Relationship between starch degradation and carbon demand for maintenance and growth in Arabidopsis thaliana in different irradiance and temperature regimes

Experiments were designed to compare the relationship between starch degradation and the use of carbon for maintenance and growth in Arabidopsis in source‐limited and sink‐limited conditions. It is known that starch degradation is regulated by the clock in source‐limited plants, which degrade their starch in a linear manner such that it is almost but not completely exhausted at dawn. We asked whether this response is maintained under an extreme carbon deficit. Arabidopsis was subjected to a sudden combination of a day of low irradiance, to decrease starch at dusk, and a warm night. Starch was degraded in a linear manner through the night, even though the plants became acutely carbon starved. We conclude that starch degradation is not increased to meet demand in carbon‐limited plants. This network property will allow stringent control of starch turnover in a fluctuating environment. In contrast, in sink‐limited plants, which do not completely mobilize their starch during the night, starch degradation was accelerated in warm nights to meet the increased demand for maintenance and growth. Across all conditions, the rate of growth at night depends on the rate of starch degradation, whereas the rate of maintenance respiration decreases only when starch degradation is very slow.     

Effects of light and prey availability on nocturnal, lunar and seasonal activity of tropical nightjars

Nightjars and their allies represent the only major group of visually hunting aerial insectivores with a crepuscular and/or nocturnal lifestyle. Our purpose was to examine how both light regime and prey abundance in the tropics, where periods of twilight are extremely short, but nightjar diversity is high, affect activity across different temporal scales. We studied two nightjar species in West African bush savannah, standard‐winged nightjars Macrodipteryx longipennis Shaw and long‐tailed nightjars Caprimulgus climacurus Vieillot. We measured biomass of potential prey available using a vehicle mounted trap and found that it was highest at dusk and significantly lower at dawn and during the night. Based on direct observations, both nightjars exhibit the most intense foraging behaviour at dusk, less intense foraging at dawn and least at night, as predicted by both prey abundance and conditions for visual prey detection. Nocturnal foraging was positively correlated with lunar light levels and ceased below about 0.03 mW m^?2. Over the course of a lunar cycle, nocturnal light availability varied markedly, while prey abundance remained constant at dusk and at night was slightly higher at full moon. Both species increased twilight foraging activity during new moon periods, compensating for the shorter nocturnal foraging window at that time. Seasonally, the pattern of nocturnal light availability was similar throughout the year, while prey availability peaked shortly after onset of the wet season and then slowly decreased over the following four months. The courtship and breeding phenology of both species was timed to coincide with the peak in aerial insect abundance, suggesting that prey availability rather than direct abiotic factors act as constraints, at least at the seasonal level. Our findings illustrate the peculiar constraints on visually orienting aerial nocturnal insectivores in general and tropical nightjars in particular and highlight the resulting nocturnal, lunar and seasonal allocation of activities.     

Non‐Parametric Entrainment by Natural Twilight in the Microchiropteran Bat,Hipposideros Speoris Inside a Cave

The study aimed to determine the influence of repeated natural dawn and dusk twilight pulses in entraining the circadian flight activity rhythm of the microchiropteran bat, Hipposideros speoris, free‐running in constant darkness in a natural cave. The bats were exposed to repeated dawn or dusk twilight pulses at eight circadian phases. All bats exposed to dawn twilight pulses were entrained by advancing transients, and the stable entrainment was reached when the onset of activity occurred about 12 h before the lights‐on of the pulses, irrespective of the initial phase at which the bats were exposed to twilight. All bats exposed to dusk twilight pulses, however, were entrained by delaying transients, and the stable entrainment was reached when the onset of activity occurred about 1.6 h after the lights‐on of the pulses. The entrainment caused by dawn and dusk twilight pulses is discussed in the context of the postulated two photoreceptors: the short wavelength sensitive (S) photoreceptors mediating entrainment via dusk twilight, and the medium wavelength sensitive (M) photoreceptors mediating entrainment via dawn twilight.     

