Photosynthetically Active Radiation Levels Research Articles

UV-B as a Photoacclimatory Enhancer of the Hermatypic Coral <i>Stylophora pistillata</i>

Photoacclimation processes are crucial for the survival of all photosynthetic organisms in the photic zone. Changes in photosynthetic active radiation (PAR) are however simultaneous to changes in UV-B radiation. The influence of UV-B levels on bio-optical and physiological parameters of deep (30 m) corals from the species Stylophora pistillata and their symbiotic algae, zooxanthellae, was examined during their gradual, stepwise acclimation to a shallow depth (3 m). Drastic exposure of deeper corals to higher UV-B levels in shallower depths is usually fatal. Hence, the acclimation process lasted 118 days and included 10 intermediate stations with an addition of similar amount of PAR at each depth transfer. Concomitantly, in an on-shore experiment, fragments from the same colonies were acclimated by changing shading nets corresponding in PAR levels to each in situ station. Since UV-B is attenuated more efficiently than PAR in seawater, the PAR: UV-B ratio changes in the depth experiment while remaining constant under the neutral density nets. This provided the opportunity to evaluate the importance of UV-B to photoacclimation. In both experiments all fragments survived, in spite of a four-fold difference in levels of PAR and a 140-fold difference in UV-B flux between the initial and final conditions. Both experimental designs resulted in reduction of zooxanthellae density, photosynthesis rates, and quantum yields of PSII, while cellular chlorophyll content remained unaffected. Zooxanthellae density and maximal photosynthetic rate was found decreased in correlation with UV-B radiation, whether it was elevated logarithmically with reducing depths or linearly with reducing shades. Conversely, quantum yields of PSII were adjusted according to the enhancement of PAR rather than UV-B. We conclude that UV-B enhances the magnitude of photoacclimation to higher PAR. This novel aspect of photoacclimation can provide the basis for our understanding of the underlying mechanisms that result in UV-related bleaching.

Fruit, yield, and vegetative growth responses to photosynthetically active radiation during oil synthesis in olive trees

Maximizing productivity in super high density and intensive olive orchards requires proper management of illumination of the canopy walls and their interior. Currently, this is difficult to achieve due to the limited knowledge about the responses to incident photosynthetically active radiation (PAR) of yield determinants and components. We determined the response functions for PAR during the oil synthesis phase of yield components (fruit dry weight and oil concentration) of fruit at a height of 2m on the canopy periphery by applying several radiation levels (3, 20, 40, and 70% of incident PAR) to the north side (S hemisphere) of well-illuminated trees. The experiment was initiated after endocarp hardening as fruit number had already been established at that time. This avoided possible confounding effects due compensation between fruit number and size. Absence of differential fruit fall in response to treatments and of changes in (endocarp+seed) dry weight after application of treatment confirmed the achievement of this objective. Fruit dry weight, oil concentration, and, consequently, yield increased linearly with mean daily PAR receipt up to a threshold of 15mol PARm−2d−1 (i.e., 40% of PAR). In treatments with irradiance levels below this threshold the fruit became the priority sinks for assimilates, although their growth rate and oil concentration were reduced. Increments in length of non-fruiting branches and of trunk cross-sectional areas were substantially reduced in response to shading. We conclude that manipulation of PAR levels during the oil synthesis phase can reduce final fruit dry weight and oil concentration, confirms the existence of upper thresholds to PAR responses for these variables, and provides evidence that fruit growth has priority in the partitioning of photosynthate over vegetative growth under low to moderate levels of PAR.

Responses of marine primary producers to interactions between ocean acidification, solar radiation, and warming

