Unilateral Blue Light Research Articles

Turgor Pressure and Phototropism inSinapis albaL. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropism in Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls. Phototropism was induced by adding 0.27 μmol m−2 s−1 unilateral blue light to a background of low pressure sodium (SOX) lamp light. Curvatures of some 6° from the vertical were reached by 60 min, the curvature rate between 20 min and 60 min being 0.14° min−1. From the axial growth rate and tissue geometry the local growth rates of illuminated and shaded sides of the hypocotyl were calculated to be 1.5 and 4.5 μmin−1 respectively. Turgor pressures of expanding cells in control plants and in the shaded and illuminated sides of the blue light illuminated hypocotyls were measured to be 0.40-0.55 MPa with a pressure probe. No changes in turgor pressure were observed on initiation of curvature. The decay of pressure in the cells of non-transpiring plants following excision indicated that the yield stress threshold of the tissue may be as low as 0.1 MPa. These results indicate that the phototropic growth response in this tissue is not mediated by changes in turgor pressure.

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.

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.

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.

Kinetic separation of phototropism from blue-light inhibition of stem elongation.

These experiments tested the hypothesis that phototropic bending arises when a light gradient across the stem differentially inhibits cell elongation because of direct inhibition of cell elongation by light (the Blaauw hypothesis). Continuous irradiation of dark-grown cucumber seedlings (Cucumis sativus L.) with unilateral blue light inhibited hypocotyl elongation within 30 s, but did not induce phototropic curvature until 4.5 h after the start of irradiation. Marking experiments showed that curvature began simultaneously at the top and bottom of the growing region. In situ measurements of the light gradient across the stem with a glass fiber optic indicated that a 5- to 6-fold difference in fluence rate was established on the two sides of the stem. The light gradient established at the start of irradiation was the same as that after 6 h of irradiation. Changes in gravitropic responsiveness during this period were also ruled out. Calculations show that the light gradient should have caused curvature which would be detectable within 30 to 60 min and which would extrapolate to the start of irradiation--if the Blaauw hypothesis were correct. The long lag for phototropism in this case indicates that rapid inhibition of cell elongation by blue light does not cause the asymmetrical growth of phototropism. Rather, phototropism is superimposed upon this separate light growth response.

Evidence Inconsistent with the Blaauw Model of Phototropism

The Blaauw model of phototropism equates the inhibition of growth at the illuminated side of a unilaterally illuminated organ with the blue light inhibition of overall organ extension evident when some shoots are exposed to uniform blue light. However, a study of the growth responses of Avena coleoptiles exposed to omnilateral, equal bilateral, unequal bilateral and unilateral blue light has revealed some light induced growth rate changes which cannot be explained by the Blaauw model. The growth responses of cells at the illuminated and shaded sides of phototropically stimulated coleoptiles seem to depend on the existence of a light gradient across the whole organ rather than the absolute levels of light at either side.

Physiology of Movements in the Stems of Seedling Pisum sativum L. cv Alaska

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.

Phytochrome Modifies Blue-light-induced Electrical Changes in Corn Coleoptiles

Unilateral blue light administered to corn coleoptile segments produces no alteration of transmembrane potential on the light side, and only a small and slow hyperpolarization on the dark side. Red light causes a 5-15 millivolt depolarization in cells on the light side causes and somewhat smaller effects on the dark side. Blue given after red causes a rapid hyperpolarization on both sides of the coleoptile. The effect of the potentiating red preirradiation is probably due to phytochrome, being largely abolished by far-red given after red, but before the blue light. The effect of prior red irradiation decays in the dark, showing a half-time of about 45 minutes at room temperature. This rapid cooperativity between phytochrome and the phototropic pigment may indicate a common locale, possibly in a membrane.

Aberrant cyclic nucleotide regulation in a behavioral mutant of Phycomyces blakesleeanus

The sporangiophore of Phycomyces blakesleeanus bends toward unilateral blue light and may transiently double its growth rate upon uniform stimulation. The sporangiophore of the mutant strain C149, mad D 120 (−), has no observable bending response and a much attenuated growth response. Uniform light stimulation of wild type produces an immediate 60% dip and recovery of cyclic AMP completed well before the growth response initiates. In C149, the same stimulation elicits a very small cyclic AMP response. In accord with this, cyclic AMP phosphodiesterase activity is shown to be altered in C149. This is the first biochemical alteration reported in a mad mutant and it may be related to the genetic lesion.

Die polaritätsinduktion bei der equisetumspore durch polarisiertes licht und partielle belichtung

Previous studies showed that in all cells, and only in those cells, whose growth could be oriented by unilateral unpolarized light, growth could also be controlled by the plane of polarized light. It was inferred that both reactions are mediated by dichroic photoreceptor molecules oriented with respect to the cell surface; they differ only in the spatial pattern of light absorption in the cell. The results in this paper agree with these general conclusions: spores of Equisetum arvense which are known to germinate away from unilateral blue light, germinate parallel to the vibration plane of polarized blue light and, if partially illuminated, out of the shaded part of the cell. The intensity effect curves for both reactions are very similar. The results suggest strongly that both effects are mediated by the same pigment molecules oriented parallel to the cell wall.