Heliotropic Leaf Movements Research Articles

A novel pattern of leaf movement: the case of Capparis spinosa L.

A novel type of heliotropic leaf movement is presented for Capparis spinosa L., a summer perennial shrub of Mediterranean and arid ecosystems. In contrast to plants that demonstrate uniform diaheliotropic and/or paraheliotropic movement for all their foliage, the alternate leaves of C. spinosa follow different movement patterns according to their stem azimuth and the side of the stem that they come from (cluster). Additionally, leaf movement for each cluster may not be uniform throughout the day, showing diaheliotropic characteristics during half of the day and paraheliotropic characteristics during the rest of the day. In an attempt to reveal the adaptive significance of this differential movement pattern, the following hypotheses were tested: (i) increase of the intercepted solar radiation and photosynthesis, (ii) avoidance of photoinhibitory conditions, (iii) amelioration of water-use efficiency and (iv) adjustment of the leaf temperature microenvironment. No evidence was found in support of the first two hypotheses. A slight difference toward a better water use was found for the moving compared with immobilized leaves, in combination with a better cooling effect.

Heliotropic leaf movement of Sophora alopecuroides L.: An efficient strategy to optimise photochemical performance

We studied the survival adaptation strategy of Sophora alopecuroides L. to habitat conditions in an arid desert riparian ecosystem. We examined the responses of heliotropic leaf movement to light conditions and their effects on plant photochemical performance. S. alopecuroides leaves did not show any observable nyctinastic movement but they presented sensitive diaheliotropic and paraheliotropic leaf movement in the forenoon and at midday. Solar radiation was a major factor inducing leaf movement, in addition, air temperature and vapour pressure deficit could also influence the heliotropic leaf movement in the afternoon. Both diaheliotropic leaf movement in the forenoon and paraheliotropic leaf movement at midday could help maintain higher photochemical efficiency and capability of light utilisation than fixed leaves. Paraheliotropic leaf movement at midday helped plants maintain a potentially higher photosynthetic capability and relieve a risk of photoinhibition. Our findings indicated the effective adaptation strategy of S. alopecuroides to high light, high temperature, and dry conditions in arid regions. This strategy can optimise the leaf energy balance and photochemical performance and ensure photosystem II function.

Detecting leaf pulvinar movements on NDVI time series of desert trees: a new approach for water stress detection.

Heliotropic leaf movement or leaf ‘solar tracking’ occurs for a wide variety of plants, including many desert species and some crops. This has an important effect on the canopy spectral reflectance as measured from satellites. For this reason, monitoring systems based on spectral vegetation indices, such as the normalized difference vegetation index (NDVI), should account for heliotropic movements when evaluating the health condition of such species. In the hyper-arid Atacama Desert, Northern Chile, we studied seasonal and diurnal variations of MODIS and Landsat NDVI time series of plantation stands of the endemic species Prosopis tamarugo Phil., subject to different levels of groundwater depletion. As solar irradiation increased during the day and also during the summer, the paraheliotropic leaves of Tamarugo moved to an erectophile position (parallel to the sun rays) making the NDVI signal to drop. This way, Tamarugo stands with no water stress showed a positive NDVI difference between morning and midday (ΔNDVImo-mi) and between winter and summer (ΔNDVIW-S). In this paper, we showed that the ΔNDVImo-mi of Tamarugo stands can be detected using MODIS Terra and Aqua images, and the ΔNDVIW-S using Landsat or MODIS Terra images. Because pulvinar movement is triggered by changes in cell turgor, the effects of water stress caused by groundwater depletion can be assessed and monitored using ΔNDVImo-mi and ΔNDVIW-S. For an 11-year time series without rainfall events, Landsat ΔNDVIW-S of Tamarugo stands showed a positive linear relationship with cumulative groundwater depletion. We conclude that both ΔNDVImo-mi and ΔNDVIW-S have potential to detect early water stress of paraheliotropic vegetation.