Anthropogenic CO2 is accumulating in the atmosphere and trapping reflected infrared radiation, resulting in warming of both terrestrial and ocean ecosystems. At the same time, the dissolution of CO2 into seawater is increasing surface ocean acidity, a process known as ocean acidification. Effects of ocean acidification on marine primary producers have been docu- mented to be stimulative, inhibitive, or neutral. Elevated CO2 and reduced pH levels can interact with solar radiation, which fluctuates over different time scales from limiting to saturating or even stressful levels, to bring about synergistic, antagonistic, or balanced effects on marine primary producers at different depths or under changing weather conditions. However, shoaling of the upper mixed layer (enhanced stratification) due to ocean warming and freshening (rain, ice melt- ing) can lead to additional photosynthetically active radiation (PAR) and ultraviolet (UV) expo- sure, which can have both benefits and costs to photosynthetic organisms. Elevated CO2 concen- trations under low or moderate levels of PAR have been shown to enhance photosynthesis or growth of both phytoplankton and macroalgae; excessive levels of PAR, however, can lead to additional inhibition of photosynthesis or growth under elevated CO2, and addition of UV radia- tion (280 to 400 nm) can increase or down-regulate such inhibition, since solar UV-B (280 to 315 nm) radiation often harms algal cells, while UV-A (315 to 400 nm) at moderate levels stimu- lates photosynthetic carbon fixation in both phytoplankton and macroalgae. In view of warming effects, increased temperatures have been shown to enhance photorepair of UV-damaged mole- cules, though it simultaneously enhances respiratory carbon loss. The net effects of ocean acidifi- cation on marine primary producers are therefore largely dependent on the photobiological con- ditions (light limitation, light or UV stress), as well as interactions with rising temperature and other variables such as altered nutrient availability. Hence, feedbacks between changing carbon- ate chemistry and solar radiation across the entire spectrum present complications to interpret and understand ocean acidification effects based on single-factor experiments.

Effects of temperature, pH, and UV radiation on alkaline phosphatase activity in the terrestrial cyanobacterium Nostoc flagelliforme

Cyanobacteria produce phosphatases in response to phosphorus deficiency as some other autotrophs. However, little has been documented on the effects of key climate change factors, such as temperature rise and solar UV radiation (280–400 nm), on cyanobacterial alkaline phosphatase activity. Here, we found that the terrestrial cyanobacterium Nostoc flagelliforme showed higher activity of the enzyme with increasing temperature and pH levels, exhibiting maximal values at 45 °C and pH 11, respectively. However, when exposed to solar radiation in the presence of UV-A (320–400 nm) and UV-B (280–320 nm), significant reduction of the enzyme activity was observed at a photosynthetically active radiation (PAR) level of 300 W m−2 (1,450 μmol photons m−2 s−1), which is equivalent or lower than the noontime level of solar PAR at the organism's habitats. UV-A and UV-A + UV-B induced about 21 and 39 % inhibition of the enzyme activity in the 3-h exposures. The decrease in the activity of phosphatase can be attributed to the UV radiation-induced inactivation of the enzyme and indirectly to the UV radiation-induced production of reactive oxygen species.

Age and light effects on seedling growth in two alternative secondary successions in central Amazonia

Background : In central Amazonia, previous low intensity land use engenders succession dominated by Cecropia spp. which proceeds at high rates; however, at higher intensity of use succession is arrested and dominated by Vismia spp. over the long-term. Factors driving these two successional pathways are unknown. Aims : We aimed to elucidate seedling growth under the two alternative successional pathways. Methods : We experimentally determined the effects of successional age and photosynthetically active radiation (PAR) on relative height growth (RHG) of nine species of shade-tolerant tree seedlings in secondary forests dominated by Cecropia and Vismia, varying in age from 1–20 years. Results : In Cecropia-dominated successions, seedling RHG decreased with increasing successional age and with associated decreasing PAR. In Vismia-dominated successions, RHG was independent of successional age and PAR, and PAR did not change with successional age, being always higher than in Cecropia stands. The RHG of seedlings was lower in Vismia- than in Cecropia-dominated stands for similar PAR levels. Conclusions : Successional age and light availability affect seedlings growth differently in the two successional pathways. Unlike in Cecropia-dominated successions, in Vismia-dominated secondary forests seedling growth is limited by factors other than light. In a scenario of increasing land use intensity, constraints to seedling development in secondary forests can reduce species diversity in human-altered landscapes.

CO2-driven seawater acidification increases photochemical stress in a green alga

Liu Y., Xu J. and Gao K. 2012. CO2-driven seawater acidification increases photochemical stress in a green alga. Phycologia 51: 562–566. DOI: 10.2216/11-65.1Increased CO2 and associated acidification in seawater, known as ocean acidification, decreases calcification of most marine calcifying organisms. However, there is little information available on how marine macroalgae would respond to the chemical changes caused by seawater acidification. We hypothesized that down-regulation of bicarbonate acquisition by algae under increased acidity and CO2 levels would lower the threshold above which photosynthetically active radiation (PAR) becomes excessive. Juveniles of Ulva prolifera derived from zoospores were grown at ambient (390 µatm) and elevated (1000 µatm) CO2 concentrations for 80 days before the hypothesis was tested. Here, the CO2-induced seawater acidification increased the quantum yield under low levels of light, but induced higher nonphotochemical quenching under high light. At the same time, the PAR level at which photosynthesis became saturated was decreased and the photosynthetic affinity for CO2 or inorganic carbon decreased in the high-CO2 grown plants. These findings indicated that ocean acidification, as an environmental stressor, can reduce the threshold above which PAR becomes excessive.

