Does light inhibit ethylene production in leaves?

The effect of light on the rate of ethylene production was monitored using two different techniques—leaf segments incubated in closed flasks versus intact plants in a flow-through open system. Three different plants were used, viz sunflower (Helianthus annuus), tomato (Lycopersicon esculentum), and soybean (Glycine max). Experiments were conducted both in the presence and absence of 1-aminocyclopropane-1-carboxylic acid (ACC).

The results obtained indicate that, in all three species studied, light strongly inhibits ethylene production when cut leaf segments are incubated in the presence of ACC in closed flasks. When ethylene measurements are made with ACC-sprayed intact plants using a continuous flow system, the effect of light on ethylene production is only marginal. In leaf segments of sunflower and soybean incubated only in distilled H₂O in closed flasks, light promotes ethylene production. In tomato, there is no difference between the rate of ethylene production between light and darkness under such conditions. When measurements are made with intact plants in a continuous flow system, the rate of ethylene production is almost identical in light and darkness, in the three plants studied.

It is concluded that the effect of light on cut leaf segments incubated in the presence of ACC in closed flasks can be attributed to the techniques used for these measurements. Light has little effect on ethylene production by intact plants in an open system.

     

Does light attract piglets to the creep area?

Hypothermia, experienced by piglets, has been related to piglet deaths and high and early use of a heated creep area is considered important to prevent hypothermia. The aims of the present study were to investigate how a newly invented radiant heat source, eHeat, would affect piglets’ use of the creep area and whether light in the creep area works as an attractant on piglets. A total of 39 sows, divided between two batches, were randomly distributed to three heat source treatments: (1) standard infrared heat lamp (CONT, n=19), (2) eHeat with light (EL, n=10) and (3) eHeat without light (ENL, n=10). Recordings of piglets’ use of the creep area were made as scan sampling every 10 min for 3 h during two periods, one in daylight (0900 to 1200 h) and one in darkness (2100 to 2400 h), on day 1, 2, 3, 7, 14 and 21 postpartum. On the same days, piglets were weighted. Results showed an interaction between treatment and observation period (P<0.05) with a lower use of the creep area during darkness compared with daylight for CONT and EL litters, but not for ENL litters. Piglets average daily weight gain was not affected by treatment, but was positively correlated with piglets’ birth weight and was lower in batch 1 compared with batch 2. Seen from the present results, neither eHeat nor light worked as an attractant on piglets; in contrast, piglets preferred to sleep in the dark and it would therefore be recommended to turn off the light in the creep area during darkness. Heating up the creep area without light can be accomplished by using a radiant heat source such as eHeat in contrast to the normally used light-emitting infrared heat lamp.     

Does light exposure make plant litter more degradable?

Many field experiments have indicated that litter decomposition in semi-arid areas may be partly or fully controlled by photodegradation. We devised a study to test our hypothesis that light exposure makes plant litter more degradable. Dry, senescent, aboveground plant litter from Miscanthus x giganteus was exposed to light including ultraviolet (UV) radiation for various lengths of time from 0 to 289 days. Weight loss was measured after exposure and appeared to be modest and did not increase with time of exposure. The litter of the longest and shortest exposure time as well as controls were then incubated with soil and moisture for 35 days and CO₂ and N₂O production were measured. The longest exposed litter degraded much faster than any other treatment during incubation with moisture, about twice as fast as the unexposed control. The shortest exposed however, degraded only slightly faster than the unexposed control. This suggests that increasing litter degradability is a more important mechanism for photodegradation than direct light-induced mass loss. N₂O production from decomposition of the exposed litter was high in the beginning, suggesting that nitrogen may be released quickly. The mechanism is probably that light exposure leaves the nitrogen in plant litter easily available to microbial utilisation upon wetting. Such a mechanism might play an important role for nutrient cycling in semi-arid areas.     

Leaf phyllotaxis: Does it really affect light capture?

The intriguing mathematical properties of leaf phyllotaxis still attract scientific attention after centuries of research. Phyllotaxis, and in particular the divergence angle between successive leaves, have been frequently interpreted in terms of maximization of light capture, although certain model simulations of light capture by vertical shoots revealed minor effects of phyllotaxis in comparison with the effect of other morphological features of the plant. However, these simulations assumed a number of simplifications, did not take into account diffuse light, and were not based on real plants with their natural range of morphological variation. This study was aimed at filling these gaps by examining the influence on light harvesting of shoot architecture and divergence angle in four species with spiral phyllotaxis (Quercus ilex, Arbutus unedo, Heteromeles arbutifolia and Daphne gnidium) with a realistic 3-D model (Y-plant). A wide range of divergence angles (from 100° to 154°) was observed within each species, with 144° being the most frequent one. These different divergence angles rendered very different vertical projections of the shoot due to contrasting patterns of leaf overlap as seen from above, but they rendered indistinguishable light interception efficiencies (Ea). Setting the leaves with an opposite-decussate phyllotaxis led, however, to a 40–50% decrease of Ea. The interplay of internode length, leaf size and shape, and leaf elevation angle led to significant species differences in Ea. Thus, only particular phyllotaxis (e.g., decussate) might be functionally inefficient under certain combinations of the various morphological variables that influence light capture of a shoot. This revised version was published online in June 2006 with corrections to the Cover Date.     

