High Photosynthetically Active Radiation Research Articles

Photosynthetic characteristics of spring ephemerals in the desert ecosystem of Dzungaria Basin, northwest China

Spring ephemerals have the highest photosynthetic rates in higher plants. Gas exchanges and related environmental factors of four typical spring ephemerals, Eremopyrum orientale, Malcolmia scorpioides, Tetracme quadricormis and Arnebia decumbens were measured in their natural environments in the Gurbantunggut Desert of the Dzungaria Basin, northwestern China. Diurnal variations in net photosynthetic rate (P N), transpiration rate (E), stomatal conductance (gs), and water use efficiency (WUE) were analyzed at individual leaf level for these species. P N of E. orientale and M. scorpioides showed a bimodal diurnal pattern, whereas that of T. quadricormis and A. decumbens showed a unimodal pattern, with the maximum value occurred at noon (around 12:00). E of all four species showed a unimodal pattern. Higher WUE in the morning was related to higher P N and lower E. The maximum value of gs appeared in early morning, and then decreased gradually during the daytime. The highest photosynthetically active radiation (PAR) occurred at 14:00, but there was a 2 h lag between the highest temperatures of air and leaf and the highest PAR. The maximum net photosynthetic rate (P Nmax) of the four spring ephemerals were 18.44, 32.03, 33.17, and 20.04 μmolCO2 m−2 s−1, respectively. The present study revealed that the midday depression of photosynthesis of E. orientale and M. scorpioides was mainly due to non-stomatal limitation, whereas that of T. quadricormis and A. decumbens was mainly due to stomatal limitation during 14:00–16:00 followed by non-stomatal limitation after 16:00. Our results illustrated that the change of E was consistent with the diurnal fluctuation of air temperature, but the diurnal change of P N was affected by the whole microclimate, i.e. temperature, relative humidity (RH), PAR, etc.

Effects of solar UV and visible radiations on the spiral structure and orientation of Arthrospira (Spirulina) platensis (Cyanophyta)

P. Li and K. Gao. 2008. Effects of solar UV and visible radiations on the spiral structure and orientation of Arthrospira (Spirulina) platensis (Cyanophyta). Phycologia 47: 573–579. DOI: 10.2216/08-19.1Morphological characteristics of Arthrospira spp. are important for product quality and harvesting efficiency. Solar ultraviolet radiation (UVR) has been previously shown to break their spirals. To further explore the relationship of morphology and UVR, we examined the spiral length, helix diameter, helical pitch and orientation of A. platensis while exposing the cells to solar radiation with or without UVR. Although solar exposures initially led to breakage of the spirals, no significant difference was found among the treatments with or without UVR. During the long-term exposures to solar radiation, the spirals became more compressed in the outdoor strain (grown under sunlight for decades) but more loosened in the indoor strain [grown under artificial photosynthetically active radiation (PAR) for decades]. Spiral orientation was altered from the right- to left-handed only in the outdoor strain, with a frequency of less than 1%. Presence of UVR enhanced such an orientation change, and regrowing under indoor low PAR (UVR-free condition) reversed the orientation to right-handed. Structure of some membrane molecules was suggested to be altered by high solar PAR and UVR.

Photosynthetic Responses to Sink–Source Manipulation in Five Peach Cultivars Varying in Maturity Date