Diurnal and annual rhythms in trees

Trees, perennial phanerophytes, display a rich variety of rhythmic phenomena. These are either due to exclusive environmental entrainment or due to the functioning of endogenous oscillators independent of the environment. Both types of rhythms are covered in this review. Purely environment controlled rhythms may be considered as a prelude to endogenous rhythms. Environment controlled rhythms discussed are (i) the diurnal rhythms of nyctinastic and heliotropic leaf movements and oscillatory phenomena of photosynthesis, such as the midday depression and Crassulacean acid metabolism (CAM), and (ii) the annual rhythms of annual growth ring formation, autumnal leaf senescence, over wintering mechanisms and flowering. Among the diurnal rhythms, nyctinastic movements and CAM are also free-running endogenous rhythms showing the operation of circadian clocks in trees. In leaf senescence, over wintering, and flowering control, photoperiod sensing is involved which suggests the participation of endogenous clocks. A question asked is if diurnal and annual rhythms are mechanistically correlated. Evidently, phenological phenomena based on photoperiodism (as dependent on measurement of night length) are co-ordinately regulated by the phytochrome system and the circadian clocks and many aspects of annual developments and over wintering are linked to photoperiodism. The existence in trees of circadian clock genes as known to be anchored in the genome of A. thaliana can be assessed by attempts of alignment with the sequenced genome of Populus or by isolating cDNA clones from trees to check them against the genome of A. thaliana. At extreme latitudes near the equator and north of the polar circle trees also display photoperiod-independent phenological phenomena. In the polar region, total irradiance of red and far red light could possibly be involved and the signalling pathway then involves phytochrome, and thus, may still be similar to that of photoperiodism. At the equator, total daily light irradiance received or sensing the dynamics of daily changes in solar irradiance are essential and it remains enigmatic whether signalling cascades are either attached to the circadian clocks in a still unknown way or totally independent of circadian clocks.

Pulvinus activity, leaf movement and leaf water-use efficiency of bush bean (Phaseplus vulgaris L.) in a hot environment

Pulvinus activity of Phaseolus species in response to environmental stimuli plays an essential role in heliotropic leaf movement. The aims of this study were to monitor the continuous daily pulvinus movement and pulvinus temperature, and to evaluate the effects of leaf movements, on a hot day, on instantaneous leaf water-use efficiency (WUEi), leaf gas exchange, and leaf temperature. Potted plants of Phaseolus vulgaris L. var. Provider were grown in Chicot sandy loam soil under well-watered conditions in a greenhouse. When the second trifoliate leaf was completely extended, one plant was selected to measure pulvinus movement using a beta-ray gauging (BRG) meter with a point source of thallium-204 (204Tl). Leaf gas exchange measurements took place on similar leaflets of three plants at an air temperature interval of 33-42 degrees C by a steady-state LI-6200 photosynthesis system. A copper-constantan thermocouple was used to monitor pulvinus temperature. Pulvinus bending followed the daily diurnal rhythm. Significant correlations were found between the leaf-incident angle and the stomatal conductance (R2 = 0.54; P < 0.01), and photosynthesis rate (R2 = 0.84; P < 0.01). With a reduction in leaf-incidence angle and increase in air temperature, WUEi was reduced. During the measurements, leaf temperature remained below air temperature and was a significant function of air temperature (r = 0.92; P < 0.01). In conclusion, pulvinus bending followed both light intensity and air temperature and influenced leaf gas exchange.

Diurnal patterns of soybean leaf inclination angles and azimuthal orientation under different levels of ultraviolet-B radiation

This paper documents the diurnal pattern of heliotropic leaf movements for four different soybean ( Glycine max) cultivars and its relationship to increased ambient levels of ultraviolet-B radiation. Considerable variation in diurnal patterns of leaf movement within and among cultivars was found, but no effect of the level of UV-B radiation was observed. The ranking of the four cultivars in order of decreasing sensitivity to UV-B radiation was the same as a ranking based on midday leaf inclination, with tolerant cultivars displaying steeper leaf angles. Plants that have the ability to reduce light interception at midday, thereby reducing light and temperature stress effects, may be better prepared to cope with the additional effects of UV-B radiation on photosynthesis, stomatal conductance, plant morphology, and growth.

Heliotropic Leaf Movement Response to H + /ATPase Activation, H + /ATPase Inhibition, and K + Channel Inhibition in Vivo

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H+/ATPase and TEA-sensitive K+ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H+/ATPase inhibitors, vanadate and erythrosin-B, and the K+ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m-2 s-1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin-B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.