Non-parametric entrainment by natural twilight in the microchiropteran bat, Hipposideros speoris inside a cave

The study aimed to determine the influence of repeated natural dawn and dusk twilight pulses in entraining the circadian flight activity rhythm of the microchiropteran bat, Hipposideros speoris, free-running in constant darkness in a natural cave. The bats were exposed to repeated dawn or dusk twilight pulses at eight circadian phases. All bats exposed to dawn twilight pulses were entrained by advancing transients, and the stable entrainment was reached when the onset of activity occurred about 12 h before the lights-on of the pulses, irrespective of the initial phase at which the bats were exposed to twilight. All bats exposed to dusk twilight pulses, however, were entrained by delaying transients, and the stable entrainment was reached when the onset of activity occurred about 1.6 h after the lights-on of the pulses. The entrainment caused by dawn and dusk twilight pulses is discussed in the context of the postulated two photoreceptors: the short wavelength sensitive (S) photoreceptors mediating entrainment via dusk twilight, and the medium wavelength sensitive (M) photoreceptors mediating entrainment via dawn twilight.     

Natural entrainment without dawn and dusk: the case of the European ground squirrel (Spermophilus citellus).

Observational data collected in the field and in enclosures show that diurnal, burrow-dwelling European ground squirrels (Spermophilus citellus) never were above ground during twilight at dawn or at dusk. The animals emerged on average 4.02 h (SD = 0.45) after civil twilight at dawn and retreated in their burrows on average 2.87 h (SD = 0.47) before civil twilight at dusk. Daily patterns of light perceived by these burrowing mammals were measured with light-sensitive radio collar transmitters in an enclosure (the Netherlands) and in the field (Hungary). The observational data are corroborated by the telemetry data, which show clear daily patterns of timing of light perception including light perceived from the burrow entrances. The first light was observed by the animals on average 3.54 h (enclosure, SD = 0.45) and 3.60 h (field, SD = 0.31) after civil twilight at dawn, whereas the final observed light was on average 3.04 h (enclosure, SD = 0.64) and 2.02 h (field, SD = 0.72) before civil twilight at dusk. Thus, the animals do not perceive the rapid natural light-dark (LD) transitions that occur at civil twilight. Instead, they generate their own pattern of exposure to light within the natural LD cycle. The classical phase response model for entrainment by light or dark pulses cannot explain how the circadian system of this species remains entrained to the external, natural LD cycle while the major LD transitions are created by its own behavior.     

The influence of light on the twilight migrations of grunts

Synopsis Behaviors that precede the daily migrations of mixed-species schools of juvenile grunts (Pomadasyidae), from patch reefs to grass beds at dusk and vice versa at dawn, are defined and utilized to ascertain the precision of the migrations. Although premigratory behaviors differ at dusk and dawn, the migrations are precise twilight events which occur at the same light intensities during dawn and dusk. Histological sections of the retina reveal that both cones and rods are fully exposed to ambient light during the migrations. Under the difficult photic conditions that prevail during migration, the retina is structured photomechanically to maximize the absorption of ambient light. Body colorations of the grunts, which consist mostly of intense colored stripes during the day, are replaced at night by cryptic melanic patterns. The precision of migration, the photomechanical movements in the retina, and the changes in body coloration are considered adaptive because they reduce predation on grunts when they migrate and are most vulnerable to attack. In support of this conclusion, the migrations take place just before the evening and just after the morning quiet period - thus they avoid that period during twilight when predation is highest in tropical fish communities.     

A telemetry study on the diurnal distribution and activity of adult pikeperch, Sander lucioperca (L.), in a riverine environment

The diurnal movements and spatial distribution of adult pikeperch, Sander lucioperca, in the Elbe River, Czech Republic was observed using radio telemetry. The hypothesis that light intensity, within four different intervals (dawn, day, dusk, night), would determine the spatial distribution of pikeperch in a riverine environment were tested across a time span of 12 months. During the day, fish were located in deep water of the main channel, moving towards shallower waters during twilight and residing in the littoral zone, closest to the riverbanks, at night. Movement activity followed the behavioural pattern in a drainage canal with maximum at twilight and minimum at night. This suggests that nocturnal positions of adult pikeperch in the shallows were not associated with hunting but more likely with resting. Handling editor: J. Cambray     