Five peach cultivars [Prunus persica (L.) Batch] with different maturity dates were subjected to sink–source manipulation by girdling to isolate 1-year-old fruit-bearing shoots. Four treatments were performed: fruit were removed (−fruit); one fruit (+1 fruit) and two fruit (+2 fruit) were kept inside two girdling cuts; and two fruit were kept outside two girdling cuts (−fruit*). Photosynthetic responses for the five cultivars were similar and did not show genotypic differences. Generally, net photosynthetic rate (Pn), stomatal conductance (g s), and transpiration rate (E) were higher, and leaf temperature (Tl) was lower in +2 fruit than in +1 fruit, followed by −fruit and −fruit* which were not different. The results also indicated that water outflow from fruit into leaves did not influence photosynthesis, and lower photosynthesis in −fruit treatment was not due to water status of source leaves influenced by removing fruit. Pn tended to increase with Tl until Tl reached a critical level. Beyond the critical temperature level, Pn generally decreased. The critical Tl was roughly identified as 34–37 °C for the five cultivars. Both higher and lower substomatal CO2 (Ci) levels occurred in −fruit and −fruit* treatments than in +1 fruit and +2 fruit treatments, indicating that decreased Pn could be due to both nonstomatal and stomatal limitations. Further analysis of the relationship between Ci and photosynthetically active radiation (PAR) showed that nonstomatal limitation under low sink demand took place mostly under high PAR. Thus, high light intensity, combined with Tl may play an important role in leaf photosynthetic regulation.

Production and biomass of picophytoplankton and larger autotrophs in Andean ultraoligotrophic lakes: differences in light harvesting efficiency in deep layers

We measured biomass and primary production of picophytoplankton (PicoPhy: 0.2–2 μm) and of autotrophic size fraction >2 μm in six deep ultraoligotrophic lakes in the Andean-Patagonian region (around 41°S) during summer stratification. Surface Photosynthetically Active Radiation (PAR) ranged from 1277 to 1849 μmol photons m−2 s−1, and the euphotic zone, generally deeper than the mixed layer, varied between 28 m and 49 m. We found a strong photoinhibiting effect of high PAR and UV-A at surface levels, whereas UV-B radiation (<320 nm) had low extra contribution in the photosynthesis inhibition. As a consequence, cell numbers, Chl a and primary production rates of both fractions increased towards deep layers in all lakes. The photosynthetic efficiency (Chl-specific production per photon unit) of both fractions increased with depth, although this increase was higher in PicoPhy, indicating a higher fitness to low-light. The per cent contribution of PicoPhy production to total production, showed an inverse significant relation with total dissolved phosphorus (TDP). Moreover our data fitted the existing database showing a significant trend towards a decrease of PicoPhy biomass and an increase of its relative contribution to total biomass with decreasing trophic state. At very low-phosphorus concentration, typical of north Patagonian lakes, we found good evidence of the competitive advantage of PicoPhy. Low-light and low TDP may interact to create the most favourable conditions for the smaller photosynthetic organisms. In conclusion, we found that at low-light and very low nutrient regime PicoPhy achieves higher photosynthetic efficiency than the larger autotrophic organisms.

Photoperiod, light quality, and irradiance effects on flowering in the alpine and prairie ecotypes of Stellaria longipes

Plants growing in vegetative shade are subjected to reductions in both red to far-red (R/FR) light ratio and in irradiance levels of photosynthetically active radiation (PAR). In this report we have attempted to uncouple the reduced R/FR ratio- and low PAR-mediated effects of shade on flowering in the ramets of Stellaria longipes Goldie s.l. (Caryophyllaceae) ecotypes collected from alpine “sun” and lower elevation prairie “shade” habitats. Both ecotypes were also tested for their flowering response (defined as the number of open flowers per ramet) to daylength. The alpine ecotype plants can best be classified as day-neutral, whereas prairie ecotype plants require long-days (LD). Under a low PAR of 115 μmol m–2·s–1 given under LD conditions at a reduced (0.7) R/FR ratio, alpine ecotype plants flowered significantly later relative to plants grown under the low PAR at a normal (1.22) R/FR ratio. In contrast, plants of the prairie ecotype flowered earlier under the reduced R/FR ratio combined with the same low PAR. Flower number per ramet differed significantly between the two ecotypes, with alpine ecotype plants developing fewer flowers under a low PAR (109 μmol m–2·s–1 irradiance) relative to a high PAR of 611 μmol m–2·s–1 (both given at a normal R/FR ratio). The prairie ecotype plants responded differently and had similar flower numbers under both low and high PARs at the normal (1.22) R/FR ratio. However, growing the prairie ecotype plants under a reduced R/FR ratio at a low PAR showed a significant increase in number of flowers. In contrast, plants grown under high (2.7) and normal (1.9) R/FR ratios combined with low PAR produced many more flowers than the alpine ecotype. Thus, the two components of shade, reduced R/FR ratio and low PAR can cause distinctly different flowering responses in sun and shade plants of S. longipes.

