Heliotropic Movements Research Articles

Do electromagnetic fields used in telecommunications affect wild plant species? A control impact study conducted in the field

Over the last three decades there has been an unprecedented increase in both the coverage of wireless communication networks and the resultant radiofrequency electromagnetic field (RF-EMF) exposure level. There is growing concern that this rapid environmental change may have unexpected consequences for living organisms. Existing research on plants has shown that RF-EMF radiation can affect their growth and development, gene expression and various metabolic activities. However, these findings are largely derived from short-time exposure of crop plants under laboratory conditions. It remains unclear to what extent plants are affected by artificial RF-EMFs in real ecosystems and what potential consequences this could have for ecosystems. This study attempts to assess these long-term effects of RF-EMF exposure on wild plants under controlled experimental field conditions. We investigated the impacts of RF-EMF exposure (866–868 MHz frequency band) from seed germination to maturation for ten common herbaceous plant species over a four-month period. The selected plant species belong to various families and have different functional and morphological traits that might affect a response to the applied RF-EMF.For most of the considered species responses to RF-EMF were undetectable or weak, and where present restricted to a single trait. Only for one species, Trifolium arvense, were effects observed at different plant development stages and for different plant characteristics. In this species RF-EMF stimulated growth and probably influenced leaf heliotropic movements, as indicated by a larger height, larger leaf area and altered leaf orientation one month after germination. However, over the growing season Trifolium arvense plants exposed to RF-EMF entered the phase of senescence earlier, which was manifested through a reduction of green leaf area and an increase in the area of discolored leaf.We conclude that the effects of RF-EMF exposure at environmentally relevant levels can be permanent and irreversible in plants growing in the open natural environment, however, these effects are restricted to specific species. This in turn suggests that future studies should examine whether the effects observed here occur also in more common Trifolium species or other legumes that are a keystone component within European grasslands. Our findings also show that Trifolium arvense could be a candidate indicator of man-made RF-EMFs in the environment.

A smart metaheuristic algorithm for solving engineering problems

Every day, the immature sunflowers generate heliotropic movements. Two growth mechanisms are controlled on the heliotropic movements of immature sunflowers. The first mechanism is the sun-tracking phenomenon, which is caused by a growth hormone called Auxin.While the second mechanism is the biological clock which is responsible for the nocturnal reorientation of the immature sunflowers at the night. By clarifying and idealizing the growth mechanisms of the immature sunflowers in nature, a new type of nature-inspired optimization algorithm, called Smart Flower Optimization Algorithm (SFOA), is proposed in this paper. The proposed algorithm has been presented in two modes: sunny and cloudy or rainy modes depending on the weather. The SFOA is benchmarked on three parts. First, a set of 15 benchmark functions on CEC 2015 is used to test the efficiency of the SFOA using statistical analysis and Wilcoxon’s test. Second, the SFOA is utilized for designing an adaptive IIR system to appropriate the unknown system. Finally, four different engineering design problems (three-bar truss, tension/compression spring, speed reducer, and welded beam) are solved by SFOA. In the first part, the SFOA is compared with well-known algorithms. The results confirm the capabilities and efficiency of the proposed algorithm in finding the optimum. The promising solutions obtained for the IIR system identification and the engineering design problems demonstrate that the SFOA significantly outperforms other algorithms and show the applicability of the proposed algorithm in solving the real-world problems with unknown search spaces as well.

The thermal ecology of flowers.

BackgroundObtaining an optimal flower temperature can be crucial for plant reproduction because temperature mediates flower growth and development, pollen and ovule viability, and influences pollinator visitation. The thermal ecology of flowers is an exciting, yet understudied field of plant biology.ScopeThis review focuses on several attributes that modify exogenous heat absorption and retention in flowers. We discuss how flower shape, orientation, heliotropic movements, pubescence, coloration, opening–closing movements and endogenous heating contribute to the thermal balance of flowers. Whenever the data are available, we provide quantitative estimates of how these floral attributes contribute to heating of the flower, and ultimately plant fitness.OutlookFuture research should establish form–function relationships between floral phenotypes and temperature, determine the fitness effects of the floral microclimate, and identify broad ecological correlates with heat capture mechanisms.