HELIOTROPIC LEAF MOVEMENT RESPONSE TO H+/ATPase activation, H+/ATPase inhibition, AND K+ CHANNEL INHIBITION IN VIVO

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H+/ATPase and TEA‐sensitive K+ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H+/ATPase inhibitors, vanadate and erythrosin‐B, and the K+ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m‐2 s‐1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin‐B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.

Dirunal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities

ABSTRACTDiurnal heliotropic leaf movements, photosynthetic gas exchange, and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) of unrestrained and horizontally restrained leaves from soybean (Glycine max cv. Cumberland) plants grown in two different water and two different nitrogen treatments were measured. Leaves of plants grown in low water or low nitrogen availability treatments displayed more pronounced diaheliotropism (solar tracking) in the afternoon and a longer period of paraheliotropism (light avoiding) at midday relative to those of well‐watered, high‐nitrogen‐grown plants. Photosaturated photosynthetic rates and the photon flux required to saturate photosynthesis were reduced by water stress and nitrogen deficiency. Compared to horizontal leaves, irradiance on orienting leaves was nearer to the breakpoint of the photosynthetic light response curve, where photosynthesis is co‐limited by ribulose biphosphate regeneration and carboxylation. This would increase the carbon return on investments of nitrogen into photosynthesis. A positive linear relationship between Fv/Fm and quantum yield of photosynthesis was measured. Leaves of low‐nitrogen‐grown plants had earlier and more prolonged reductions in Fv/Fm at midday compared to leaves of high nitrogen grown plants of the same water treatment. Within the same water and nitrogen treatment, horizontally restrained leaves had lower midday Fv/Fm in relation to orienting leaves. Nitrogen deficiency and water stress enhanced this difference such that horizontally restrained leaves of low water and low nitrogen grown plants had earlier and longer midday depressions in Fv/Fm.

Heliotropic Leaf Movements in Common Beans Controlled by Air Temperature

Heliotropic leaf movements were examined in common beans (Phaseolus vulgaris cv Blue Lake Bush) under outdoor and laboratory conditions. Heliotropic leaf movements in well-watered plants were partly controlled by temperature, and appeared to be independent of atmospheric humidity and CO(2) concentration. When environmental conditions were held constant in the laboratory, increased air temperature caused bean leaves to orient more obliquely to a light source. Ambient CO(2), intercellular CO(2), and net photosynthesis were not correlated with the temperature-induced changes in heliotropic movements, nor did they significantly affect these movements directly. The effect of air temperature on leaf movements need not be mediated through a change in leaf water potential, transpiration, or leaf conductance. Air temperature modified laminar orientation in light through its effect on tissue temperature in the pulvinal region, not that of the lamina or petiole. However, under darkness the temperature effects on leaf movements were not expressed. Active heliotropic movements in response to air temperature allowed lamina temperature to remain close to the thermal optimum of photosynthesis. This temperature effect underlies a commonly observed pattern of leaf movements under well-watered conditions: a tendency for leaves to face the sun more obliquely on hot days than cool days.

Effect of water stress on the canopy architecture and spectral indices of irrigated alfalfa

Diurnal measurements of alfalfa leaf distribution and crop canopy reflectance were made in stressed and unstressed alfalfa plots in Phoenix, AZ. Measurements were made on three dates in January, May, and November. Results showed that heliotropic leaf movements were directly affected by crop water status. Under well-watered conditions, alfalfa leaves tended to track the sun throughout the day, in both the azimuthal and zenithal directions. In the stressed plots, diurnal tracking ability diminished as the day progressed and the canopy assumed a more vertical profile due to leaflet cupping. The stressed canopy then had a lower spectral reflectance (measured at nadir) than the unstressed canopy, in both the red and near-IR wavebands. The Perpendicular Vegetation Index (PVI) decreased with the stress-induced change in architecture while the near-IR/red ratio remained relatively constant. As a result, the near-IR / red ratio was more successful in estimating alfalfa biomass than was the PVI. These results emphasize the need to account for stress-induced architectural differences in plant canopies when using remotely sensed spectral data.