PAR acclimation and UVBR-induced DNA damage in Antarctic marine microalgae

Whether or not it is related to stratospheric ozone depletion, Antarctic microalgae expe- rience ultraviolet-B radiation (UVBR) stress in situ, leading to decreased photosynthetic performance, DNA damage, and/or altered community composition. UVBR vulnerability is known to be species specific, but may also be affected by a range of environmental growth conditions, including the light history of the cells. This study investigates the influence of acclimation to photosynthetically active radiation (PAR) on the vulnerability to UVBR-induced DNA damage in Antarctic microalgae. Chaeto- ceros dichaeta, Pyramimonas gelidicola, Phaeocystis antarctica and Polarella glacialis were accli- mated to 5 PAR levels, after which growth rate, pigment composition, malondialdehyde (MDA, a general indicator of oxidative stress) and UV-absorbing compounds were measured. Photoacclimated cultures were then exposed to a single UVBR treatment and the accumulation of UVBR-induced DNA damage was determined by the number of cyclobutane pyrimidine dimers (CPDs). Acclimation to increasing irradiance enhanced both the xanthophyll to chlorophyll a ratio and xanthophyll de- epoxidation in all species. Increased cellular MDA levels were found at the highest irradiance in all species except P. gelidicola. Yet, growth rates were only reduced at the 2 lowest PAR levels. P. antarctica and P. glacialis showed a strong linear induction of UV-absorbing compounds at increasing PAR intensities, whereas P. gelidicola showed no induction of these compounds. The UVBR treatment induced CPDs in P. gelidicola only, and CPD levels were elevated at the highest PAR acclimation intensities. Thus, sensitivity to UVB-induced CPD accumulation was species specific, and, counterintuitively, acclimation to high PAR increased the sensitivity of P. gelidicola to UVB- induced DNA damage.

Physiological Effects of Kaolin Applications in Well-irrigated and Water-stressed Walnut and Almond Trees

Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions. A 6% kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, psi(s)), gas exchange (i.e. light-saturated CO2 assimilation rate, Amax; stomatal conductance, g(s)), leaf temperature (T(l)) and physiological relationships in treated and control trees were then measured and compared. In both species, kaolin did not affect the daily course of psi(s) whereas it reduced Amax by 1-4 micromol CO2 m(-2) s(-1) throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g(s) in any situation. Consequently, intercellular CO2 concentration (C(i)) was always greater in treated trees than in controls, suggesting that the reduction of Amax with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T(l)) by about 1-3 degrees C and leaf-to-air vapour pressure difference (VPD(l)) by about 0.1-0.7 kPa. Amax was lower at all values of g(s), T(l) and VPD(l) in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63% of the incident PAR to reach the leaf. The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.

Responses of seedling transplants to environmental variations in contrasting habitats of Central Amazonia

In the Central Amazon we investigated whether seedling performance (survival, and relative growth rates in height and leaf numbers) was affected by initial seedling size (height and leaf numbers) in habitats that varied in their degree of human disturbance: cattle pasture, young secondary forest, 1-ha forest fragment and old-growth forest. Additionally, effects of photosynthetically active radiation (PAR), litter standing crop (LSC) and insect herbivory were evaluated 12 mo after transplantation in seedlings from the native canopy trees Chrysophyllum pomiferum, Micropholis venulosa and Pouteria caimito. Seedling performance changed rank across the understorey environment depending on species. Seedlings of Chrysophyllum thrived in all conditions but under high PAR, Micropholis thrived only in intermediate light conditions, whereas Pouteria thrived under high PAR. Effects of initial seedling size, PAR and herbivory after 1 y were specific to species, whereas LSC had no effect on performance. Initially larger seedlings resulted in lower survival for Chrysophyllum and Pouteria. Herbivory affected seedling performance in all species. Negative effects of herbivory were intensified under low PAR. Overall, our results showed that, as seedlings, species of the same family and characteristic of old-growth forests respond differently to the environmental constraints present in contrasting human-disturbed conditions. Larger seedlings may not always present greater tolerance to physical and biotic mortality risks.

