Dose-response curves for radish seedling phototropism

The Effect of Phosphate on the Activity and Sensitivity of Nutritropism toward Ammonium in Rice Roots

Understanding how plants determine growth direction from environmental cues is important to reveal optimal strategies in plant survival. Nutritropism is the directional growth of plant roots towards nutrient sources. Our previous study showed that an NH4+ gradient stimulates nutritropism in the lateral roots, but not in the main roots, of a rice cultivar. In the present study, we report nutritropism in the main roots of rice accessions among the World Rice Core Collection, including WRC 25. We investigated the effects of components in nutrient sources on nutritropism in WRC 25. Nutritropism in main roots was stimulated by NH4+ and significantly enhanced by Pi. We found that roots required more NH4+ stimulation for nutritropic responses in the presence of higher Pi, meaning that Pi desensitized root nutritropism. These results indicate that Pi acts as an activator and a desensitizer in nutritropism. Such a regulation of nutritropism would be important for plants to decide their optimum growth directions towards nutrient sources, gravity, moisture, or other stimuli.

Leaf angle in Chenopodium album is determined by two processes: induction and cessation of petiole curvature

Petiole curvature is important in regulating light interception by the leaf. To dissect the determination processes of leaf angle, we irradiated the lamina or petiole of Chenopodium album L. with either one or two spots of actinic light, after dark adaptation. When the abaxial side of the petiole was irradiated with blue light, the petiole curvature increased, and under continuous irradiation, the curvature continued for up to 6 h. The rate of curvature increased with increasing blue light intensity. The curvature induced by irradiation of the abaxial side with blue light ceased when the adaxial side of the petiole was simultaneously irradiated with either blue or red light. When an inhibitor for photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, was applied to the adaxial side of the petiole, the cessation of curvature caused by blue light was only weakly inhibited, while the cessation caused by red light was markedly inhibited. When the adaxial side of the petiole was irradiated alternately with red and far-red light, the far-red light antagonized the cessation of curvature caused by the red light. These results clearly show that the petiole curvature is controlled by two processes, the induction and the cessation of curvature. At least three photoreceptor systems, blue-light receptor, photosynthesis and phytochrome, are involved in the reactions.

Pulse-induced phototropisms in oat and maize coleoptiles

Phototropisms induced by a pulse (1-30 seconds) of blue light in red-light-grown coleoptiles of oats (Avena sativa L.) and maize (Zea mays L.) were investigated in terms of fluence-response relationships and time courses. Phototropic stimulation was made by a laser beam (457.9 nanometers), allowing application of high-fluence pulses. The phototropic fluence-response curves for oats and maize revealed two peaks in the positive range, thus indicating the occurrence of two separable pulse-induced positive responses. The response at low fluences corresponded to the ;first positive curvature.' The response at high fluences was very small in oats, but was large in maize. Reciprocity was valid in this high-fluence response (tested only for maize), indicating that it is distinct from the so-called ;second positive curvature.' In oats, the trough between the two positive responses fell into the negative range. This negative response, corresponding to the ;first negative curvature,' showed time courses distinct from those of ;first positive curvature:' the negative response was induced after a longer time lag and developed with a more gradual increase of the rate of bending. The maximal rate of the negative response was as high as one-half of that of first positive curvature. In maize, the trough between the two responses was in the positive range, and the time-course result revealed no apparent response counteracting the positive responses. Physiological and ecological implications of the pulse-induced phototropisms are discussed.

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.

Phototropism in Hypocotyls of Radish : III. Influence of Unilateral or Bilateral Illumination of Various Light Intensities on Phototropism and Distribution of cis- and trans-Raphanusanins and Raphanusamide