Do we taste fat?

Sense of taste informs the body about the quality of ingested foods. Five sub-modalities allowing the perception of sweet, salty, sour, bitter, and umami stimuli are classically depicted. However, the inborn attraction of mammals for fatty foods raises the possibility of an additional orosensory modality devoted to fat perception. For a long time, dietary lipids were thought to be detected only by trigeminal (texture perception), retronasal olfactory, and post-ingestive cues. This minireview analyses recent findings showing that gustation also plays a significant role in dietary lipid perception.     

Does bacteriorhodopsin energize the membranes of animal mitochondria under light?

A suspension of freeze-thawed mitochondria mixed with purple membranes from Halobacteria was illuminated with visible light. It was found that the light exposure prevented inhibition of succinate oxidation. The illumination also led to a decrease in inhibition of the rate of ferricyanide reduction by rat liver mitochondria in the presence of succinate. Both phenomena are explained by the fact that oxalacetate inhibition of succinate dehydrogenase is prevented by light-induced energization of mitochondrial membranes due to the contact with purple membranes.     

Does light intensity modify the effect mayfly grazers have on periphyton?

1. A factorial experiment was conducted in artificial outdoor streams to quantify the effects of irradiance (two levels) and two mayfly grazers (four densities of each) on periphytic community structure. The mayflies were Ecdyonurus venosus (Heptageniidae), a grazer using brushing mouthparts, and Baetis spp. (Baetidae) a grazer which uses mandibles and maxilla to scrape and gather periphyton. The experiment ran for 16 days. 2. Grazer densities in channels approximated those existing in a shoreline habitat in the River Sihl, Switzerland. Light treatments were natural (daily mean = 810 μmol m^–2 s^–1) and shaded (daily mean = 286 μmol m^–2 s^–1). 3. Higher irradiance increased total algal abundance by a factor of 4. Algae most affected were prostrate/motile and erect diatoms, filamentous chlorophytes and Hydrurus foetidus. 4. Both species of mayfly reduced periphytic and algal biomass. Mayfly–mayfly interactions, however, were associated with statistical increases in algal biovolume and chlorophyll-a content, indicating that the two grazers may have interfered with one another as their densities increased. The mayfly–mayfly interaction did not influence periphytic ash-free dry mass (AFDM). Light modified the influence of Ecdyonurus such that this mayfly produced greater reductions in algal biovolume under high irradiance. 5. Despite efforts to exclude other grazers, chironomids colonized experimental channels. Chironomid biomass was approximately eight times less than mayflies across treatments and was positively correlated with all measures of periphytic abundance, suggesting that these grazers were responding to periphyton rather than controlling it. Chironomids were also associated with an increase in the abundance of diatoms having a prostrate/motile physiognomy. The only physiognomy to show a negative relationship with chironomid biomass was the thallus type, a form which comprised less than 1% of the algal biovolume across channels. 6. Ecdyonurus and Baetis had distinct influences on algal physiognomy. Ecdyonurus, for example, reduced adnate, stalked and Achnanthes-type physiognomies, but was associated with a significant increase in the abundance of filamentous chlorophytes (primarily Ulothrix sp.). Baetis reduced erect, Achnanthes-type and thallus physiognomies. Neither mayfly influenced the abundance of prostrate/motile diatoms; a physiognomy that comprised 21% of the algae in channels. 7. Light and mayfly interactions affected algal community structure. The interaction of Ecdyonurus with light had a negative effect on erect diatoms, filamentous chlorophytes and the thallus physiognomy, but a positive effect on stalked and Achnanthes-type physiognomies. Baetis interacting with light had a positive effect on adnate diatoms. 8. Although both mayfly taxa influenced periphytic community structure, physiognomy was not a good predictor of algal susceptibility to grazing. The type of substratum to which an alga is attached (detritus or algal filaments vs hard surfaces) and location within the periphytic matrix may be better indicators of vulnerability to grazing than physiognomy.     

Does conditioned taste aversion learning in the pond snail Lymnaea stagnalis produce conditioned fear?