The diurnal metabolism of leaf starch

Starch is a primary product of photosynthesis in leaves. In most plants, a large fraction of the carbon assimilated during the day is stored transiently in the chloroplast as starch for use during the subsequent night. Photosynthetic partitioning into starch is finely regulated, and the amount of carbohydrate stored is dependent on the environmental conditions, particularly day length. This regulation is applied at several levels to control the flux of carbon from the Calvin cycle into starch biosynthesis. Starch is composed primarily of branched glucans with an architecture that allows the formation of a semi-crystalline insoluble granule. Biosynthesis has been most intensively studied in non-photosynthetic starch-storing organs, such as developing seeds and tubers. Biosynthesis in leaves has received less attention, but recent reverse-genetic studies of Arabidopsis (thale cress) have produced data generally consistent with what is known for storage tissues. The pathway involves starch synthases, which elongate the glucan chains, and branching enzymes. Remarkably, enzymes that partially debranch glucans are also required for normal amylopectin synthesis. In the last decade, our understanding of starch breakdown in leaves has advanced considerably. Starch is hydrolysed to maltose and glucose at night via a pathway that requires recently discovered proteins in addition to well-known enzymes. These sugars are exported from the plastid to support sucrose synthesis, respiration and growth. In the present review we provide an overview of starch biosynthesis, starch structure and starch degradation in the leaves of plants. We focus on recent advances in each area and highlight outstanding questions.     

Trehalose induces the ADP-glucose pyrophosphorylase gene, ApL3, and starch synthesis in Arabidopsis

In Arabidopsis, genes encoding functional enzymes for the synthesis and degradation of trehalose have been detected recently. In this study we analyzed how trehalose affects the metabolism and development of Arabidopsis seedlings. Exogenously applied trehalose (25 mM) strongly reduced the elongation of the roots and, concomitantly, induced a strong accumulation of starch in the shoots, whereas the contents of soluble sugars were not increased. When Arabidopsis seedlings were grown on trehalose plus sucrose (Suc), root elongation was restored, but starch still accumulated to a much larger extent than during growth on Suc alone. The accumulation of starch in the shoots of trehalose-treated seedlings was accompanied by an increased activity of ADP-glucose pyrophosphorylase and an induction of the expression of the ADP-glucose pyrophosphorylase gene, ApL3. Even in the presence of 50 mM Suc, which itself also slightly induced ApL3, trehalose (5 mM) led to a further increase in ApL3 expression. These results suggest that trehalose interferes with carbon allocation to the sink tissues by inducing starch synthesis in the source tissues. Furthermore, trehalose induced the expression of the beta-amylase gene, AT-beta-Amy, in combination with Suc but not when trehalose was supplied alone, indicating that trehalose can modulate sugar-mediated gene expression.     

Starch turnover: pathways, regulation and role in growth

Many plants store part of their photosynthate as starch during the day and remobilise it to support metabolism and growth at night. Mutants unable to synthesize or degrade starch show strongly impaired growth except in long day conditions. In rapidly growing plants, starch turnover is regulated such that it is almost, but not completely, exhausted at dawn. There is increasing evidence that premature or incomplete exhaustion of starch turnover results in lower rates of plant growth. This review provides an update on the pathways for starch synthesis and degradation. We discuss recent advances in understanding how starch turnover and the use of carbon for growth is regulated during diurnal cycles, with special emphasis on the role of the biological clock. Much of the molecular and genetic research on starch turnover has been performed in the reference system Arabidopsis. This review considers to what extent information gained in this weed species maybe applicable to annual crops and perennial species.     

Photoperiodism and the photic environment of the pitcher-plant mosquito,Wyeomyia smithii

Summary Wyeomyia smithii Coq. (Diptera: Culicidae) overwinters as a larva in a state of diapause which is initiated, maintained, and terminated by photoperiod. Both in the dawn and in the dusk, diapausing larvae are photoperiodically most sensitive to blue light (390–450 nm) with a shoulder in response in the bluegreen and green (480–540 nm) region of the spectrum. The saturation curves for response to blue light in the dusk has a steeper slope than for response to blue-green and green light in the dusk, suggesting two distinct pigments or pigment complexes underly photoperiodic response in W. smithii.The photic environment of W. smithii during twilight is rich in yellow-green light but sufficient light is available at 390–540 nm to trigger photoperiodic response early during morning civil twilight and to sustain response until late in evening civil twilight. Comparison of action spectra with spectra of available light indicates that the zenith angles of the sun which would result in 50% response are 95°48 and 94°52 in the dawn and dusk, respectively. Using these zenith angles to approximate daylength in nature provides a resonable prediction of development in the field.The flux density of photons necessary to elicit 50% development a 454 nm is about 9×10⁷ photons/cm² s in the dawn and 3×10⁸ photons/cm² s in the dusk. This high degree of sensitivity enables W. smithii to cue to the rapidly changing light intensity which occurs around the nautical-civil twilight transition. At the same time, the chromophore is not likely to be stimulated by the light of the full moon.     