UV effects on photosynthesis, growth and acclimation of an estuarine diatom and cryptomonad

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 300:53-62 (2005) - doi:10.3354/meps300053 UV effects on photosynthesis, growth andacclimation of an estuarine diatom and cryptomonad Elena Litchman1,2,3,*, Patrick J. Neale1 1Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA2School of Biology, 310 Ferst Drive, Georgia Institute of Technology, Atlanta, Georgia 30332, USA3Present address: Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 49060, USA *Email: litchman@msu.edu ABSTRACT: Ultraviolet (UV) radiation is a significant stressor in aquatic environments and can inhibit primary productivity of phytoplankton. The effects of UV depend on many factors, including phytoplankton community composition and acclimation status. Using spectrally resolved biological weighting functions (BWFs), we determined sensitivity of photosynthesis and acclimation to UV in a common estuarine diatom, Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, and cryptomonad, Cryptomonas sp. Cryptomonas sp. grown under high PAR (photosynthetically active radiation) (250 µmol quanta m–2 s–1) was significantly more sensitive to photoinhibition in the UV-B part of the spectrum (280 to 320 nm) than T. pseudonana under high PAR. Growth under low irradiance (25 µmol quanta m–2 s–1) increased sensitivity of T. pseudonana. After a week-long exposure to moderate UV radiation, sensitivity of Cryptomonas sp. declined, while sensitivity of T. pseudonana did not change. Growth rates and chlorophyll a-specific absorption decreased in both species. Based on the BWFs obtained in this study, we predict 11 to 26% UV inhibition of depth-integrated primary production by these species under summer conditions in a shallow, turbid temperate estuary. KEY WORDS: Estuarine phytoplankton · UV radiation · Photosynthesis · Photoinhibition · Biological weighting functions · Acclimation · Thalassiosira pseudonana · Cryptomonas sp. Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 300. Online publication date: September 16, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