Daily heliotropic movements assist gas exchange and productive responses in DREB1A soybean plants under drought stress in the greenhouse

Drought stress is one of the most severe environmental constraints on plant production. Under environmental pressures, complex daily heliotropic adjustments of leaflet angles in soybean can help to reduce transpiration losses by diminishing light interception (paraheliotropism), increase diurnal carbon gain in sparse canopies and reduce carbon gain in dense canopies by solar tracking (diaheliotropism). The plant materials studied were cultivar BR 16 and its genetically engineered isoline P58, ectopically overexpressing AtDREB1A, which is involved in abiotic stress responses. We aimed to follow the movements of central and lateral leaflets in vegetative stages V7-V10 and reproductive stages R4-R5, integrating the reversible morphogenetic changes into an estimate of daily plant photosynthesis using three-dimensional modeling, and to analyze the production parameters of BR 16 and P58. The patterns of daily movements of central leaflets of BR 16 in V7-V10 and R4-R5 were similar, expressing fewer diaheliotropic movements under drought stress than under non-limiting water conditions. Daily heliotropic patterns of lateral leaflets in V7-V10 and R4-R5 showed more diaheliotropic movements in drought-stressed P58 plants than in those grown under non-limiting water conditions. Leaf area in R4-R5 was generally higher in P58 than in BR 16. Drought significantly affected gas exchange and vegetative and reproductive architectural features. DREB1A could be involved in various responses to drought stress. Compared with the parental BR 16, P58 copes with drought through better compensation between diaheliotropic and paraheliotropic movements, finer tuning of water-use efficiency, a lower transpiration rate, higher leaf area and higher pod abortion to accomplish the maximum possible grain production under continued drought conditions.

Architectural Characteristics and Heliotropism May Improve Spray Droplet Deposition in the Middle and Low Canopy Layers in Soybean

Leaf angle and plant architecture may determine the penetration of phytosanitary products in the low layer of soybean [Glycine max (L.) Merr.] plants, make the diseases control difficult, and promote losses in crop productivity. Strategies should be studied to increase the efficiency of deposition of phytosanitary products inside the canopy. The hypothesis of this work was that reversible leaf movements and plant architecture may change the penetration of spray droplets into the low layer of soybean plants. We evaluated whether plant architectural characteristics affect spray droplet deposition and monitored if heliotropic movements can increase the efficiency of applications of plant protection products. Four soybean cultivars were investigated. Plant architectural characteristics were evaluated in the V6, V9, R2, and R4 phenological stages, interception of photosynthetically active radiation was evaluated in the V9, R2, and R4 stages, and heliotropic movements were evaluated in the V6 and R4 stages. Sprayings were performed at 0800, 1100, 1400, and 1700 h in the R4 stage. Occurrence of diaheliotropic movements was greater in the V6 than in the R4 stage. The number of branches per plant in the R4 stage was 8, 14, 6, and 14, the branch size was 10.6, 25.8, 12.5, and 21.4 cm, and the leaf area index was 4.2, 4.7, 3.7, and 4.7 for cultivars ‘BMX Ativa RR’, ‘NA 5909 RG’, ‘95R51’, and ‘BMX Potência RR’, respectively. Those architectural characteristics played the most important role in droplet penetration into the low canopy layer. Droplet deposition in the low canopy layer was greater at daylight hours close to noon, showing that the paraheliotropism occurring in those daily hours is more helpful to droplet penetration deeper into the canopy than in other hours.

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.

Heliotropic responses of soybean cultivars at three phenological stages and under two water regimes

The objectives of this work were to determine the heliotropic movements of the upper trifoliates for two soybean cultivars, BR 16 and Embrapa 48, during a daily cycle, in three phenological stages and two water regimes, and to estimate the impact of irrigation and daily leaflet movements on agronomic characteristics and grain yield. Heliotropic movements were studied in three phenological stages: V4-V6, V7-V10, and R5 in irrigated and non-irrigated plots. For each stage, the leaflet elevation and azimuth were measured hourly. Under a low (V4-V6 stage) and mid (V7-V10 stage) leaf area index (LAI) the diaheliotropism was slightly more frequent and intensive in non-irrigated than in irrigated plants, only at early morning and late afternoon hours. At R5 stage (high LAI) the paraheliotropism of superior trifoliates was predominant and more intensive in non-irrigated plants. The heliotropic movements are correlated to carbon gain, but not to environment (light intensity or temperature), for measurements at 11h. 'Embrapa 48' expresses greater paraheliotropism than 'BR 16' at high LAI, while 'BR 16' displays lower heliotropic plasticity under irrigation. In spite of significant heliotropic differences, genotype and water availability treatments did not influence the final grain yield.