In conditioned taste aversion (CTA) training performed on the pond snail Lymnaea stagnalis, a stimulus (the conditional stimulus, CS; e.g., sucrose) that elicits a feeding response is paired with an aversive stimulus (the unconditional stimulus, US) that elicits the whole-body withdrawal response and inhibits feeding. After CTA training and memory formation, the CS no longer elicits feeding. We hypothesize that one reason for this result is that after CTA training the CS now elicits a fear response. Consistent with this hypothesis, we predict the CS will cause (1) the heart to skip a beat and (2) a significant change in the heart rate. Such changes are seen in mammalian preparations exposed to fearful stimuli. We found that in snails exhibiting long-term memory for one-trial CTA (i.e., good learners) the CS significantly increased the probability of a skipped heartbeat, but did not significantly change the heart rate. The probability of a skipped heartbeat was unaltered in control snails given backward conditioning (US followed by CS) or in snails that did not acquire associative learning (i.e., poor learners) after the one-trial CTA training. These results suggest that as a consequence of acquiring CTA, the CS evokes conditioned fear in the conditioned snails, as evidenced by a change in the nervous system control of cardiac activity.     

Does light explain damselfish Chromis viridis abundances observed over coral colonies?

A single autonomous video camera was used to record the abundances of Chromis viridis over a branching Acropora sp. colony eight times per day over a period of 50 days. The poor explanatory power of global radiation suggests the need for recording the light really available to the fish, especially in the UV range. The increasing number of C. viridis observed with increasing wind along shore and water level may correspond to individuals swimming further from their shelter in order to get closer to the food carried by the water currents.     

Does artificial light pollution impair problem‐solving success in peafowl?

Behavioral innovations allow animals to adjust their behavior to solve novel problems. While innovative behavior can be important for animals living in new environments, anthropogenic pollution may limit their ability to adapt by impairing cognition or motivation. In particular, exposure to light pollution at night can cause sleep deprivation and may, therefore, hinder innovative behavior. To test this hypothesis, we examined experimentally whether exposure to acute light pollution impacts problem‐solving success in peafowl (Pavo cristatus). After peafowl were exposed to artificial light pollution for one night, they were presented with a problem‐solving task in which they could extract food by piercing the lid of an unfamiliar food bowl. Their problem‐solving success was unrelated to short‐term light pollution exposure. Other factors, including persistence, sex of the bird, and moon illumination, influenced their success in solving the task. The results suggest that short‐term exposure to light pollution does not limit behavioral innovation, but long‐term studies are necessary to further probe this question.     

Does light pollution alter daylength? A test using light loggers on free-ranging European blackbirds (Turdus merula)

Artificial light at night is one of the most apparent environmental changes accompanying anthropogenic habitat change. The global increase in light pollution poses new challenges to wild species, but we still have limited understanding of the temporal and spatial pattern of exposure to light at night. In particular, it has been suggested by several studies that animals exposed to light pollution, such as songbirds, perceive a longer daylength compared with conspecifics living in natural darker areas, but direct tests of such a hypothesis are still lacking. Here, we use a combination of light loggers deployed on individual European blackbirds, as well as automated radio-telemetry, to examine whether urban birds are exposed to a longer daylength than forest counterparts. We first used activity data from forest birds to determine the level of light intensity which defines the onset and offset of daily activity in rural areas. We then used this value as threshold to calculate the subjective perceived daylength of both forest and urban blackbirds. In March, when reproductive growth occurs, urban birds were exposed on average to a 49-min longer subjective perceived daylength than forest ones, which corresponds to a 19-day difference in photoperiod at this time of the year. In the field, urban blackbirds reached reproductive maturity 19 day earlier than rural birds, suggesting that light pollution could be responsible of most of the variation in reproductive timing found between urban and rural dwellers. We conclude that light at night is the most relevant change in ambient light affecting biological rhythms in avian urban-dwellers, most likely via a modification of the perceived photoperiod.     

Does green light influence the fluorescence properties and structure of phototrophic biofilms?

Artificial illumination can harm works of art by inducing the development of photosynthetic biofilms. With the aim of preventing biodeterioration or esthetic damage to such surfaces, we evaluated and compared the effects of illuminating biofilms formed by Gloeothece membranacea (cyanobacteria) and Chlorella sorokiniana (Chlorophyta) using exclusively white or green light.     

Does light cause internal cancers? The problem and challenge of an ubiquitous exposure

Visible light of sufficient intensity and duration inhibits melatonin biosynthesis, and experimental studies suggest that melatonin may protect against cancer. From a public health point of view it is important to verify or falsify the hypothesis that artificial light--or even sunlight itself--suppresses melatonin production sufficiently to increase the risk of developing cancers of internal organs in man. Epidemiology is a discipline that can contribute to in-vivo verification of experimental findings. But when attempting to study the effects of light on man, epidemiologists are faced with a major problem: the ubiquitous nature of natural and anthropogenic light, which renders everyone, everywhere exposed. The challenge is to identify populations with demonstrable varying exposures to light. This paper summarizes how recent epidemiological investigations have sought to tackle the problem by studying shift-workers, blind people and Arctic residents. It is suggested that future studies should test the underlying assumptions regarding endocrine responses to light, i.e., that melatonin levels are reduced among shift-workers, and that they are increased among the blind and those who live in the Arctic. A systematic investigation of exposure-response relationships could be based on "light dosimetry by geography". Such a study is envisaged by European researchers who aim to study melatonin and other hormones in samples from healthy general populations that are differentially exposed to light by virtue of varying ambient photoperiods. Further methodologic options for prospective and retrospective epidemiologic studies are suggested. It is concluded that the biologically plausible link between ubiquitous light, hormones and the development of very frequent malignancies such as breast cancer and prostate cancer should be investigated rigorously by additional well-designed epidemiological research.     