PRECISION STRESSING BY UV-B RADIATION TO IMPROVE QUALITY OF SPINACH UNDER PROTECTED CULTIVATION

Plants possess a number of defence systems against plant-stress related inflictions such as natural, but at times excessive UV-B radiation. One protection mechanism is the altered synthesis of antioxidative substances like flavonoids, carotenoids, and ascorbic acid. Antioxidants have gained wide recognition for their health promoting actions in human body metabolism. Plants grown under protected cultivation do not receive any UV-B light leading to reduced amounts of secondary metabolites as compared to field crops. We concluded that plants will generate higher amounts of valuable antioxidants when exposed to UV-B light from fixtures installed in a greenhouse. Therefore, seven-week old spinach plants were treated with doses of 0, 1, 2 and 6 kJ m d UV-BBE (biologically effective) radiation for two weeks and compared to PAR-radiated (photosynthetically active radiation) and non-radiated (daylight only) plants. The sum of PAR was maintained at 14.9 mmol m d for all radiated treatments. Biomass was increased by PAR and by increasing UV-B doses. Chlorophyll fluorescence measurements indicated improved photosynthetic capacities due to PAR and moderate UV-B, while it was reduced at 6 kJ m d UVBBE. The content of carotenoids, flavonoids and ascorbic acid was increased by PAR and 1 and 2 kJ m d UV-BBE but was reduced at 6 kJ m d UV-BBE. Antioxidative capacity (AC) of carotenoid-extracts was investigated: Moderate UV-B treatment lead to higher AC compared to PAR treatment. Under conditions of high PAR, low and moderate UV-B radiation improved plant growth and the content of valuable secondary plant substances. INTRODUCTION Quality of vegetables plays an increasing role in human diet and for marketing of horticultural produce. As important determinants of internal quality we consider flavor, texture, minerals, vitamins (e.g. vitamin C) as well as bioactive substances (e.g. carotenoids). Plants are the major source of ascorbate (Vitamin C) in the human diet as humans are unable to synthesize this substance. Ascorbate is the most abundant soluble antioxidant in plant cells among secondary plant metabolites which also possess antioxidative properties. Since many diseases have been related to the occurrence of reactive oxygen species, plant derived antioxidants have gained importance for human nutrition in recent years (Kauer and Kapoor, 2001). Plant metabolism and as a consequence synthesis of vitamins and antioxidants is affected by many environmental parameters. Light conditions under cultivated protection are different from open field due to UV-B and other light/wavelength absorption of the plant coverage. In order to improve the nutritional value of greenhouse vegetables we started an initial experiment on UV-B exposure to spinach since little is known about the effect of dose and duration of UV-B on growth and metabolism in vegetables. We postulate that plants will generate higher amounts of valuable compounds when exposed to UV-B light from lamps installed in a greenhouse. MATERIALS AND METHODS Spinach (Spinacea oleracea L.) ‘Poncho‘ was cultivated in the glasshouse in pots Proc. VII IS on Prot. Cult. Mild Winter Climates Eds. D.J. Cantliffe, P.J. Stoffella & N. Shaw Acta Hort. 659, ISHS 2004 202 (12 cm, 2 plants pot) with peat, ebb-flood irrigated with complete nutrient solution (Flory 9 specifications, EC 2 mS cm) and temperatures set at 19/17°C. On November 7, nine weeks after sowing, the plants were subjected to differing radiation treatments for two weeks (Table 1). Treatments were no additional lighting (standard growing practise, SGP), additional photosynthetically active radiation (PAR), 1 kJ md UV-BBE (field grown condition, FGC), 2 kJ md UV-BBE (UV intensive, INT), 6 kJ md UV-BBE (UV permanent, PER). The treatments contained 150 plants and were not replicated. The UV lamp was a high-pressure metal-halide gas lamp (400 W, MHL400/E40, BLV Licht) emitting UV-B at >294 nm (Figure 1) and the daily doses were obtained by 4, 8 and 24 hours exposure at equal intensity. Since the UV lamp also emits PAR, the resulting different daily PAR sums were supplemented to 14.9 mol md by horticultural PAR lamps (400 W, SON-T AGRO, Philipps) from 7-17 h daily which is equal in quantity to the PAR treatment. The lighted treatments were separated from each other and from SGP by non-transmitting black PE-sheet walls. Plant weight and quality parameters were assessed before and at the end of the treatment period. Fresh weight was cumulated from 10 pots and dry matter content was determined in four replicates by oven drying. Chlorophyll fluorescence (CF) evaluation during growth was done before radiation, after one and two weeks of radiation, using two middle aged leaves/plant of the same 10 plant pots each time. Additionally, chlorophyll fluorescence of 20 detached leaves was measured after one week of storage at 1°C. The fluorescence parameters Fv/Fm and Fo were determinated with PAM-fluorometer (Walz) by emitting a single saturation light pulse on dark adapted tissue of the spinach plants at temperature of 18°C.

Quantification of photosynthetically active radiation inside sunlit growth chambers

Outdoor growth chambers allow plants to be grown under sunlight while other environmental variables such as air temperature and CO 2 concentration are controlled. Photosynthetically active radiation (PAR) inside these sunlit chambers could differ from ambient levels due to attenuations and reflections by the glazing materials. This study identified diurnal patterns and distribution of PAR inside various types of sunlit growth chambers, including soil-plant-atmosphere-research (SPAR) units using single-point and line quantum sensors. In comparison with the ambient levels, higher PAR was measured when reflections from the adjacent walls overlapped at specific locations in the SPAR unit. Within the crop growing area located in the northern part of a SPAR unit, daily integrals of PAR were between 93% and 105% of the ambient PAR on a clear day. A gradient in daily PAR existed, increasing from south to north inside the chamber. On a cloudy day, PAR within the crop growing area was between 92% and 95% of ambient PAR. A mathematical model was developed to predict incident PAR inside sunlit chambers. This model accounted for solar geometry, direct and diffuse radiations, chamber geometry, and the optical properties of the glazing material. The model was capable of simulating the diurnal patterns of PAR inside various types of sunlit chambers. The model predicted that the mean daily PAR inside sunlit chambers of acrylic sheets would usually be within 5% of the ambient PAR. Spatial distribution of the predicted daily PAR ranged from 75% to 120% of ambient levels inside various types of sunlit chambers including SPAR and open-top chambers.

