PROTECTION OF THE D1 PHOTOSYSTEM II REACTION CENTER PROTEIN FROM DEGRADATION IN ULTRAVIOLET RADIATION FOLLOWING ADAPTATION OF Brassica napus L. TO GROWTH IN ULTRAVIOLET-B

Effect on essential oil components and wedelolactone content of a medicinal plant Eclipta alba due to modifications in the growth and morphology under different exposures of ultraviolet-B

In the present study sensitivity of a medicinal plant Eclipta alba L. (Hassk) (False daisy) was assessed under intermittent (IT) and continuous (CT) doses of elevated ultraviolet-B (eUV-B). Eclipta alba is rich in medicinally important phytochemical constituents, used against several diseases. The hypothesis of this study is that alterations in UV-B dose may modify the quantity and quality of medicinally valuable components with changes in the morphological and physiological parameters of test plant. To fulfill our hypothesis IT and CT of eUV-B (ambient ± 7.2 kJ m^-2 day^-2) was given for 130 and 240 h respectively to assess the impact of UV-B stress. Growth and physiological parameters were adversely affected under both the treatments with varying magnitude. The observation of leaf surfaces showed increase in stomatal and trichome densities suggesting the adaptive resilience of the plants against UV-B. Besides, biosynthesis of wedelolactone, a major medicinal compound of E. alba was observed to be stimulated under UV-B exposure. The essential oil content was reduced under IT while increased under CT. A total of 114 compounds were identified from oil extract of E. alba. n-Pentadecane (25.79%), n-Octadecane (12.98%), β-Farnesene (9.43%), α-Humulene (4.95%) (E)-Caryophyllene (4.87%), Phytol (4.25%), α-Copaene (2.26%), Humulene epoxide (1.46%), β-Pinene (1.07) and β-Caryophyllene oxide (1.06%) were identified as major components of oil. CT induced the synthesis of some medicinally important compounds such as α-terpineol, δ-cadinene, linolenic acid, methyl linoleate and myristic acid amide. Hence, the study revealed that continuous UV-B exposure of low intensity could be helpful for commercial exploitation of essential oil in E. alba.

Red and Blue Light Promote the Accumulation of Artemisinin in Artemisia Annua L

Artemisinin, which has been isolated from Artemisia annua L., is the most effective antimalarial drug and has saved millions of lives. In addition, artemisinin and its derivatives have anti-tumor, anti-parasitic, anti-fibrosis, and anti-arrhythmic properties, which enhances the demand for these compounds. Improving the content of artemisinin in A. annua is therefore becoming an increasing research interest, as the chemical synthesis of this metabolite is not viable. Ultraviolet B and C irradiation have been reported to improve the artemisinin content in A. annua, but they are harmful to plant growth and development. Therefore, we screened other light sources to examine if they could promote artemisinin content without affecting plant growth and development. We found that red and blue light could enhance artemisinin accumulation by promoting the expression of the genes that were involved in artemisinin biosynthesis, such as amorpha-4,11-diene synthase (ADS) and cytochrome P450 monooxygenase (CYP71AV1) genes. Thus, in addition to being the main light sources for photosynthesis, red and blue light play a key role in plant secondary metabolism, and optimizing the combination of these light might allow for the productionof artemisinin-rich A. annua.

Responses of wheat (Triticum aestivum) and turnip (Brassica rapa) to the combined exposure of carbaryl and ultraviolet radiation

The increase of ultraviolet (UV) radiation reaching the Earth's surface as a result of increased ozone layer depletion has affected crop production systems and, in combination with pesticides used in agricultural activities, can lead to greater risks to the environment. The impact of UV radiation and carbaryl singly and in combination on Triticum aestivum (wheat) and Brassica rapa (turnip) was studied. The combined exposure was analyzed using the MixTox tool and was based on the conceptual model of independent action, where possible deviations to synergism or antagonism and dose-ratio or dose-level response pattern were also considered. Compared with the control, carbaryl and UV radiation individually led to reductions in growth, fresh and dry weight, and water content for both species. Combined treatment of UV and carbaryl was more deleterious compared with single exposure. For T. aestivum length, no interaction between the 2 stressors was found (independent action), and a dose-level deviation was the best description for the weight parameters. For B. rapa, dose-ratio deviations from the conceptual model were found when length and dry weight were analyzed, and a higher than expected effect on the fresh weight (synergism) occurred with combined exposure.