When etiolated radish (Raphanus sativus var. hortensis f. gigantissimus Makino) hypocotyls were subjected to a continuous unilateral illumination with white fluorescent light at 0.05, 0.1, or 1 watt per square meter, the suppression of the growth rate on the lighted side depended on the light intensity. The growth rate at the shaded side was only a little affected by the illumination at 0.05 and 0.1 watt per square meter but considerably suppressed by that at 1 watt per square meter. Upon a continuous unequal bilateral illumination, the growth rate was more strongly suppressed on the side of the higher intensity than on the side of the lower one, resulting in phototropic curvature toward the light source of the higher intensity. It was calculated from correlation analysis of light intensity and growth rate that, on an average, 6.9% of the irradiation applied to one side reached the opposite side. The amounts of cis- and trans-raphanusanins and raphanusamide in hypocotyls subjected to unilateral or unequal bilateral illumination increased much more at the side of the lighted or the higher intensity than at the opposite side. The present study demonstrates that phototropism in radish hypocotyl is correlated with and we conclude caused by a gradient of growth inhibition in the hypocotyl, depending on irradiation-induced amounts of cis- and trans-raphanusanins and raphanusamide.

Redistribution of growth during phototropism and nutation in the pea epicotyl

First positive phototropism of the third internode of intact, 5-d-old pea (Pisum sativum L.) seedlings, grown under continuous, dim red light, showed maximal response following a photon fluence of 3 μmol·m(-2) blue light. Greater or lesser fluences (with irradiation time 100 s or less) caused less bending, no response being detectable above 300 or below 0.03 μmol·m(-2). Bilateral irradiation with blue light caused no detectable inhibition of growth rate over that range of fluences. The linear nutation of the pea third internode was shown to be driven by a balanced oscillation of growth rate such that the overall growth rate was little changed during the oscillation. Phototropic stimulation changed neither the amplitude nor the period of nutation. Nutation and phototropism probably regulate growth independently. Phototropism in response to the optimal blue light fluence was caused by concomitant depressed growth on the irradiated side and stimulated growth on the shaded side of the bending internode. These results are consistent with the Cholodny-Went hypothesis which states that unilateral blue light induces a lateral redistribution of a growth regulator.

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.

Physiology of Movements in the Stems of Seedling Pisum sativum L. cv Alaska : III. Phototropism in Relation to Gravitropism, Nutation, and Growth

Phototropic response in etiolated pea (Pisum sativum L. cv Alaska) seedlings is poor. However, the curvature induced by unilateral blue light can be hastened and increased in magnitude by a previously administered red light pulse followed by several hours of darkness. Phytochrome is involved in the red light effect. Phototropic response was almost completely inhibited by removal of the apical bud and hook, but it was restored if exogenous indole-3-acetic acid was applied apically to the cut stump. Therefore, the stem contains both the phototropic photoreceptor and response mechanism. Perception of gravity and gravitropic response were also localized in the stem, but gravitropism was scarcely inhibited by decapitation. It was also observed that the kinetics and curvature pattern of gravitropism differed greatly from those of phototropism. Like phototropism, stem nutation required auxin and was promoted by red light. Unlike phototropism, photoenhanced nutational curvature required the apical hook and was propagated as a wave down the stem. Naphthylphthalamic acid inhibited, in order of decreasing effect, nutation, phototropism/gravitropism, and growth. Phototropism, gravitropism, and nutation appear to represent distinct forms of stem movement with fundamental differences in the mechanisms of curvature development.

The role of cotyledons in phototropism of de-etiolated seedlings

Simulated "phototropic" curvatures caused by differential masking of the cotyledons of de-etiolated seedlings exposed to white light are unconnected with true phototropism. In Cucumis sativus L. and Helianthus annuus L. such curvatures result from a red-light-induced inhibition coming from the exposed cotyledon. True phototropic bending in these species under long-term exposure to fairly high irradiances (as in nature) is a response to blue light. It occurs even when cotyledons are completely covered. These results show that the cotyledons do not perceive the phototropic stimulus and need not be illuminated for phototropism to occur.

Stem sensitivity and ethylene involvement in phototropism of mung bean

A system is described for the examination of phototropism in the epicotyl of a dicot seedling, mung bean (Phaseolus aureus Roxb.), under conditions approximating nature, including the use of intact, nonetiolated plants exposed to elevated, continuous, white, unilateral light. It is found that in this system perception of the phototropic stimulus by the leaves alone cannot account for the curvature, and that exposure of the stem is also necessary. The phototropic response was found to be strongly altered in nonintact plants. Hypobaric treatment indicates that ethylene may participate in phototropism, possibly by acting as an inhibitor of auxin transport.