Light and growth rate effects on crop and weed responses to nitrogen

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Photosynthetically active radiation regimes in a southern African savanna environment

Global solar radiation ( K↓) and photosynthetically active radiation (PAR) were measured at Lusaka, Zambia from April to December 2000. Daily K↓ and PAR followed a similar trend and the seasonal cycle was primarily determined by extraterrestrial radiation and cloudiness. The daily PAR /K↓ ratio also exhibited seasonal variation. The mean daily ratio was 1.992 mol MJ −1; the maximum daily ratio was 2.414 mol MJ −1 and the minimum daily ratio was 1.778 mol MJ −1. Generally, high PAR /K↓ ratios were associated with low clearness indices and low ratios with high clearness indices. The maximum daily ratio occurred in December, when dense cloud associated with the intertropical convergence zone (ITCZ) prevailed. The minimum daily ratio occurred in September when predominantly cloudless conditions, associated with the subtropical highs, coincided with regional biomass burning enhancements to atmospheric turbidity.

Daily Irradiance and Feedback Inhibition of Photosynthesis at Elevated Carbon Dioxide Concentration in Brassica oleracea

The fundamental cause of down-regulation of photosynthesis at elevated carbon dioxide concentration (EC) is thought to be a slower rate of utilization of saccharides than their stimulated rate of production, but there are few studies directly supporting this idea under field conditions. We hypothesized that within Brassica oleracea, down-regulation would not occur in kohlrabi because it has a large sink for saccharides in an enlarged stem, but would occur in collards, which lack this sink. Field tests were consistent with this hypothesis. In collards, the degree of down-regulation of photosynthesis in plants grown at EC varied depending on the daily integral of photosynthetically active radiation (PAR) of the day prior to the measurement of photosynthetic capacity, as did leaf saccharide content. However, EC did not result in lower leaf contents of chlorophyll, soluble protein, ribulose-1,5-bisphosphate carboxylase, or nitrate in collards, nor was there any evidence of a triose phosphate utilization rate limiting photosynthesis. Experiments in controlled environment chambers confirmed that there was a threshold response for the down-regulation of photosynthesis in collards at EC to the PAR of the previous day, with down-regulation only occurring above a minimum daily integral of PAR. Down-regulation of photosynthesis could be induced in plants grown at ambient carbon dioxide by a single night at low temperature or by a single day with high PAR and EC. In the controlled environment study, the degree of down-regulation of photosynthesis was highly correlated with leaf glucose, fructose, and sucrose contents, and less well correlated with starch content. Hence down-regulation of photosynthesis at EC in collards in the field represented feedback inhibition from the accumulation of soluble saccharides and day-to-day variation in its occurrence was predictable from the weather.

Photobiological characteristics and photosynthetic UV responses in two Ulva species (Chlorophyta) from southern Spain