Proteins with high turnover rate in barley leaves estimated by proteome analysis combined with in planta isotope labeling

Protein turnover is a key component in cellular homeostasis; however, there is little quantitative information on degradation kinetics for individual plant proteins. We have used (15)N labeling of barley (Hordeum vulgare) plants and gas chromatography-mass spectrometry analysis of free amino acids and liquid chromatography-mass spectrometry analysis of proteins to track the enrichment of (15)N into the amino acid pools in barley leaves and then into tryptic peptides derived from newly synthesized proteins. Using information on the rate of growth of barley leaves combined with the rate of degradation of (14)N-labeled proteins, we calculate the turnover rates of 508 different proteins in barley and show that they vary by more than 100-fold. There was approximately a 9-h lag from label application until (15)N incorporation could be reliably quantified in extracted peptides. Using this information and assuming constant translation rates for proteins during the time course, we were able to quantify degradation rates for several proteins that exhibit half-lives on the order of hours. Our workflow, involving a stringent series of mass spectrometry filtering steps, demonstrates that (15)N labeling can be used for large-scale liquid chromatography-mass spectrometry studies of protein turnover in plants. We identify a series of abundant proteins in photosynthesis, photorespiration, and specific subunits of chlorophyll biosynthesis that turn over significantly more rapidly than the average protein involved in these processes. We also highlight a series of proteins that turn over as rapidly as the well-known D1 subunit of photosystem II. While these proteins need further verification for rapid degradation in vivo, they cluster in chlorophyll and thiamine biosynthesis.

Photoinhibition of Photosystem II

Photoinhibition of Photosystem II (PSII) is the light-induced loss of PSII electron-transfer activity. Although photoinhibition has been studied for a long time, there is no consensus about its mechanism. On one hand, production of singlet oxygen ((1)O(2)) by PSII has promoted models in which this reactive oxygen species (ROS) is considered to act as the agent of photoinhibitory damage. These chemistry-based models have often not taken into account the photophysical features of photoinhibition-like light response and action spectrum. On the other hand, models that reproduce these basic photophysical features of the reaction have not considered the importance of data about ROS. In this chapter, it is shown that the evidence behind the chemistry-based models and the photophysically oriented models can be brought together to build a mechanism that confirms with all types of experimental data. A working hypothesis is proposed, starting with inhibition of the manganese complex by light. Inability of the manganese complex to reduce the primary donor promotes recombination between the oxidized primary donor and Q(A), the first stable quinone acceptor of PSII. (1)O(2) production due to this recombination may inhibit protein synthesis or spread the photoinhibitory damage to another PSII center. The production of (1)O(2) is transient because loss of activity of the oxygen-evolving complex induces an increase in the redox potential of Q(A), which lowers (1)O(2) production.

Tissue-specific distribution of secondary metabolites in rapeseed (Brassica napus L.)

Four different parts, hypocotyl and radicle (HR), inner cotyledon (IC), outer cotyledon (OC), seed coat and endosperm (SE), were sampled from mature rapeseed (Brassica napus L.) by laser microdissection. Subsequently, major secondary metabolites, glucosinolates and sinapine, as well as three minor ones, a cyclic spermidine conjugate and two flavonoids, representing different compound categories, were qualified and quantified in dissected samples by high-performance liquid chromatography with diode array detection and mass spectrometry. No qualitative and quantitative difference of glucosinolates and sinapine was detected in embryo tissues (HR, IC and OC). On the other hand, the three minor compounds were observed to be distributed unevenly in different rapeseed tissues. The hypothetic biological functions of the distribution patterns of different secondary metabolites in rapeseed are discussed.

UV-A enhanced growth and UV-B induced positive effects in the recovery of photochemical yield in Gracilaria lemaneiformis (Rhodophyta)

The effects of solar UV radiation (280-400 nm) on growth, quantum yield and pigmentation in Gracilaria lemaneiformis were investigated when the thalli were cultured under solar radiation with or without UV for a period of 15 days. Presence of UV-A (315-400 nm) enhanced the relative growth rate, while UV-B (218-315 nm) inhibited it. The positive effect of UV-A and negative effect of UV-B counteracted to result in an insignificant impact of UVR on growth. During the noon period, both UV-A and UV-B resulted in the decrease of maximum quantum yield (Fv/Fm), but UV-B aided in the recovery of the yield in the late afternoon, reflecting that UV-B might be used as a signal in photorepair processes. UV induced the accumulation of UV-absorbing compounds (UVAC) to defend against the harmful UVR. However, the accumulation of UVAC took a much longer time compared to that previously reported, which was probably due to the lower levels of solar radiation and water temperature in the early spring period. Unknown UV-absorbing compounds (UVAC), which peaked at 265 nm, probably the precursor of MAAs (UVAC(325)), accumulated under moderate levels of solar radiation and were transformed to MAAs under higher solar radiation.

