Flower heliotropism of <Emphasis Type="Italic">Anemone</Emphasis> <Emphasis Type="Italic">rivularis</Emphasis> (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.     

Flower heliotropism in an alpine population of Ranunculus acris (Ranunculaceae): effects on flower temperature,insect visitation,and seed production

Some plants in arctic and alpine habitats have heliotropic flowers that track the sun. This results in a heating of the flower's interior, which may improve the possibilities for insect pollination and seed production. Here, I examine whether flower heliotropism in an alpine population of the self-incompatible Ranunculus acris L. (Ranunculaceae) enhances pollinator visitation and seed production. Flowers of Ranunculus acris tracked the sun during the day. Tracking accuracy was greatest during the middle of the day. The temperature elevation in flowers was negatively correlated with the flower's angle of deviation from the sun. Despite the increased temperature, insects did not discriminate among flowers on the basis of their angle of deviation from the sun, or tend to stay longer in the flowers aligned closest towards the sun. A tethering experiment was conducted on three groups of plants flowering at different times in the 1993 season and on one group the following season. Manipulation plants were constrained not to track the sun, whereas control plants tracked the sun naturally. Solar tracking had no effect on seed:ovule ratio, seed mass, or abortion rate in any of the groups. There is probably a very narrow range of weather conditions (cold, sunny, and calm) where flower heliotropism may enhance visitation rate to flowers and their seed production.     

The ecological role of orientation in tropical convolvulaceous flowers

Flowers of Ipomoea pes-caprae and Merremia borneensis show a preferred orientation, pointing in the general direction of the sun but not exactly tracking the sun. They demonstrated no diurnal heliotropism but strong seasonal heliotropism. The possible effects of this non-random orientation on the heat balance of the flower and the possible consequences on pollination were studied by measuring gynoecium temperature and insect visitation. Differently treated flowers were used to measure gynoecia temperature along with the microclimate: intact flowers, flowers with corollas removed, flowers with the stile and stamens removed, shaded flowers, and flowers constrained to be facing away from the sun. The lowest gynoecium temperature was achieved when the flowers were not constrained and not greased. It is concluded that the natural position of the flower, as well as transpiration, ensures that the temperature of the gynoecium does not reach dangerous levels. Insects preferentially visited sunlit flowers that were free to adopt their natural orientation.     

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 CO₂ 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 CO₂, intercellular CO₂, 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.     

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.     

Solar furnaces or swamp coolers: costs and benefits of water use by solar-tracking flowers of the alpine snow buttercup, Ranunculus adoneus

Solar tracking or heliotropism simultaneously raises organ temperature and light interception. For leaves and flowers carbon gain is maximized at the expense of water loss. In this study I explore how costs and benefits associated with water use by solar-tracking flowers of the alpine snow buttercup, Ranunculus adoneus change with ambient temperature. First, I test whether heliotropism increases the water cost of reproduction in the snow buttercup under extant alpine conditions. I then explore whether water use for evaporative cooling in solar-tracking flowers reduces the risk of over-heating as temperatures increase. Solar tracking, by elevating floral temperature and irradiance causes a 29% increase in water uptake by flowers. Gas exchange measurements suggest that the extra water taken up by solar-tracking flowers is released through transpiration. Transpirational cooling in turn allows solar-tracking flowers to gain advantages of enhanced light interception and warmth while reducing the risk of over-heating. Transpiration reduces excess temperature in solar-tracking flowers, but at a water cost. Results show that even in cool alpine habitats, flower heliotropism has water costs to balance its reproductive advantages. Plants with solar-tracking flowers may tolerate hotter conditions if soil moisture is plentiful, but not under drought.     