Effect of temperature and light on the photosynthesis as measured by chlorophyll fluorescence of cultured Eucheuma denticulatum and Kappaphycus sp. (Sumba strain) from Indonesia

The photosynthetic performance of two Indonesian carrageenophytes (Solieriaceae), Eucheuma denticulatum and Kappaphycus sp. (so-called Sumba strain), was investigated under a variety of temperature and light conditions regarding their mariculture performance. A pulse amplitude modulated-chlorophyll fluorometer (Diving-PAM) was used to generate rapid light curves (RLCs) to provide estimates of the relative electron transport rates (rETR) for over 10 temperatures (i.e., from 16 to 34 °C) and at nine levels of photosynthetic active radiation, which ranged from 0 to 1,000 μmol photons m−2 s−1. Underwater irradiance in a cultivation area was also measured at the collection site in South Sulawesi, Indonesia. The initial slope (α), photoinhibition coefficient (β), and the coefficient of maximum photosynthesis assuming no photoinhibition (γ) was calculated by fitting the RLCs to a nonlinear model of the form \( {\text{rETR}} = \gamma \left( {1 - \exp \left( { - \frac{\alpha }{\gamma }{\text{PAR}}} \right)} \right)\left( {\exp \left( { - \frac{\beta }{\gamma }{\text{PAR}}} \right)} \right) \) using a two-level hierarchical Bayesian model. The experiments revealed that E. denticulatum and Kappaphycus sp. required temperatures ranging from 23 to 32 °C and 22 to 33 °C to maintain high rates of photosynthetic activity, respectively. Clearly, both species appear to be well-adapted to the natural light and temperature conditions at the cultivation site, and we expect the results of this study will be useful for the design and sustainable management of similar mariculture activity.

Shade light interaction with salicylic acid in regulating growth of sun (alpine) and shade (prairie) ecotypes of Stellaria longipes

The possible involvement of salicylic acid (SA) in a typical growth response of plants to shade light was investigated using the model system Stellaria longipes L. Goldie. The prairie (shade) ecotype of S. longipes is from foothills grassland habitat where it grows under shrubs or among taller grasses. The plants of this ecotype responded, as expected, with increased growth under lower red to far-red (R/FR) ratio and reduced photosynthetically active radiation (PAR). By contrast, the alpine (sun) ecotype is from an open sunny habitat, where canopy shade is a non-factor. The plants of this ecotype failed to respond with increased growth under a lower R/FR ratio, but had increased growth under a reduced PAR level. To examine the possible role of SA in shade light-mediated growth, the two main components of shade light, R/FR ratio and PAR, were uncoupled, and a series of experiments were performed by measuring the endogenous SA content and the plant response to exogenous SA concentrations. Contrary to the alpine plants, the prairie plants had increased endogenous SA content and higher shoot biomass accumulation under a low R/FR ratio treatment compared with normal or high R/FR ratios. Both alpine and prairie plants responded to a low PAR treatment with a decrease in endogenous SA content and an increase in shoot biomass accumulation, but the magnitude of this response was higher in prairie plants. Based on the results of this study, we conclude that shoot SA content is differentially regulated by both R/FR ratio and PAR signals, and SA may contribute, in ecotype specific manner, to growth changes in plants subjected to changing light environments.

Light interception principally drives the understory response to boxelder invasion in riparian forests

Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning.

Spatial variability of photosynthetically active radiation in European beech and Norway spruce

The vertical and horizontal variability of solar radiation within a mature European beech ( Fagus sylvatica L.)–Norway spruce ( Picea abies [L.] Karst) mixed stand in Southern Germany is investigated. A large dataset with more than one million spectral measurements of photon fluence rates at six vertical levels within the stand is analyzed with respect to tree species, meteorological sky conditions, and the influence of solar elevation angle on canopy penetration. Irradiance probability density functions of the photosynthetically active waveband are used to describe the three-dimensional radiation field. For a quantification of umbra, penumbra, and sunfleck frequencies, in-canopy fractions of photon fluence rates within the photosynthetically active waveband are investigated. Different phenological stages of beech and their effects on the in-canopy light climate are compared. The results show that during overcast conditions (OVC) fractions of photosynthetically active radiation (PAR) are higher at all canopy levels than during clear sky (CS) conditions due to their exclusively diffuse character. The lowest median PAR level of less than 1% of above-canopy PAR can be observed in the shade crown of beech and at ground level. More PAR can penetrate the canopy at a higher solar elevation under CS conditions. This effect is more pronounced for spruce than for beech due to the conical crown shape of the conifers that allows photons from higher angles to enter the gaps inbetween trees in contrast to the more homogeneously closed beech canopy. Solar elevation is not an important factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing species or different sky conditions. The frequency of sunflecks differs more by species and by the vertical position within the canopy than by sky condition.

