Stem phototropism of trees: a possible significant factor in determining stem inclination on forest slopes

BACKGROUND AND AIMS: The main stems of trees on forest slopes incline down the slope to various extents that are characteristic of the species. The inclination has been explained as an active response to a horizontally asymmetrical light environment, but the contributing physiological mechanisms are unknown. The present study tested the hypothesis that stem phototropism, gravitropism, or a combination of the two determines the inclination of tree stems on forest slopes. METHODS: Cryptomeria japonica, Pinus densiflora, Quercus myrsinaefolia and Q. serrata were studied. Measurements were made of stem inclination of mature trees on forest slopes in uniform plantations of each species, and changes in stem inclination of potted seedlings in response to illumination treatments (unilateral or overhead) and inclination treatments (artificially inclined or erect). Indices of phototropic and gravitropic responsiveness were evaluated for each species, calculated from the change in stem inclination in response to artificial inclination with unilateral or overhead illumination. KEY RESULTS: Stem inclination on forest slopes varied significantly among species: Q. serrata inclined most in the down-slope direction, C. japonica inclined the least, and P. densiflora and Q. myrsinaefolia were intermediate. The change in stem inclination of seedlings in each treatment varied significantly among species. One-year-old stems of Q. serrata and 2-year-old stems of Q. myrsinaefolia bent toward the light source. Interspecific variation in the change in stem inclination in response to the unilateral illumination or that in the index of phototropic responsiveness was strongly correlated with the variation in stem inclination on forest slopes. CONCLUSIONS: The orientation of woody stems that have finished elongation can be actively controlled by phototropism. Interspecific variation in phototropic responsiveness of trees is a possible significant determinant of interspecific variation in stem inclination on forest slopes.     

Role of leaves in phototropism

Experiments with green seedlings of sunflower (Helianthus annuns L.) indicate the existence of a phototropic mechanism which involves the leaves or cotyledons, and which can produce an asymmetry of auxin content without the involvement of lateral auxin transport, the classic explanation of phototropism in etiolated seedlings. The basic lines of evidence for the leaf-mediated tropism are: 1) darkening of one cotyledon will cause curvature of the stem toward the lighted cotyledon: 2) the darkened cotyledon sustains an enhanced growth rate in the stem below it: 3) conversely, light suppresses the growth-stimulating effects of a single cotyledon: and 4) more diffusible auxin is obtained from the stem below darkened cotyledons than below lighted ones.     

The sporelings of Lejeunea caespitosa Lindenb.

Abstract

Sporelings of Lejeunea caespitosa Lindenb. usually form a primary protonema of a single row of four cells, and a secondary protonema at first of two rows, and eventually of four rows of cells. The juvenile stem with two dorsal and three ventral rows of cells produces simple undivided underleaves and elobulate primary leaves or, later, lobulate juvenile leaves of which well developed lobules nearly equal the lobes in size. Bifid underleaves are formed as soon as the stem acquires four ventral rows of cells.     

Control of hypocotyl phototropism by phytochrome in a dicotyledonous seedling (Sesamum indicum L.)

Abstract The phototropic response in stems of higher plants is brought about by blue/UV light. The problem studied here is to what extent long-wavelength light, which is absorbed by phytochrome, affects the phototropic response. A refined measurement of phototropism — a curvature index — was applied to the hypocotyl of the sesame seedling (Sesamum indicum L.). The time course of the phototropic response was followed in continuous unilateral weak blue light (B, 460 nm, 8 mW m^?2). Long term red light (R) pretreatments, operating through phytochrome, strongly increase the rate and extent of the phototropic response once it is elicited by unilateral B, while the pretreatments decrease the sensitivity towards B. If a R pulse is given immediately prior to the onset of unilateral B, the rate of the response is strongly reduced compared to the time course of curvature observed when the pretreatment was terminated with a long wavelength far-red light (FR) pulse. R and FR were then applied simultaneously with unilateral B to manipulate the status of the phytochrome system during actual curvature. It was found that a low Pfr/P ratio (established by FR) stimulates the phototropic response far above the control (B alone), while a high Pfr/P ratio (established by R) reduces the response below the control. During bending a positive effect of phytochrome on the rate and extent of the phototropic response, which is saturated at a low level of Pfr, appears to be counteracted by an inhibitory effect which dominates at higher levels of Pfr, such as established by omnilateral R. However, if R is applied unilaterally from the same direction as B, R increases the rate of curvature. Apparently the sesame seedling is capable of detecting the direction of R relative to the direction of B. While a mechanistic explanation of these effects cannot be advanced at present, it is clear that the seedling is capable of super-imposing information about the actual light conditions during bending on a ‘memory’ of the light conditions prior to the onset of bending. Thus, the previous as well as the actual light conditions determine its phototropic responsiveness.     