Leaf photosynthetic and solar-tracking responses of mallow, Malva parviflora, to photon flux density

Malva parviflora L. (mallow) is a species that occupies high-light habitats as a weedy invader in orchards and vineyards. Species of the Malvaceae are known to solar track and anecdotal evidence suggests this species may also. How M. parviflora responds physiologically to light in comparison with other species within the Malvaceae remains unknown. Tracking and photosynthetic responses to photon flux density (PFD) were evaluated on plants grown in greenhouse conditions. Tracking ability was assessed in the growth conditions and by exposing leaves to specific light intensities and measuring changes in the angle of the leaf plane. Light responses were also determined by photosynthesis and chlorophyll fluorescence. Leaves followed a heliotropic response which was highly PFD-dependent, with tracking rates increasing in a curvilinear pattern. Maximum tracking rates were up to 20°h⁻¹ and saturated for light above 1300 μmol (photons) m⁻² s⁻¹. This high-light saturation, both for tracking (much higher than the other species), and for photosynthesis, confirmed mallow as a high-light demanding species. Further, because there was no photoinhibition, the leaves could capture the potential of an increased carbon gain in higher irradiance by resorting to solar tracking. Modelling suggested the tracking response could increase the annual carbon gain by as much as 25% compared with leaves that do not track the sun. The various leaf attributes associated with solar tracking, therefore, help to account for the success of this species as a weed in many locations worldwide.     

Black wildebeest seek shade less and use solar orientation behavior more than do blue wildebeest

Many ungulates, including wildebeest, seek shade and orient their bodies relative to incoming solar radiation in order to reduce environmental heat loads. Blue (Connochaetes taurinus) and black wildebeest (Connochaetes gnou), which co-exist artificially in some reserves in South Africa, are thought to adopt different thermoregulatory behaviors to mitigate high environmental heat loads. However, whether or not blue and black wildebeest use different behaviors to reduce heat loads in regions where they co-occur has never previously been examined. We compared the shade seeking and solar orientation behavior of free-ranging blue and black wildebeest in summer at three locations in South Africa where both species co-occur. We found that blue wildebeest exhibited more shade seeking behavior than did black wildebeest at all times of day, at all study sites. Black wildebeest remained in the sun but were more likely than blue wildebeest to orient their bodies parallel to the sun at all study sites, a behavior which reduces the amount of surface area exposed to incoming radiation. Black wildebeest were most likely to employ parallel solar orientation during the hottest times of the day when the sun was not directly overhead (i.e., solar noon ± 1 hour). We thus demonstrate that co-occurring blue and black wildebeest use different thermoregulatory behaviors to reduce high heat loads. It is possible that the lack of shade in the historical distribution of black wildebeest led to selective pressure for reliance on solar orientation. Differences in thermoregulatory behavior can affect species-specific heat loads, habitat use, body mass, fitness and grazing activity. Such differences may also allow blue and black wildebeest to inhabit separate microclimates within the same habitat, provided there is sufficient heterogeneity in vegetation structure, potentially facilitating reproductive isolation.     

Sunny-side up: flower heliotropism as a source of parental environmental effects on pollen quality and performance in the snow buttercup, Ranunculus adoneus (Ranunculaceae)

Floral traits affect mating success via their influence on the microenvironment in which sexual reproduction occurs as well as their impact on pollinator attraction. Here we investigate the importance of flower heliotropism as a source of parental environmental effects on pollen quality and performance. Flowers of the snow buttercup, Ranunculus adoneus, closely track the sun's rays. We experimentally restrained flowers to test for effects of heliotropism on pollen quality and performance after pollination. When equivalent amounts of pollen were transferred to recipient pistils, pollen from solar-tracking donor flowers exhibited a 32% advantage in germination compared to pollen from stationary (tethered) donor flowers. By the end of anthesis, pistils of tracking flowers contained 40% more germinating pollen grains and 44% more pollen tubes midway down the style than pistils of stationary ones. Solar tracking had no direct effect on pollen tube growth. The greater amount of germinating pollen in tracking flowers accounted for the treatment effect on pollen tube density. A survey of pollen receipt and pollen germination in naturally tracking flowers indicated that solar tracking primarily affects pollen tube density by promoting pollen germination rather than pollen deposition. We conclude that flower heliotropism, by enhancing the paternal environment for pollen development and the maternal environment for pollen germination, represents a source of positive parental environmental effects on pollen performance in snow buttercups.     