Phytochemical Changes Induced by Different Nitrogen Supply Forms and Radiation Levels in Two Leafy Brassica Species

The effect of three different nitrogen (N) supply forms differing in their ammonium-to-nitrate (NH(4):NO(3)) ratio (100% NH(4), 50% NH(4) + 50% NO(3), 100% NO(3)) under three different levels of daily photosynthetic active radiation (PAR) (low, 5.0; medium, 6.8; high, 9.0 mol m(-2) day(-1)) on a range of desirable health-promoting phytochemicals in Brassica rapa subsp. nipposinica var. chinoleifera and Brassica juncea was determined. The 100% NH(4) supply under medium PAR levels led to the highest concentration of glucosinolates based on a low nitrogen/sulfur ratio as well as high levels of carotenoids in the leaves of both Brassica species. However, the 100% NH(4) supply under low and medium PAR levels resulted in low concentrations of flavonoids based on high N concentration in the leaves. Thus, the data provided here have strong implications for crop management strategies aimed at optimizing both the concentration and composition of a range of phytochemicals.

Field investigations of apparent optical properties of ice cover in Finnish and Estonian lakes in winter 2009

A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February-April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74-0.92 in winter, going down to 0.18-0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14-25 m -1 ; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1-0.4 and 1-5 m -1 . Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light.

Combined effects of solar UV radiation and CO2-induced seawater acidification on photosynthetic carbon fixation of phytoplankton assemblages in the South China Sea

2010 We carried out short term pCO2/pH perturbation experiments in the coastal waters of the South China Sea to evaluate the combined effects of seawater acidification (low pH/high pCO2) and solar UV radiation (UVR, 280–400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Under photosynthetically active radiation (PAR) alone treatments, reduced pCO2 (190 ppmv) with increased pH resulted in a significant decrease in the photosynthetic carbon fixation rate (about 23%), while enriched pCO2 (700 ppmv) with lowered pH had no significant effect on the photosynthetic performance compared to the ambient level. The apparent photosynthetic efficiency decreased under the reduced pCO2 level, probably due to C-limitation as well as energy being diverged for up-regulation of carbon concentrating mechanisms (CCMs). In the presence of UVR, both UV-A and UV-B caused photosynthetic inhibition, though UV-A appeared to enhance the photosynthetic efficiency under lower PAR levels. UV-B caused less inhibition of photosynthesis under the reduced pCO2 level, probably because of its contribution to the inorganic carbon (Ci)-acquisition processes. Under the seawater acidification conditions (enriched pCO2), both UV-A and UV-B reduced the photosynthetic carbon fixation to higher extents compared to the ambient pCO2 conditions. We conclude that solar UV and seawater acidification could synergistically inhibit photosynthesis.

The interaction of light irradiance with ethylene in regulating growth of Helianthus annuus shoot tissues

Shade light found in ecological niches where plants are growing under a canopy or in proximity of taller neighbouring vegetation consist mainly of two separate light signals: low red to far-red ratio and low photosynthetically active radiation (PAR). The effect of the latter on the growth of 7-day old sunflower shoots was examined by assessing hypocotyl, cotyledon and leaf tissue growth under three varying PAR levels: near-normal of 1,000 μmol m−2 s−1, low of 100 μmol m−2 s−1 and very low of 10 μmol m−2 s−1. Then, the possible interaction between PAR signaling and ethylene in regulating growth of these sunflower tissues was investigated. The results showed that gradual decrease in PAR level increases hypocotyl elongation and decreases ethylene evolution. However, gradual decrease in PAR level decreases cotyledon and leaf growth and increases ethylene evolution. Thus it seems possible that PAR regulation of shoot growth is mediated by changes in ethylene evolution in tissue specific manner. This hypothesis was supported by experiments with the ethylene releasing factor, ethephon, and the ethylene biosynthesis inhibitor, AVG, as well as by transfer experiments where sunflower seedlings were transferred from one PAR regime to another with subsequent growth and ethylene measurements.