Flower heliotropism of Anemone rivularis (Ranunculaceae) in the Himalayas: effects on floral temperature and reproductive fitness

The flower heliotropism of Anemone rivularis (Ranunculaceae) was investigated on the Yulong Snow Mountain, near Lijiang in the northwest of the Yunnan province of China. We discovered that tepals in this species drive the peduncles to track the sun, and that this flower heliotropism was likely an adaptation for parental environmental effects on reproductive fitness. In brief, A. rivularis flowers retained sun-tracking behavior following removal of pistils and stamens, but lost heliotropic movement, if tepals were removed. Light is the major factor to affect floral heliotropism, the tepal-received light signal was in the blue frequency. Meanwhile, the peduncles were found to bend significantly on the top portion of both control flowers and those lacking pistils and stamens, but instead of keeping a vertical peduncle orientation in flowers with tepals removed. Furthermore, the floral temperature was steadier, and seed sizes and numbers were greater for control flowers than for flowers with tepals removed. Therefore, we conclude that the tepals trigger the flower heliotropism in A. rivularis and play an important role on not only increasing but also keeping optimal thermal condition of flower interior. We further conclude that flower heliotropism enhances the pollen viability and seed production, resulting in higher reproductive success for this alpine species.

The Pulvinus Endodermal Cells and their Relation to Leaf Movement in Legumes of the Brazilian Cerrado*

Legume pulvini have a clearly delimited endodermis, whose variable content has been associated with the velocity and type of leaf movement: pulvini in leaves with fast nastic movement contain starch grains; pulvini in leaves with slow nastic movements have calcium oxalate crystals as well as starch grains in the endodermis. However, the studies carried out to date have involved few legume species. This study therefore purported to examine the consistency of this hypothesis in other legumes. Thus, the structure and content of the pulvinus endodermal cells of nine legumes of the Brazilian cerrado, with different types and velocities of leaf movement, were investigated: slow nyctinastic and heliotropic movements ( BAUHINIA RUFA, COPAIFERA LANGSDORFFII, SENNA RUGOSA - Caesalpinioideae; ANDIRA HUMILIS and DALBERGIA MISCOLOBIUM - Faboideae; STRYPHNODENDRON POLYPHYLLUM - Mimosoideae), slow heliotropic movement ( ZORNIA DIPHYLLA - Faboideae), and fast seismonastic and slow nyctinastic and heliotropic movements ( MIMOSA RIXOSA and MIMOSA FLEXUOSA - Mimosoideae). Samples were prepared following standard plant anatomy and ultrastructure techniques. The endodermis of all the species contains starch grains. In the species displaying only slow movements, calcium oxalate prismatic crystals were observed in addition to starch grains, except in ZORNIA DIPHYLLA. In conclusion, oxalate crystals occur only in endodermal cells of pulvini that display slow movements, while starch grains are always present in pulvinus endodermal cells of plants with any kind of movement.

Digital measurement of heliotropic leaf response in soybean cultivars and leaf exposure to solar UVB radiation

Inconsistencies in reported sensitivities of soybean cultivars [ Glycine max (L.) Merr.] to enhanced ultraviolet-B (UVB) irradiance may in part be due to differences in the radiative environment of the experimental conditions or differences in exposure due to heliotropic response. In order to examine the impact of heliotropic movement on UVB exposure of the soybean upper trifoliate, leaf position was electronically recorded and inclination and azimuthal position of the leaves calculated for three soybean cultivars—Bay, York, and Williams 82—under greenhouse and field conditions. The UVB exposures of the top trifoliate of three soybean cultivars were modeled using anisotropic and isotropic sky radiance distributions. The Williams 82 cv. produced the greatest variation in leaf angle with solar zenith angle of the three cultivars, averaging between 19 and 47°, compared with a lower 10–27° range for York and Bay cultivars. The incidence angle data for the upper trifoliates of the field plants were not significantly different from the greenhouse plants. For clear sky conditions, overall exposure differences indicated that the Bay cultivar receives more UVB than the York or Williams 82 cultivars, in large part due to the higher variation in heliotropic movement shown by the Bay cultivar in response to the sun location.