Consequences of flower heliotropism for reproduction in an alpine buttercup (Ranunculus adoneus)

Summary The flowers of the alpine snow buttercup Ranunculus adoneus track the sun's movement from early morning until mid-afternoon. Individual blooms last up to a week: younger female stage flowers show greater solar tracking fidelity than older hermaphrodite or dehisced flowers. Flowers aligned parallel to the sun's rays reach mean internal temperatures several degrees Celsius above ambient air temperature. As a flower's angle of deviation from the sun increases beyond 45 degrees, internal flower temperature is significantly reduced. Fly pollinators are seen disproportionately often on flowers aligned with the sun; this is due, in part, to their greater residence time on tracking flowers. Fly visitation is important to fecundity. When flies were excluded from flowers, some selfed seeds were matured, but total seed production was much less than in either handoutcrossed or open-pollinated flowers. Flowers that were tethered at random angles to prevent solar tracking set fewer, smaller seeds than unmanipulated control flowers. Reductions in seed weight were statistically consistent at different times in the season, but tethering influenced seed number per flower most strongly in the early season. Effects of tethering on maternal reproduction could be due to either pollinator diserimination, post-pollination developmental processes, or both. In a second series of experiments using fluorescent dye particles as pollen analogs, tethered donor flowers dispersed dye to as many recipients as paired control flowers, suggesting that solar tracking fidelity may have little effect upon this component of male reproduction.     

Far-Red Light Reflection from Green Leaves and Effects on Phytochrome-Mediated Assimilate Partitioning under Field Conditions

The influence of plant spacing and row orientation on spectral distribution of light received by growing soybean (Gylcine max [L.] Merr.) plants was measured under field conditions. Light absorption, reflection and transmission of individual leaves showed that most of the blue and red was absorbed while most of the far-red was either reflected or transmitted. Plants growing in the field received different ratios of far-red relative to red, depending on nearness and/or orientation of other vegetation. Plants grown in close-spaced rows, or high population densities, received higher far-red/red ratios than did those grown in wide rows, or sparse populations. Heliotropic movements of the leaves also contributed to the far-red reflection patterns associated with row orientation. Under field conditions, differences in far-red/red ratios associated with nearness of competing vegetation became more pronounced with low solar angle near the end of the day. Plants exposed to far-red for 5 minutes at the end of each day in controlled environments, and those grown in close-spaced rows in the field, developed longer internodes and fewer branches. Red, far-red photoreversibility in the controlled environment study indicated involvement of phytochrome. Dry matter partitioning among plant components in the field was related to far-red/red light ratio received during growth and development.     

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

Diurnal 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.     

Non-random leaf orientation in Lactuca serriola L.

Abstract Leaf shapes and leaf orientation of Lactuca serriola serriola and serriola integrifolia were studied. Leaf shapes in L. serriola serriola differed greatly from those of L.serriola integrifolia, but leaf surface areas were similar. In exposed habitats, leaf orientation of cauline leaves of both forms was non-random, with leaves almost vertical and tending to orient with their lamina normal to the east and west. In the shade, cauline leaves oriented randomly. An experiment demonstrated that the orientation of leaves did not change significantly once they were fully expanded. The leaf orientation in L. serriola affected the diurnal distribution of solar irradiance intercepted by a leaf. Peak solar radiation fluxes are incident on the rosette leaves at midday, but on the cauline leaves the peak solar radiation flux occurs early in the morning and again late in the afternoon. The significance of this unusual leaf orientation is discussed in relation to water loss and carbon gain.     

Near-infrared orientation of Mozambique tilapia Oreochromis mossambicus

Light plays a pivotal role in animal orientation. Aquatic animals face the problem that penetration of light in water is restricted through high attenuation which limits the use of visual cues. In pure water, blue and green light penetrates considerably deeper than red and infrared spectral components. Submicroscopic particles and coloured dissolved organic matter, however, may cause increased scattering and absorption of short-wave components of the solar spectrum, resulting in a relative increase of red and infrared illumination. Here we investigated the potential of near-infrared (NIR) light as a cue for swimming orientation of the African cichlid fish (Cichlidae) Oreochromis mossambicus. A high-throughput semi-automated video tracking assay was used to analyse innate behavioural NIR-sensitivity. Fish revealed a strong preference to swim in the direction of NIR light of a spectral range of 850-950nm at an irradiance similar to values typical of natural surface waters. Our study demonstrates the ability of teleost fish to sense NIR and use it for phototactic swimming orientation.     