背联体贻贝棘尾虫的形态,形态发生及其调节现象

背联体贻贝棘尾虫的每一虫腹面含有相当于正常棘尾虫的腹面纤毛系统,背联两虫任意一侧属于一虫的背面有4列背触毛,它们的排列分布相似于正常棘昆虫的第1—4列背触毛,另一虫背面打2列背触毛,它们相似于正常棘尾虫的第5、6列背触毛。结果表明,背联体棘尾虫是其中两虫各以背面第4列和第5列背触毛之间的皮层区相联接形成的。也有的背联体中背部皮层联接区有变化。无性分裂中背联两虫皮层纤毛结构的形态发生相似于正常棘尾虫,并且两者其皮层纤毛器如口围带、额腹横棘毛、左、右缘棘毛和背触毛等相应结构的发育是同步进行的,推测背联两虫的皮层发育既是相对独立的,又有某种机制控制着相互间的协调。背联体棘昆虫在无性生殖周期中总是经历着一个调节成单体的过程,认为这于背联两虫都具有一套结构功能正常的运动胞器(特别是口围带),而产生向不同方向运动的“不协调”的力有关。     

Cellular basis for the automorphic curvature of rice coleoptiles on a three-dimensional clinostat: possible involvement of reorientation of cortical microtubules

Coleoptiles of rice (Oryza sativa L.) show a spontaneous (automorphic) curvature toward the caryopsis under microgravity conditions. The possible involvement of the reorientation of cortical microtubules in automorphic curvature was studied in rice coleoptiles grown on a three-dimensional clinostat. When rice seedlings that had been grown in the normal gravitational field were transferred to the clinostat in the dark, cortical microtubules of epidermal cells in the dorsal side of the coleoptiles oriented more transversely than the ventral side within 0.5 h. The rotation on the clinostat also increased the cell wall extensibility in the dorsal side and decreased the extensibility in the ventral side, and induced automorphic curvature. The reorientation of cortical microtubules preceded the changes in the cell wall extensibility and the curvature. The irradiation of rice seedlings with white light from above inhibited microtubule reorientation and changes in the cell wall extensibility, as well as curvature of coleoptiles. Also, colchicine, applied to the bending region of coleoptiles, partially inhibited the automorphic curvature. These results suggest that reorientation of cortical microtubules is involved in causing automorphic curvature in rice coleoptiles on the clinostat.     

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.     

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.     

Blue Light-Induced Phosphorylation of a Plasma Membrane-Associated Protein in Zea mays L

Blue light induces a variety of photomorphogenic responses in higher plants, among them phototropic curvature, the bending of seedlings toward a unidirectional light source. In dark-grown coleoptiles of maize (Zea mays L.) seedlings, blue light induces rapid phosphorylation of a 114-kD protein at fluence levels that are sufficient to stimulate phototropic curvature. Phosphorylation in response to blue light can be detected in vivo in coleoptile tips preincubated in 32Pi or in vitro in isolated membranes supplemented with [[gamma]-32P]ATP. Phosphorylation reaches a maximum level in vitro within 2 min following an inductive light pulse, but substantial labeling occurs within the first 15 s. Isolated membranes remain activated for several minutes following an in vitro blue light stimulus, even in the absence of exogenous ATP. Phosphoamino acid analysis of the 114-kD protein detected phosphoserine and a trace of phosphothreonine. The kinase involved in phosphorylating the protein in vitro is not dependent on calcium. The 114-kD protein itself has an apparent binding site for ATP, detected by incubating with the nonhydrolyzable analog, 5[prime]-p-fluorosulfonyl-benzoyladenosine. This result suggests that the 114-kD protein, which becomes phosphorylated in response to blue light, may also be capable of kinase activity.     

Red-light-induced positive phototropism in Arabidopsis roots

The interaction between light and gravity is critical in determining the final form of a plant. For example, the competing activities of gravitropism and phototropism can determine the final orientation of a stem or root. The results reported here indicate that, in addition to the previously described blue-light-dependent negative phototropic response in roots, roots of Arabidopsis thaliana (L.) Heynh. display a previously unknown red-light-dependent positive phototropic response. Both phototropic responses in roots are considerably weaker than the graviresponse, which often masks phototropic curvature. However, through the use of mutant strains with impaired gravitropism, we were able to identify a red-light-dependent positive phototropic response in Arabidopsis roots. The red-induced positive phototropic response is considerably weaker than the blue-light response and is barely detectable in plants with a normal gravitropic response. Received: 22 May 2000 / Accepted: 3 July 2000     