The effects of far-red light on plant growth and flavonoid accumulation in Brassica napus in the presence of ultraviolet B radiation

Flavonoid induction is regulated by complex signal transduction pathways involving cryptochrome, phytochrome and UVB photoreceptors. Previously, we identified the UVB-inducible flavonoids in Brassica napus cv. Topas leaves and showed that UVA affected accumulation of the quercetin (Q) and kaempferol (K) glycosides (Wilson et al. [2000] Photochem. Photobiol. 73, 678-684). In this study, we examined the effects of far-red light (FR, 700-780 nm) on UVB-mediated flavonoid accumulation in B. napus. Plants were grown under photosynthetically active radiation (PAR, 400-700 nm, 150 micromol m(-2) s(-1)) plus a moderate level of FR (35 micromol m(-2) s(-1)) for 14 days, and then transferred to five different irradiation regimes (PAR +/- [UVA + UVB] + moderate, intermediate or low fluence FR) for 4 days. Kinetics of flavonoid accumulation were assessed via HPLC. Accumulation of flavonoids, in general, was suppressed by increasing the amount of FR in the spectrum. Furthermore, addition of UVB (290-320 nm) to the spectrum altered the flavonoid composition by causing significant changes in the quantities of individual flavonoids. The relative levels of acylated K glycosides were diminished whereas the relative levels of nonacylated Q glycosides increased dramatically. With UVB exposure there was a five-fold increase in the Q:K ratio. In contrast, increasing the level of FR in the presence of UVB decreased the Q:K ratio by half.

Reduction of UV-B radiation causes an enhancement of photoinhibition in high light stressed aquatic plants from New Zealand lakes

Anthropogenic stratospheric ozone depletion causes an increase of UV-B radiation impinging on the earth surface, which is a threat to plants not adapted to higher UV-B irradiances. Investigations were undertaken with aquatic plants from New Zealand, where UV-irradiances are naturally higher due to the southern latitude, to compare with former results of polar species. The experiments reported in this study were undertaken with plants collected from different lakes of the South Island, with different UV transparencies. Photoinhibition was induced under controlled conditions using a sun simulator, which mimicked the natural underwater radiation spectrum. Photosynthetic activity during high light stress, and during recovery in dim light, was determined in vivo by measuring fluorescence changes, using a PAM fluorometer device. A comparison of different species showed that the extent to which UV causes an additional decrease of photosynthetic performance during high light stress varies according to the depth of growth and UV transparency of the water body. This observation fits with previous studies. However, a new finding was that some species were even more strongly inhibited when UV-B was filtered out of the simulated sun spectrum, indicating a supporting effect of the short UVR wavelength range against photoinhibition. These results were also confirmed by field experiments under natural radiation conditions. Thus, UV-B does not solely cause negative effects on photosynthesis, but it may even support recovery processes in aquatic plants adapted to a high UV-radiation environment. The latter is in contrast to earlier studies, in which UV-B radiation was considered causing only harmful effects on photosynthesis of aquatic plants.

Sensitivity of rice to ultraviolet-B radiation

BACKGROUND

Depletion of the stratospheric ozone layer leads to an increase in ultraviolet-B (UVB: 280-320 nm) radiation reaching the earth's surface, and the enhanced solar UVB radiation predicted by atmospheric models will result in reduction of growth and yield of crops in the future. Over the last two decades, extensive studies of the physiological, biochemical and morphological effects of UVB in plants, as well as the mechanisms of UVB resistance, have been carried out.

SCOPE

In this review, we describe recent research into the mechanisms of UVB resistance in higher plants, with an emphasis on rice (Oryza sativa), one of the world's most important staple food crops. Recent studies have brought to light the following remarkable findings. UV-absorbing compounds accumulating in the epidermal cell layers have traditionally been considered to function as UV filters, and to play an important role in countering the damaging effects of UVB radiation. Although these compounds are effective in reducing cyclobutane pyrimidine dimer (CPD) induction in plants exposed to a challenge exposure to UVB, certain levels of CPD are maintained constitutively in light conditions containing UVB, regardless of the quantity or presence of visible light. These findings imply that the systems for repairing DNA damage and scavenging reactive oxygen species (ROS) are essential for plants to grow in light conditions containing UVB.