Photosynthetic responses of two heliotropic legumes from contrasting habitats

Abstract Photosynthetic responses to light, temperature and leaf‐to‐air water vapour concentration deficit for Strophostyles helvola (L.) Ell. in an open beach site, and Amphicarpa bracteata (L.) Ell. in two deciduous forest sites were quantified. Photosaturated rates of net CO2 assimilation were 52.1 ± 4.6, 11.0 ± 1.6 and 4.1 ± 0.3 μmol m−2s−2 for plants in beach, roadside and closed canopy sites, respectively. In terms of photosynthesis, plants in the beach site were more tolerant of higher leaf temperatures and water vapour concentration deficits than were plants in forested sites. Heliotropic leaf movements in the beach site reduced calculated total daily transpiration by 2%, increased total daily carbon gain by 8% and reduced the transpiration ratio by 9% relative to an horizontal leaf. During long‐term sunflecks in forested sites, heliotropism reduced transpiration by 15%, increased carbon gain by 71% and reduced the transpiration ratio by 50% relative to an horizontal leaf. We hypothesize that heliotropic leaf movements in mesic, high‐light, low‐nitrogen habitats may increase carbon return on nitrogen investment in photosynthesis, while heliotropism in canopy gaps may represent a morphological mechanism to reduce damage to low‐light acclimated photosynthetic systems during long‐term sunflecks.

RAPID LEAF MOVEMENT, MICROCLIMATE, AND WATER RELATIONS OF TWO TEMPERATE LEGUMES IN THREE CONTRASTING HABITATS

Diurnal heliotropic leaf movements, microclimate, stomatal conductance and leaf water potential of two leguminous species, Strophostyles helvola and Amphicarpa bracteata, were measured in three different habitats over two growing seasons. The habitats occurred along an environmental gradient from an open, sandy beach to a closed canopy deciduous forest understory. At the beach site, heliotropism in S. helvola resulted in higher irradiances in morning and afternoon hours and lower irradiances during midday periods compared to an horizontal leaf. In an exposed forest site A. bracteata responded within minutes to penetration of the direct solar beam by orienting its leaves to steep angles. In contrast, plants in closed canopy locations showed little diurnal leaf movement. The combined results of leaf energy budget calculations and plant physiological responses suggest that ecological ramifications of these movements vary with habitat. We hypothesize that heliotropism in open habitats increases water use efficiency and maximizes carbon returns on plant investment in photosynthesis. Conversely, we hypothesize that leaf movements in understory habitats represent a morphological mechanism to avoid thermal damage, photoinhibition of the photosynthetic apparatus, and water stress associated with high irradiances.

Rapid Leaf Movement, Microclimate, and Water Relations of Two Temperate Legumes in Three Contrasting Habitats

Diurnal heliotropic leaf movements, microclimate, stomatal conductance and leaf water potential of two leguminous species, Strophostyles helvola and Amphicarpa bracteata, were measured in three different habitats over two growing seasons. The habitats occurred along an environmental gradient from an open, sandy beach to a closed canopy deciduous forest understory. At the beach site, heliotropism in S. helvola resulted in higher irradiances in morning and afternoon hours and lower irradiances during midday periods compared to an horizontal leaf. In an exposed forest site A. bracteata responded within minutes to penetration of the direct solar beam by orienting its leaves to steep angles. In contrast, plants in closed canopy locations showed little diurnal leaf movement. The combined results of leaf energy budget calculations and plant physiological responses suggest that ecological ramifications of these movements vary with habitat. We hypothesize that heliotropism in open habitats increases water use efficiency and maximizes carbon returns on plant investment in photosynthesis. Conversely, we hypothesize that leaf movements in understory habitats represent a morphological mechanism to avoid thermal damage, photoinhibition of the photosynthetic apparatus, and water stress associated with high irradiances.

Diurnal variations of vegetation canopy structure

The significance and magnitude of diurnal variations of vegetation canopy structure are reviewed. Diurnal leaf inclination-azimuth angle distributions of a soybean and cotton canopy were documented using a simple measurement technique. The precision of the measurements was on the order of + or -5 deg for the inclination and + or -14 deg for the azimuth. The experimental results and a review of the literature showed that this distribution can vary significantly on a diurnal basis due to vegetation type, heliotropic leaf movement, environmental conditions, and vegetation stress. The study also showed that it is erroneous to treat two separate distributions of azimuth and inclination angles rather than one three-dimensional distribution of leaf orientation. The latter distribution needs to be routinely collected in studies which document variations of diurnal spectral reflectance with changes in solar zenith angle.