on the dorsal surface and two rows of large leaves inserted laterally. Stem tips exhibit a differential phototropic response. When stem tips are placed in their normal horizontal orientation and the dorsal surfaces are illumianted, the tips bend only 20” below the horizon and away from light. Stem curvature is limited to a zone 450 μm long located 1.5 mm behind the shoot apex. The dorsal cortical cells within this zone of curvature are about 1.44 times longer than the ventral cortical cells. Illumination of the ventral surface of the stem tips elicits a strong phototropic response. The stems bend from 123–158° below the horizon and toward light, and the zone of curvature increases in length to 10 mm of the explant. The curvature is large enough so that the previously shaded dorsal leaves of the stem tips become redirected toward the light. This phototropic response is promoted by white and blue light, whereas red or far‐red light has no effect. When stem tips are cultured in total darkness, the length of the zone of curvature is 8.0 mm but the stems bend only 50–67°. Treatment of the small dorsal leaves with phenylacetic acid inhibits phototropic curvature, and the phototropic response is unaffected by gravity.

Phototropism of Selaginella: The Differential Response to Light

Selaginella kraussiana (Kunze) A. Braun has a plagiotropic dorsiventral stem with two rows of small leaves on the dorsal surface and two rows of large leaves inserted laterally. Stem tips exhibit a differential phototropic response. When stem tips are placed in their normal horizontal orientation and the dorsal surfaces are illumianted, the tips bend only 20” below the horizon and away from light. Stem curvature is limited to a zone 450 μm long located 1.5 mm behind the shoot apex. The dorsal cortical cells within this zone of curvature are about 1.44 times longer than the ventral cortical cells. Illumination of the ventral surface of the stem tips elicits a strong phototropic response. The stems bend from 123–158° below the horizon and toward light, and the zone of curvature increases in length to 10 mm of the explant. The curvature is large enough so that the previously shaded dorsal leaves of the stem tips become redirected toward the light. This phototropic response is promoted by white and blue light, whereas red or far-red light has no effect. When stem tips are cultured in total darkness, the length of the zone of curvature is 8.0 mm but the stems bend only 50–67°. Treatment of the small dorsal leaves with phenylacetic acid inhibits phototropic curvature, and the phototropic response is unaffected by gravity.