Effect of nitrogen form and radiation on growth and mineral concentration of two Brassica species

Three greenhouse experiments were carried out to determine the growth, yield, nitrate, total N and S concentration in shoots, and water uptake of hydroponically grown Brassica rapa L. subsp. nipposinica var. chinoleifera and Brassica juncea L. In each experiment, daily photosynthetically active radiation (PAR) level was 5.0 mol m −2 (low), 6.8 mol m −2 (medium) or 9.0 mol m −2 (high). Plants were supplied with nutrient solutions having equal N concentrations of 11 mM in different forms: 100% NH 4, 50% NH 4 + 50% NO 3, and 100% NO 3. Nitrogen supplied as 100% NH 4 reduced fresh and dry shoot biomass, leaf area, and leaf number in both Brassica species, especially at low and medium PAR levels. In both Brassica species, S concentrations were highest, while nitrate concentrations were lowest in leaves of plants grown at N supplied as 100% NH 4. No differences in leaf nitrate concentrations were observed between 50% NH 4 + 50% NO 3 and 100% NO 3 treatments. Low and high PAR levels increased the nitrate concentrations and decreased the N/S ratio in leaves of both crops compared to medium PAR level. Fresh shoot biomass was maximized in Brassica rapa when PAR level was above the medium value and nitrate was supplied in the nutrient solution as NO 3 or as a mixture of 50% NO 3 and 50% NH 4. The highest fresh shoot biomass of Brassica juncea was observed in all nutrient solution treatments at high PAR level.

Effect of Jetty Pillar Orientation on Scleractinian Corals

This study investigated the effect of orientation of the Pulau Tioman Marine Park jetty pillars on the underwater light intensity, zooxanthellae density and chlorophyll content of Acropora selago (branching coral) and Cyphastrea japonica (encrusting coral). There was significant difference in Photosynthetically Active Radiation (PAR) level between orientations of jetty pillars. However, the coral cover showed no significant difference between orientations suggesting that corals are able to grow if minimum PAR level is available. In Cyphastrea japonica, the difference in zooxanthellae density was significant between pillar orientations but Acropora selago showed otherwise. This difference in C. japonica was due to their zooxanthellar adaptation to different PAR levels. The chlorophyll a and c2 content of both coral species between pillar orientations showed no significant difference. Zooxanthellae density and chlorophyll content of A. selago were almost similar compared between the jetty and the natural reef at Kampung Air Batang suggesting that the jettyA¢â‚¬â„¢s corals are viable species. Thus, we suggest that the construction of jetties in corals reef areas should consider pillar design and orientation in order to provide hard substrate and help to rehabilitate corals in the damaged construction area.

Particle Film Mechanisms of Action That Reduce the Effect of Environmental Stress in ‘Empire’ Apple

Heat stress is a limiting factor of plant productivity throughout the world and kaolin-based particle films (PF) have demonstrated that the reflective nature of the resulting plant surface can increase plant productivity primarily by reducing temperature in fruit, leaf, and canopy. The purpose of this study was to evaluate the environmental mechanisms and related physiological responses of ‘Empire’ apple (Malus ×domestica) gas exchange at the canopy level to PF treatments to identify those parameters key to plant response and increased plant productivity. Trees received either no treatment or season-long applications of a PF treatment and each treatment either received no supplemental irrigation or full replacement of evapotranspiration. Studies were begun in 1998 and continued to 2007. Fruit number and fruit weight were measured in all years. Whole canopy carbon dioxide assimilation rates (A) were measured in apple for a 4-year period to determine the relationship with incoming light and vapor pressure deficit (VPD) levels. The photosynthetic response to the irrigation and PF treatments varied between years as a result of environmental variation in VPD and photosynthetically active radiation (PAR) levels. There was a unique treatment response for PAR levels greater than 1600 μmol·m−2·s−1 in which the combination of PF and irrigation maintained midday A at maximum levels compared with other treatments although A was reduced by increasing VPD. This response indicated that although VPD limited A, the combination of PF and adequate water could maintain maximum A rates at full sun levels during the midday period and minimize the midday depression of A that is commonly observed and reduces the daily carbon accumulation. The increased carbon accumulation during the midday period was likely partitioned to the fruit. Increased fruit weight resulting from the PF treatment, compared with the control, was positively correlated with the growing season air temperature and VPD indicating that as the environment becomes hotter and/or drier, the magnitude of the PF response increased as a result of the reduced leaf and fruit temperature and the subsequent physiological effect. The PF treatments reduced radiation and heat load on exposed leaves enabling them to better regulate leaf temperature and improved the light distribution inside the canopy resulting in increased carbon gain at the whole plant scale. Fruit hue angle was reduced and red color improved by PF treatments in 5 of 10 years. The use of PF may be an effective substitute for evaporative cooling to reduce solar injury and to improve apple quality through increased fruit weight. The results indicate that benefits of PF treatments would occur in agroecosystems with large VPDs and high temperatures and that use of irrigation would further enhance the benefits at high PAR levels.