The effect of different wavebands of artificial UV (UVB and UVA) and photosynthetically active radiation (PAR) was assessed in two species of the genus Ulva, U. olivascens and U. rotundata, from southern Spain in order to test for possible differences in acclimation of photosynthesis. Both species share similar morphology but are subject to different light environments: U. rotundata is an estuarine alga, inhabiting subtidal locations, while U. olivascens is an intertidal, sun-adapted organism. Algae were exposed to three different UV conditions, PAR + UVA + UVB, PAR + UVA and PAR for 7 d. Short-term exposure (6 h) was also carried out, using two PAR levels, 150 and 700 μmol m −2 s −1. Pigment contents and photosynthesis vs. irradiance curves from oxygen evolution were used to contrast sun- and shade adaptation between these species. O 2-based net photosynthesis ( P max) and PAM-chlorophyll fluorescence (optimal quantum yield, F v/ F m) were used as parameters to evaluate photoinhibition of photosynthesis in the experiments. The results underline different photobiological characteristics among species: the subtidal U. rotundata had higher contents of pigments (Chl a, Chl b and carotenoids) than the sun-adapted U. olivascens, which resulted in higher thallus absorptance and P– I parameters characterized by higher photosynthetic efficiency at limiting irradiances ( α) and lower saturating points for photosynthesis ( E k ). After 7 d exposure, photoinhibition of F v/ F m was close to 40–45% in both species. Differences between UV treatments were seen in U. rotundata after 5 d and after 7 d in U. olivascens, in which PAR + UVA impaired strongly photosynthesis (80%). Such patterns were correlated with a progressive decrease in pigment contents, specially chlorophylls. In short-term (6 h) exposures, combinations of UVA + UVB and high PAR level resulted in high rates of photoinhibition of chlorophyll fluorescence (68–92%) in U. rotundata, whereas in U. olivascens photoinhibition ranged between 42% and 53%. Photoinhibition under low PAR combined to UV radiation was lower than observed under high PAR. Net O 2– P max revealed similar response among the species, with maximal photoinhibition rates close to 60% in algae incubated under high PAR + UVA + UVB. In the case of UV exposure in combination with low PAR, the highest photoinhibition rates were measured in U. rotundata.

Behavioural reactions of Antarctic krill ( Euphausia superba Dana) to ultraviolet and photosynthetically active radiation

We used broad, shallow and deep, narrow aquarium tanks to examine the behavioural responses of Antarctic krill to photosynthetically active radiation (PAR) and ultraviolet (UV) radiation. Groups of animals were exposed to different levels of radiation and to gradients of radiation and their mean position in the tanks were recorded photographically. Results from the horizontal tank (length 2.4 m) showed that krill moved from areas of high levels (7 W m −2) of UVA exposure (320–400 nm) to areas of low exposure (0.1 W m −2), differing in irradiance by two orders of magnitude. In addition, a similar reaction was observed in krill exposed to a PAR gradient of similar magnitude (30–5 W m −2). There was no observed reaction to UVB wavelengths (280–320 nm). Krill in the vertical tank (height 1.5 m) remained evenly distributed throughout the tank when exposed to both high and low levels of PAR. Additional exposure to UVA wavelengths from the surface caused krill to increase their depth in the tank significantly, to areas of lower irradiance. Addition of UVB radiation had no effect on the mean depth of krill in the tank. The implications of these findings are that krill may avoid regions of high UVA or PAR and consequently would reduce their exposure to UVB and therefore the risk of UVB-induced damage, such as disruption to DNA.

Oxidative stress and enzymatic scavenging of superoxide radicals induced by solar UV-B radiation in &lt;i&gt;Ulva&lt;/i&gt; canopies from southern Spain

The generation of reactive oxygen species (ROS) and scavenging of the superoxide radical by superoxide dismutase (SOD) was studied in mat-like canopies of the green macroalga Ulva rotundata Bliding in a tidal brine pond system in southern Spain. Artificial canopies were covered with different cut-off filters, generating different radiation conditions. ROS and SOD were assessed after three days of exposure. ROS induced lipid peroxidation depended on the position of individual thalli within the canopy and on radiation conditions. Samples exposed to the full solar spectrum were most affected, whereas samples either exposed to photosynthetically active radiation (PAR) alone or UV radiation without PAR exhibited fewer peroxidation products. The activity of SOD appeared to be controlled by the impinging UV-A and UV-B radiation and also increased in response to oxidative stress. The results provide evidence for additive effects of high PAR and UV-B under field conditions and support the previously proposed hypothesis that UV-B effects are mediated by an inhibition of the xanthophyll cycle, which increases ROS production and, consequently, causes oxidative damage to components of the photosynthetic machinery, such as proteins and pigments.