Flower Heliotropism and Floral Heating of Five Alpine Plant Species and the Effect on Flower Visiting in Ranunculus montanus in the Austrian Alps

During the growing seasons in 1994 and 1995 flower heliotropism and its effects on floral biology of several plants in the Grossglockner region (Salzburg, Austria) at 2230-2340 m a.s.l. was investigated. Ranunculus montanus, R. alpestris, Pulsatilla alpina, Callianthemum coriandrifolium (Ranunculaceae), and Leucanthemopsis alpina (Asteraceae) all have bowl and disc flowers (or inflorescences) that are heliotropic during periods of direct solar radiation. Temperatures of the air inside the flowers were above those of the ambient air for all five species. The maxima of excess temperature (means of 10 flowers or inflorescences) were between 2.0°C (C. coriandrifolium) and 6.2°C (P. alpina). Comparative temperature measurements in flowers of R. montanus showed that during sunny or slightly cloudy periods the air within those having uninhibited heliotropic movements was on average 0.7°C (in individual cases up to 1.3°C) warmer than in flowers fixed to face north. For the self-incompatible R. montanus, heliotropism affected the frequency and residence time of flower visitors: unimpeded heliotropic flowers were more attractive and so were visited longer and more often by insects than those oriented away from the sun.

Flower Heliotropism and Floral Heating of Five Alpine Plant Species and the Effect on Flower Visiting in Ranunculus montanus in the Austrian Alps

During the growing seasons in 1994 and 1995 flower heliotropism and its effects on floral biology of several plants in the Grossglockner region (Salzburg, Austria) at 2230-2340 m a.s.l. was investigated. Ranunculus montanus, R. alpestris, Pulsatilla alpina, Callianthemum coriandrifolium (Ranunculaceae), and Leucanthemopsis alpina (Asteraceae) all have bowl and disc flowers (or inflorescences) that are heliotropic during periods of direct solar radiation. Temperatures of the air inside the flowers were above those of the ambient air for all five species. The maxima of excess temperature (means of 10 flowers or inflorescences) were between 2.0°C (C. coriandrifolium) and 6.2°C (P. alpina). Comparative temperature measurements in flowers of R. montanus showed that during sunny or slightly cloudy periods the air within those having uninhibited heliotropic movements was on average 0.7°C (in individual cases up to 1.3°C) warmer than in flowers fixed to face north. For the self-incompatible R. montanus, heliotropism affected the frequency and residence time of flower visitors: unimpeded heliotropic flowers were more attractive and so were visited longer and more often by insects than those oriented away from the sun.

Potential Effect of Soybean Heliotropism on Ultraviolet‐B Irradiance and Dose

Assessing the potential for decreases in crop yield due to stratospheric ozone loss and concomitant increases in ultraviolet‐B (UVB) radiation requires knowledge of the characteristics of UVB irradiance above and within crops and the sensitivity of crops to the detrimental effects of the irradiance. Evaluations of the tolerance of many crops to increases in UVB irradiance have been made. The tolerance or intolerance of a crop to UVB irradiance is in part a result of physical characteristics of the plant. This study evaluated both the distribution of irradiance and the effects of plant leaf heliotropic movement of soybean [Glycine max (L.) Merr.] cultivars on the irradiance received on the leaf surface. Measurements of the above‐canopy UVB irradiance on planes having a wide range of slope and aspect were made under cloud‐free skies in a soybean field during the summer of 1990. Results showed that the relative irradiance was strongly dependent on the leaf inclination angle and could be modeled by a simple function having a relative irradiance accuracy of 0.14 (root mean square error); with greater model error at small incidence angles to the sun than at large incidence angles. The developed model was then used to evaluate the influence of heliotropy on the UVB dose received on the terminal leaflet. These results suggest that heliotropy in the leaflet acts to reduce irradiance. Since the more tolerant soybean cultivars appear to orient leaflets away from the sun, results suggest that the classification of UVB tolerance may be due in part to the capacity for paraheliotropic response reducing the UVB radiation received.

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.