Photosynthetically active and UV radiation act in an antagonistic way in regulating buoyancy of Arthrospira ( Spirulina) platensis (cyanobacterium)

Buoyancy provided by gas vesicles has been suggested to play an important role in regulating vertical distribution and nutrient acquisition in cyanobacteria. However, little is known about how changes in UV radiation (UVR, 280–400 nm) would affect the buoyancy. We have shown here that the floatation activity of the economically important cyanobacterium Arthrospira platensis (D-0083) decreased with increased photosynthetic rates associated with increased photosynthetically active radiation (PAR), but it decreased less in the presence of UVR, which resulted in inhibitory effects. When the cells were grown under isoenergetic levels of solar PAR or UVR alone, they migrated downward under PAR but maintained buoyant under UVR. The buoyancy regulation of A. platensis depended on the exposed levels of PAR as well as UVR, which affected photosynthesis and growth in an antagonistic way. The buoyancy of A. platensis in water columns is likely to be dependant on diurnal photosynthetic performance regulated by solar radiation, and can hardly be considered as an active strategy to gain more energy during sunrise/sunset or to escape from harmful irradiation during the noon period.

Use of Extenday reflective groundcover in production of ‘Gala’ apples (Malus domestica) in New Brunswick, Canada: 1. Impact on canopy microclimate and leaf gas exchange

A field study was conducted in New Brunswick, Canada to assess the impact of a reflective groundcover (Extenday) on ‘Gala’ (Malus domestica) apple tree microclimate (diurnal and seasonal distributions of incident and reflected photosynthetically active radiation (PAR) within the canopy, seasonal canopy air temperature and relative humidity (RH), soil temperature and moisture) and leaf gas exchange (net photosynthesis (Pn), stomatal conductance (gs), intercellular CO2 (Ci), and transpiration (E)). When installed between tree rows from full‐bloom until 3 weeks post‐harvest, the 3‐m wide reflective groundcover did not influence incident PAR within the tree canopy, but enhanced the reflection and distribution of light into all canopy levels (upper, middle, lower), relative to uncovered sod. Within the lower canopy, overall relative PAR levels were increased from 29% full‐sun to 39% full‐sun (on sunny days) and levels of reflected PAR were 5–9‐fold greater (over the season), with use of the reflective groundcover. On a seasonal basis, canopy RH was reduced by 3.8 ± 1.2%, and soil moisture was increased by 5.5 ± 0.28% to 7.7 ± 0.52%, with use of the reflective groundcover; canopy air temperature was unaffected. Soil temperature beneath the reflective groundcover was as much as 2.1–2.5°C lower than that below uncovered sod, with the differential greatest earlier in the growing season and decreasing over time. Under light‐saturated conditions, the reflective groundcover did not alter leaf temperature or leaf gas exchange or lead to photoinhibition.

Light and spectral properties as determinants of C:N:P-ratios in phytoplankton

Abstract Light is a major determinant not only for carbon (C)-fixation in autotrophs, but also for the cellular proportions of major elements like C, nitrogen (N) and phosphorus (P). High intensities of photosynthetically active radiation (PAR) increase C:P-ratios in experiments with arctic marine and freshwater phytoplankton species. While high levels of PAR promote high autotrophic productivity, the increased C:P may invoke a “paradox of enrichment” effect since this means lower stoichiometric food quality for herbivores. In contrast, exposure to ultraviolet radiation (UVR) gave reduced cellular C:P-ratios (and N:P) in phytoplankton. This was partly owing to a strong reduction in C-fixation under UVR, but also due to enhanced uptake of P, presumably in response to increased demands for nucleotide repair under UVR stress. The net outcome of these opposing effects will depend on optical properties and mixing depth in the water column. These stoichiometric responses could cause deviations from Redfield ratio in phytoplankton as well as affecting biogeochemical cycling and trophic transfer efficiency in aquatic food-webs.