Influence of light quality and photoperiod on flowering of Cyclamen persicum Mill. cv. ‘Dixie White’

The effect of light quality (spectral quality) and photoperiod (day length) were studied on flowering of Cyclamen persicum cv. Dixie White. Light generated from light emitting diodes (LED) i.e. monochromatic blue (10 or 12 h per day), monochromatic red (10 or 12 h per day), blue plus red (10 or 12 h per day) and fluorescent lights were used in these studies. It was found that blue plus red LEDs improved flower induction in cyclamen, the number flower buds and open flowers being highest in the plants grown under blue plus red LED (10 h per day). Blue and red LEDs alone reduced the flowering response. Peduncle length (flower stalk length) and blooming period of flowers were also influenced by light qualities and photoperiod treatments. Peduncle length was 23.8 cm on plants grown under red LED (12 h per day) treatment but 14 cm on plants grown under fluorescent light. Blooming period of flowers grown under fluorescent light was 20 d, whereas it was 40 d with the plants grown under red LEDs (10 h per day). The results indicate that flowering and subsequent growth of cyclamen can be controlled by manipulating light quality and lighting period.     

Solar tracking response to drought in a desert annual

Summary The responses to drought of the solar tracking winter annualLupinus arizonicus (Wats.) were examined under field and laboratory growth regimes. Under drought conditions tracking movements were maintained until the plant reached the wilting point. The leaves and leaflets were observed to cup in response to decreases in the xylem water potential. This resulted in a negative, but linear relationship between the cosine of the angle of incidence of the direct solar beam upon the leaf and water potential. Leaf conductance was also significantly related to xylem water potential in natural populations. Leaf and leaflet cupping seem to be responding directly to changing plant water status, resulting in reduced solar radiation load to drought stressed plants. It is hypothesized that this property may have adaptive consequences in relation to the water relations, energy budget, and carbon balance of the plant.     

Frequency-dependent selection by pollinators: mechanisms and consequences with regard to behaviour of bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae)

Bombus terrestris , a typical pollinating insect species, was offered artificial flowers of two different corolla colours with the same sucrose solution reward in an array. Common colours were significantly preferred, and the strength of the frequency-dependent response increased as a result of learning. There were also frequency-independent biases towards blue flowers, probably because blue flowers appeared more conspicuous to bumblebees than yellow flowers, and the degree of preference for blue was greater when flowers had low nectar rewards. Flower-to-flower movements by individual bumblebees between flowers were non-random, were biased to movements within the same flower colour, and were also dependent on morph frequency. The mechanisms governing flower selection in bumblebees are discussed. Pollinators foraging similarly in a natural situation would induce positive frequency-dependent selection, assortative mating, and directional selection on different corolla colour morphs of the plant population being visited, resulting in stabilizing selection for a single flower colour.     

Chloroplast movements in fern leaves: correlation of movement dynamics and environmental flexibility of the species

The movements of chloroplasts in response to varying levels and wavelengths of incident light were investigated in leaves of four fern species: Adiantum capillus-veneris, Adiantum caudatum, Adiantum diaphanum and Pteris cretica. In all of the species studied blue light induced chloroplast redistribution resulting in face and profile patterns that were typical of low and high fluence rates, respectively. Fluence rate response characteristics and the kinetics of transmission changes accompanying these blue-light-induced movements were similar to those observed in the leaves of higher plants. Only in A. capillus-veneris was the distribution of chloroplasts affected by red light. The response was of the weak-light type, irrespective of the light intensity. The most effective fluence rate for red light was found to be below 7·2 μmol m^–2 s^–1 (1 W m^–2). The effect of red light was far-red reversible, indicating phytochrome involvement. Chloroplast responses were more dynamic in A. capillus-veneris and P. cretica, the two species that exhibited higher environmental flexibility.     

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.