UV‐induction of sun‐screening pigments in lichens

Summary • Induction of the pigments parietin and melanin by specific wavelength ranges of solar radiation was studied in two widespread foliose lichens: the ubiquitous suntolerant Xanthoria parietina , from which the orange parietin can be extracted in living thalli by acetone, and naturally melanin-free shade-adapted specimens of the old forest species Lobaria pulmonaria . • Parietin-free X. parietina and melanin-free L. pulmonaria were exposed to solar radiation in the field under filter foils transmitting or absorbing natural UV-radiation (< 400 nm). Synthesis of both pigments depended on the presence of UV-radiation. • A growth chamber experiment showed that the induction of parietin and melanin synthesis required the presence of UV-B (280‐320 nm). UV-A (320‐400 nm) induced a very small synthesis only, whereas photosynthetically active radiation (PAR) was not inductive. Melanins and parietin were only synthesized in thalli being hydrated during UV-exposures, suggesting that induction required active metabolism. • The photosystem II (PSII) of X. parietina was not adversely affected by any applied radiation treatment, regardless of the hydration status during exposure, whereas L. pulmonaria was susceptible both to high PAR- and UV-irradiances in the desiccated state.

Tree seedling canopy responses to conflicting photosensory cues.

Light with decreased red:far-red (R:FR) ratios may signal neighbor presence and trigger plant developmental responses. There is some evidence that plant canopies forage towards increased R:FR ratios, but it is unclear to what extent R:FR versus the total amount of photosynthetically active radiation (PAR) influences canopy foraging responses among forest trees. The objective of this study was to examine the relative importance of PAR and R:FR as photosensory cues leading to tree canopy foraging responses. Seedlings of Betula papyrifera Marshall (paper birch) were grown in an experimental garden. Each seedling was germinated and grown in its own shading structure and exposed to two spatially separated light environments, in a factorial design of PAR and R:FR. Plant canopy foraging was evaluated at the end of one growing season in terms of canopy displacement, canopy area, leaf number, direction of stem lean, petiole aspect, and lamina aspect with respect to experimental light treatments. Leaf number and canopy area were greater on the high PAR sides of plants, irrespective of the R:FR treatment. Seedling canopies were displaced towards the direction of high PAR, but this relationship was not significant across all treatments. Petiole aspect was random and showed no significant directedness towards any of the light treatments. Lamina aspect and the direction of stem lean were distributed towards the direction of high PAR, irrespective of the R:FR treatment. Overall, first-year B. papyrifera seedlings used PAR, rather than R:FR ratio, as a photosensory cue for canopy light foraging.

Excretion of coumarins by the Mediterranean green alga Dasycladus vermicularis in response to environmental stress

The internal contents and excretion of 3,6,7-trihydroxycoumarin (THC), a UV-absorbing substance, were measured in the green alga Dasycladus vermicularis from southern Spain under different conditions of photosynthetically active radiation (PAR), temperature and salinity in the laboratory. Data were correlated to changes in the effective quantum yield of photosynthesis using PAM chlorophyll fluorometry. The concentrations of THC in the thallus were not significantly affected by the different experimental treatments after 24 h of exposure. However, excretion of THC was induced rapidly by high and low PAR irradiances, as well as by high temperature and by low salinity. In some cases, enhanced dissolved THC concentration was correlated to increasing photoinhibition of chlorophyll fluorescence. The results suggest that, apart from the previously reported induction by UV radiation, the excretion of THC in D. vermicularis responds to multiple environmental factors. Both excreted and extracted forms of THC were characterized by means of high-performance liquid chromatography, indicating some changes in the molecule structure as observed by differences in the maximum of the absorption peak. However, no oxidation of the molecule occurs in the excretion process, since a high antioxidant capacity of THC was evident, underlying its possible involvement in the scavenging of oxidized radicals.