Spatial separation of light perception and growth response in maize root phototropism

Although the effects of gravity on root growth are well known and interactions between light and gravity have been reported, details of root phototropic responses are less documented. We used high-resolution image analysis to study phototropism in primary roots of Zea mays L. Similar to the location of perception in gravitropism, the perception of light was localized in the root cap. Phototropic curvature away from the light, on the other hand, developed in the central elongation zone, more basal than the site of initiation of gravitropic curvature. The phototropic curvature saturated at approximately 10 micromoles m-2 s-1 blue light with a peak curvature of 29 +/- 4 degrees, in part due to induction of positive gravitropism following displacement of the root tip from vertical during negative phototropism. However, at higher fluence rates, development of phototropic curvature is arrested even if gravitropism is avoided by maintaining the root cap vertically using a rotating feedback system. Thus continuous illumination can cause adaptation in the signalling pathway of the phototropic response in roots.     

NPH3- and PGP-like genes are exclusively expressed in the apical tip region essential for blue-light perception and lateral auxin transport in maize coleoptiles

Phototropic curvature results from differential growth on two sides of the elongating shoot, which is explained by asymmetrical indole-3-acetic acid (IAA) distribution. Using 2 cm maize coleoptile segments, 1st positive phototropic curvature was confirmed here after 8 s irradiation with unilateral blue light (0.33 μmol m(-2) s(-1)). IAA was redistributed asymmetrically by approximately 20 min after photo-stimulation. This asymmetric distribution was initiated in the top 0-3 mm region and was then transmitted to lower regions. Application of the IAA transport inhibitor, 1-N-naphthylphthalamic acid (NPA), to the top 2 mm region completely inhibited phototropic curvature, even when auxin was simultaneously applied below the NPA-treated zone. Thus, lateral IAA movement occurred only within the top 0-3 mm region after photo-stimulation. Localized irradiation experiments indicated that the photo-stimulus was perceived in the apical 2 mm region. The results suggest that this region harbours key components responsible for photo-sensing and lateral IAA transport. In the present study, it was found that the NPH3- and PGP-like genes were exclusively expressed in the 0-2 mm region of the tip, whereas PHOT1 and ZmPIN1a, b, and c were expressed relatively evenly along the coleoptile, and ZmAUX1, ZMK1, and ZmSAURE2 were strongly expressed in the elongation zone. These results suggest that the NPH3-like and PGP-like gene products have a key role in photo-signal transduction and regulation of the direction of auxin transport after blue light perception by phot1 at the very tip region of maize coleoptiles.     

古尔班通古特沙漠白梭梭枝干光合及其影响因素

植物枝干光合（P_g）固定其自身呼吸所释放的CO₂，有效减少植物向大气的CO₂排放量。以古尔班通古特沙漠优势木本植物白梭梭（Haloxylon persicum）为研究对象，利用LI-COR 6400便携式光合仪与特制光合叶室（P-Chamber）相结合，观测白梭梭叶片、不同径级枝干的光响应及光合日变化特征；同时监测环境因子（大气温湿度、光合有效辐射、土壤温度及含水量等）与叶片/枝干性状指标（叶绿素含量、含水量、干物质含量、碳/氮含量等），揭示叶片/枝干光合的主要影响因子；采用破坏性取样，量化个体水平上叶片与枝干的总表面积，阐明枝干光合对植株个体碳平衡的贡献。研究结果显示：（1）白梭梭叶片叶绿素含量是枝干叶绿素含量的12-16倍，各径级枝干叶绿素含量差异不显著；（2）枝干光饱和点低于叶片，枝干不同径级（由粗至细），暗呼吸速率和枝干光合逐渐减小；（3）光合有效辐射、土壤含水量和空气温湿度是影响叶片光合的主要因子，对枝干光合无显著影响；（4）枝干光合可以固定其自身呼吸产生CO₂的73%，最高可达90%，枝干光合固定CO₂约占个体水平固碳量的15.4%。研究结果表明，忽视枝干光合的贡献来预测未来气候变化背景下荒漠生态系统碳过程，可能存在根本性缺陷，并且在估算枝干呼吸时，需要考虑枝干是否存在光合作用，以提高枝干呼吸的准确性。     

Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using ¹⁴C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of ³H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.     