Circadian control of root elongation and C partitioning in Arabidopsis thaliana

Plants grow in a light/dark cycle. We have investigated how growth is buffered against the resulting changes in the carbon supply. Growth of primary roots of Arabidopsis seedlings was monitored using time‐resolved video imaging. The average daily rate of growth is increased in longer light periods or by addition of sugars. It responds slowly over days when the conditions are changed. The momentary rate of growth exhibits a robust diel oscillation with a minimum 8–9 h after dawn and a maximum towards the end of the night. Analyses with starch metabolism mutants show that starch turnover is required to maintain growth at night. A carbon shortfall leads to an inhibition of growth, which is not immediately reversed when carbon becomes available again. The diel oscillation persists in continuous light and is strongly modified in clock mutants. Central clock functions that depend on CCA1/LHY are required to set an appropriate rate of starch degradation and maintain a supply of carbon to support growth through to dawn, whereas ELF3 acts to decrease growth in the light period and promote growth in the night. Thus, while the overall growth rate depends on the carbon supply, the clock orchestrates diurnal carbon allocation and growth.     

The diel activity pattern of mountain hare (Lepus timidus) on managed heather moorland in Scotland

The research presented in this paper provides an insight into the behavioral ecology of mountain hares on heather moorland in the Lammermuir Hills of southeast Scotland. We examine the seasonal and diel activity patterns using camera traps over a period of 12 months. The rate of camera detections was calculated for the different divisions of the 24‐hr cycle (daylight, dusk, night, and dawn). During autumn and winter (October–February), the activity pattern was crepuscular with greater activity at dusk than at dawn. Daylight activity was relatively low, and there was a regular pattern of small peaks of activity during the night. In spring and summer (March–September), peaks of crepuscular activity remained evident but daylight activity was much more prevalent than during autumn and winter, and night activity was lower. We discuss the problematic definition of twilight and present an explanation for seasonal changes in the pattern of diel activity that is linked to the reproductive cycle of the mountain hare.     

Foraging rhythm of great crested grebes Podiceps cristatus adjusted to diel variations in the vertical distribution of their prey Osmerus eperlanus in a shallow eutrophic lake in The Netherlands

Summary Great crested grebes Podiceps cristatus only foraged for an hour or two during dawn and again during dusk on Lake IJsselmeer in August-September. During this time of the year the adult birds are in wing moult and temporarily unable to fly. The food of grebes consisted almost exclusively of smelt Osmerus eperlanus, the most numberous pelagic fish. Simultaneous sonar registrations and trawl net fishing showed that smelt moved to the water surface during the twilight periods. During day and night they were concentrated near the bottom. We argue that grebes have the best foraging opportunities during twilight when much of their prey is near the surface, where light intensities allow the fish to be detected and captured. When the smelt are in the upper water layers the distance to the covered to get the prey (i.e. diving time and cost) is also least.     

Activity patterns of the lesser horseshoe bat Rhinolophus hipposideros at summer roosts

Dusk to dawn observations, using a bat detector and occasionally an image intensifier, were made outside two nursery roosts of lesser horseshoe bats Rhinolophus hipposideros from late May to September. Emergence was correlated with sunset but delayed by extended twilight. Light intensity was important in triggering departure and cloud cover advanced it. Light-testing behaviour was invariably undertaken, in the form of brief flights out and back into the roosts. The exit from one roost was shaded by trees and exploratory flights were generally more extended there. Heavy rain inhibited emergence. There was almost always intermittent activity throughout the night, with many individuals returning and departing, and no indication of seasonal or overnight peaks. A bat detector inside a third roost confirmed overnight observations at the other two. Some bats often returned to the roost for the night before dawn. Dawn return was linked to sunrise, prolonged twilight in midsummer hastening it. Colony size varied appreciably over periods of a few days and even overnight. There is some limited evidence that increased colony size, perhaps through social interaction, may have influenced timing of departure at dusk and return at dawn.