Does pupil constriction under blue and green monochromatic light exposure change with age?

Many nonvisual functions are regulated by light through a photoreceptive system involving melanopsin-expressing retinal ganglion cells that are maximally sensitive to blue light. Several studies have suggested that the ability of light to modulate circadian entrainment and to induce acute effects on melatonin secretion, subjective alertness, and gene expression decreases during aging, particularly for blue light. This could contribute to the documented changes in sleep and circadian regulatory processes with aging. However, age-related modification in the impact of light on steady-state pupil constriction, which regulates the amount of light reaching the retina, is not demonstrated. We measured pupil size in 16 young (22.8±4 years) and 14 older (61±4.4 years) healthy subjects during 45-second exposures to blue (480 nm) and green (550 nm) monochromatic lights at low (7×10(12) photons/cm2/s), medium (3×10(13) photons/cm2/s), and high (10(14) photons/cm2/s) irradiance levels. Results showed that young subjects had consistently larger pupils than older subjects for dark adaptation and during all light exposures. Steady-state pupil constriction was greater under blue than green light exposure in both age groups and increased with increasing irradiance. Surprisingly, when expressed in relation to baseline pupil size, no significant age-related differences were observed in pupil constriction. The observed reduction in pupil size in older individuals, both in darkness and during light exposure, may reduce retinal illumination and consequently affect nonvisual responses to light. The absence of a significant difference between age groups for relative steady-state pupil constriction suggests that other factors such as tonic, sympathetic control of pupil dilation, rather than light sensitivity per se, account for the observed age difference in pupil size regulation. Compared to other nonvisual functions, the light sensitivity of steady-state pupil constriction appears to remain relatively intact and is not profoundly altered by age.     

Does far-red light affect growth and mycorrhizas of Scots pine seedlings grown in forest soil?

We studied the response of Scots pine (Pinus sylvestris L.) to supplementary far-red sidelight in seedlings grown in a forest soil substrate without additional nutrient supply. Our aim was to determine possible changes in the accumulation and allocation of dry weight and mineral nutrients and the presence of mycorrhizas. Half of the seedlings were grown in light conditions simulating reflected far-red light (FR) from neighbouring plants and the other half were controls not receiving additional FR. PAR irradiance was kept constant in both treatments. At the first harvest (41 d of treatment), FR+ had no effect on stem height, biomass accumulation or allocation. However, at the end of the experiment (93 d of treatment), an increase in stem extension rate and stem dry weight was observed in FR+ seedlings when compared to controls. Both control and treated plants had several morphological types of ectomycorrhizas, but no effect of FR+ on the frequency of these morphotypes was observed. Nor was the concentration of ergosterol or estimated mycorrhizal fungal biomass affected. Nutrients were more responsive to the light quality treatment: P concentration in roots and N and P contents in stems and roots were higher in FR+ plants than in control seedlings. These results are in contrast to those of a previous study in which the root system of pine seedlings, which were fertilised and had less developed mycorrhizas, was reduced by FR+. This revised version was published online in June 2006 with corrections to the Cover Date.     

Does complementary resource use enhance ecosystem functioning? A model of light competition in plant communities

Recent experiments on grassland ecosystems have shown that biodiversity can enhance ecosystem processes such as plant biomass production. Functional complementarity is generally regarded as the main class of mechanisms generating these effects of biodiversity on ecosystem functioning. Although intuitively appealing and supported by some data, the complementarity hypothesis has been little explored theoretically using mechanistic approaches. Here, we present a simple dynamical model for a light-limited terrestrial ecosystem to assess the effects of species diversity on light competition and total biomass in plant communities. Our model shows that competitive relaxation (reduction in average light competition intensity) due to differences in foliar architecture among species enhances total plant biomass in mixtures, but that competitive imbalance (generated by the variance of the average light competition intensity experienced by different species) can either reinforce the effect of competitive relaxation or counteract it and contribute to reducing total plant biomass. Thus, complementary resource use is not enough to increase total plant biomass in species-rich communities; competitive balance among species also plays an important role. We propose an operational measure of light-use complementarity using empirical field data on light absorption to test the presence of complementarity in natural plant communities.