Correlation of Blue Light-Induced Phosphorylation to Phototropism in Zea mays L

The physiology of light-induced phototropic curvature has been studied extensively in coleoptiles of grasses, particularly in Avena and Zea mays L. In Z. mays L., we have found that, in addition to curvature, blue light also induces rapid phosphorylation of a 114-kD protein in the tips of coleoptiles, and, in a previous report, we reported several characteristics of the phosphorylated substrate protein and kinase (J.M. Palmer, T.W. Short, S. Gallagher, W.R. Briggs [1993] Plant Physiol 102: 1211-1218). Here, we compare the phosphorylation response to several known aspects of phototropism physiology. Blue light-induced phosphorylation occurs only in the upper portion of the coleoptile and is absent from the node and mesocotyl. The specific activity of phosphorylation is highest in the extreme apical portion of the tip, which is also the site of maximal sensitivity to phototropic stimuli (A. W. Galston [1959] In Physiology of Movements, Encyclopedia of Plant Physiology, Springer, Berlin). Fluence-response determinations indicate that light dosage levels that stimulate curvature also stimulate phosphorylation. However, the threshold for inducing detectable phosphorylation in maize cannot be matched to the threshold for curvature induction. The recovery of sensitivity to phototropic stimuli after exposure to high fluences of light occurs with kinetics that are very similar to those for recovery of the phosphorylation response after a previous high-fluence light exposure. In addition, wavelengths of light in the blue and near-ultraviolet regions of the spectrum that maximally stimulate phototropic curvature also maximally stimulate in vitro phosphorylation in maize. The pattern of stimulation matches the absorption spectra of flavoproteins, which have been proposed as candidates for blue light photoreceptors.     

Kinetics and mechanics of stem gravitropism in Coprinus cinereus

Using video recordings we have completed the first kinetic analysis of mushroom stem gravitropism. The stem became gravireceptive after completion of meiosis, beginning to bend within 30 minutes of being placed horizontal. Stem bending first occurred in the apical 15% of its length, then the position of the bend moved rapidly towards the base, traversing 40% of stem length in 2.5 h. Meanwhile, the stem elongated by 25%, mostly in its upper half but also in basal regions. If the apex was pinned horizontally the stem base was elevated but overshot the vertical, often curling through more than 300 degrees. When the base was pinned to the horizontal (considered analogous to the normal situation), 90% of the initial bend was compensated as the stem brought its apex accurately upright, rarely overshooting the vertical. The apex had to be free to move for this curvature compensation to occur. Stems transferred to a clinostat after some minutes gravistimulation showed curvature which increased with the length of initial gravistimulation, indicating that continued exposure to the unilateral gravity vector was necessary for continued bending. Such gravistimulated stems which bent on the clinostat subsequently relaxed back towards their original orientation. Reaction kinetics were unaffected by submergence in water, suggesting that mechanical events do not contribute, but submerged stems bent first at the base rather than apex. In air, the gravitropic bend appeared first near the apex and then moved towards the base, suggesting basipetal movement of a signal. In water, the pattern of initial bending was changed (from apex to base) without effect on kinetics. Taken together these results suggest that bending is induced by a diffusing chemical growth factor (whose extracellular propagation is enhanced under water) which emanates from the apical zone of the stem. The apex is also responsible for regulating compensation of the bend so as to bring the tip to the vertical. The nature of this latter stimulus is unknown but it is polarized (the apex must be free to move for the compensation to occur) and it may not require reference to the unilateral gravity vector.     

Phototropic Curvature in Phycomyces

The distribution of curvature and of bending speed along the cell's growing region are studied during steady state phototropic bending. At the start, elemental bending speed parallels the known axial distribution of growth rate. Hence regional phototropic sensitivity is initially determined by the local growth rate, and unilateral visible light acts proportionally at all levels of the growth zone. In the later course of bending, the bending speed distribution shifts downward instead of progressing upward in step with the cell's elongation. Furthermore, during phototropic inversion reversed bending begins high in the growth zone and progresses downward while normal bending continues below. These spatial and temporal changes in the distribution of differential growth are considered to be due to a fixed rate of supply of material used in growth that is transported from lower regions of the cell and asymmetrically distributed within the growth zone.     

Phototropic Stimulation can Shift the Gradient of Crown Root Emergence in Maize

The crown roots in the coleoptilar node of maize emerge asymmetrically: emergence at the dorsal flank of the node (opposite to the caryopsis) precedes emergence at the ventral flank (facing the caryopsis). This asymmetry can be altered by phototropic stimulation: emergence of crown roots is delayed in the lighted flank and promoted in the shaded flank causing an inversion of the endogenous asymmetry. The curvature induced by the phototropic stimulation is transient, the effect on crown root emergence, in contrast, persists. This stable effect is not a consequence of curvature per se and becomes irreversibly fixed between one and two hours after stimulation. The emergence of crown roots depends on directional signalling from the coleoptile to the node. The data are discussed in terms of a stable blue light induced transverse polarity of the coleoptile that can imprint a stable asymmetry upon the coleoptilar node guiding the emergence of crown roots.     

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.     

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.