Physiological Responses of Intsia bijuga Trees to Drought Stress

Intsia bijuga is an important native tree on Guam, and is being promoted as an urban forestry tree. Container-grown I. bijuga trees were subjected to a drying cycle (50% of mean water loss replaced daily) to determine physiological responses to drought. Early to mid-morning gas exchange began to decline compared to well-watered plants on day 12, and quickly declined thereafter. Net CO2 assimilation (A) was close to zero by day 29 and became negative by day 36. Chlorophyll fluorescence of drought-stressed trees was not different from that of well-watered trees on numerous days of measurement throughout the drying cycle. Unlike leaflets of well-watered trees, leaflets of the stressed trees exhibited heliotropic movement to avoid direct exposure to the sun. The stressed trees did not respond with any change in osmotic potential of leaflets following rehydration. Carbon dioxide-use efficiency of stressed trees was reduced to 66% of well-watered trees. The most profound response following rewatering (day 37) was leaf shedding. All trees shed some leaves, and 33% of the trees shed the entire canopy. On the trees that retained some leaves, A returned to that of the control trees by day 13 of recovery.

Ecological Significance of Flower Heliotropism in the Spring Ephemeral Adonis ramosa (Ranunculaceae)

Flower heliotropism could increase reproductive success in Adonis ramosa by increasing pollination, fertilization success, and/or seed development. Mean gynoecium temperature of heliotropic flowers was 5.5°C warmer than the ambient. Under natural conditions, seed set was restricted by pollen limitation. Activity of pollinating insects was temperature-dependent and the visiting frequency was higher in heliotropic flowers than in non-heliotropic ones. As optimal temperature for pollen germination and pollen tube growth was higher than the ambient temperature, heliotropic warming might facilitate greater fertilization success after pollination. Experiments combining hand-pollination and petal removal showed that both seed set and individual seed weight decreased when heliotropic effects were removed. Thus, all three hypotheses were supported in this' study. Advantages of out-crossing through heliotropic movements was not clear in this study, because there was no significant difference in individual seed weight between selfed and out-crossed flowers

GEOMETRY OF HELIOTROPIC AND NYCTINASTIC LEAF MOVEMENTS

Leaves that show nyctinastic and helionastic movements are usually more steeply inclined during the night and near midday than in the early morning or afternoon. But leaf movement is complex and there are many different combinations of leaf folding, axial rotation, or inclination of the midrib that result in changes in total leaf inclination. The complexity of leaf movement and amount of total leaf inclination can be described by plotting axial rotation or folding angle as a function of the angle of inclination of the midrib. The use of this method of description of leaf movement indicates that the variation between types of leaf movement may be continuous and not simply classifiable as diaheliotropic or paraheliotropic. Furthermore, this paper introduces a new method for characterization of the geometry of leaf movement that uses correlation between axial rotation and elevation as a tool for identification and quantification of leaf movement.

The effects of nitrogen, light and water availability on tropic leaf movements in soybean (Glycine max)

Abstract. Rapid, tropic leaf movements and photo‐synthetic responses of the heliotropic plant, soybean, Glycine max cv. Cumberland, grown under two different nitrogen, three different light and two different water treatments were examined. Measurements of leaf orientation during midday periods outdoors, and tropic reorientation of leaflets in response to vertical illumination indoors, revealed a positive, linear relationship between leaf water potential and the cosine of the angle of incidence between the leaf and the direct beam of the excitation light. This relationship was altered by nitrogen availability, such that a lower cosine of incidence (lower leaf irradiance) for a given leaf water potential was measured for plants grown under low nitrogen compared to those grown under high nitrogen. Additionally, plants grown under low nitrogen and low water availability showed more rapid rates of leaf movement compared to plants receiving high levels of these resources. Light regime during growth had no effect on the relationship between the cosine of incidence and leaf water potential. Reduced water and nitrogen availabilities during growth resulted in lower photosaturated rates of photosynthesis and stomatal conductance, as well as alterations in the relationship between these parameters. Thus, higher values for the ratio of intercellular CO2/ambient CO2 were measured for low‐N grown plants (higher nitrogen use efficiencies) and lower values of this ratio for water stressed plants (higher water use efficiencies). The results show that environmental growth conditions other than water availability have the potential to modify leaf orientation responses to vectorial light in heliotropic legumes such as soybean. This has implications for the potential of heliotropic movements to minimize environmental stress‐induced damage to the photosynthetic apparatus, and to modulate leaf‐level resource use efficiencies.

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.