CONCLUSION

CPD photolyase activity is a crucial factor determining the differences in UVB sensitivity between rice cultivars. The substitution of one or two bases in the CPD photolyase gene can alter the activity of the enzyme, and the associated resistance of the plant to UVB radiation. These findings open up the possibility, in the near future, of increasing the resistance of rice to UVB radiation, by selective breeding or bioengineering of the genes encoding CPD photolyase.

Simulated solar irradiation with enhanced UV-B adjust plastid- and thylakoid-associated polyamine changes for UV-B protection

Polyamines have been described to protect against numerous oxidative stresses in plants. Increasing UV-B radiation (280-315 nm) in the biosphere may also induce an increase in radical formation in tissues. This study employed the tobacco cultivars Bel B and Bel W3 to describe possible protective functions of polyamines against UV-B radiation in sun light simulators (GSF/Munich) with natural diurnal fluctuations of simulated UV-B. Polyamine measurements on a whole leaf basis in isolated chloroplasts and thylakoids were paralleled to photosynthetic and respiration rates, photosynthetic efficiency, leaf thickness and photosynthetic pigment compositions. The study revealed that an increase of polyamines, and especially of putrescine level in thylakoid membranes upon elevated UV-B exposure comprises one of the primary protective mechanisms in the photosynthetic apparatus of the tobacco variety Bel B against UV-B radiation. The tobacco cultivar Bel W3, sensitive to ozone, was also proved to be sensitive to UV-B. This sensitivity is attributed to its incapability to enhance putrescine level in thylakoid membranes. After prolongation of UV-B exposure, when endogenous plant balances are being gradually restored, due to secondary responses, (e.g., biosynthesis of carotenoids and of additional flavonoids) and the plant is adapting to the altered environmental conditions, then the polyamine level is being reduced. Thus, we can discriminate the UV-B induced stress period from a UV-B acclimation period.

The effects of UVB radiation on temperate southern hemisphere forests

The temperate forests of the southern hemisphere are the most likely forests to be affected by increased levels of ultraviolet-B (UVB) radiation resulting from reduced ozone. The review describes these forests and then discusses the morphological changes, physiological effects, and protection mechanisms, particularly UV absorbing compounds that result from present day and increasing UVB radiation. Possible avenues for future research are explored.

Contrasting strategies for UV-B screening in sub-Arctic dwarf shrubs

The content and distribution of UV-absorbing phenolic compounds was investigated in leaves of three species of Vaccinium co-existing at a site in north Sweden. Vaccinium myrtillus L., Vaccinium vitis-idaea L., and Vaccinium uliginosum L. exhibit markedly different strategies, in terms of localization and content of leaf phenolics and in their responses to UV-B enhancement. Plants were exposed to either ambient radiation or to enhancement of UV-B corresponding to 15% (clear sky) depletion of stratospheric ozone for approximately 10 years prior to commencement of this study. Vaccinium myrtillus contained the highest concentration of methanol-extractable UV-B-absorbing compounds, which was elevated in plants exposed to enhanced UV-B. Fluorescence and confocal laser scanning microscopy showed that these compounds were distributed throughout the leaf, and were particularly concentrated in chlorophyll-containing cells. In V. vitis-idaea, most phenolic compounds were cell wall-bound and concentrated in the walls of the epidermis; this pool increased in response to UV-B enhancement. It is suggested that these two plants represent extreme forms of two divergent strategies for UV-B screening, the different responses possibly being related to leaf longevity in the two species. The response of V. uliginosum was intermediate between the other two, with high concentrations of cell wall-bound phenolics in the epidermis but with this pool decreasing, and the methanol-soluble pool tending to increase, after exposure to enhanced UV-B. One explanation for this response is that this plant is deciduous, like V. myrtillus, but has leaves that are structurally similar to those of V. vitis-idaea.

Effects of ultraviolet radiation on plant cells

Recent measurements of ozone levels have led to concern that the stratospheric ozone layer is being depleted as a result of contamination with man-made chlorofluorocarbons. Concomitantly, the amounts of solar UV-B radiation reaching the Earth's surface is increasing. UV-B radiation has been shown to be harmful to living organisms, damaging DNA, proteins, lipids and membranes. Plants, which use sunlight for photosynthesis and are unable to avoid exposure to enhanced levels of UV-B radiation, are at risk. Thus, mechanisms by which plants may protect themselves from UV radiation are of particular interest. This review will summarizes the main aspects of ultraviolet radiation on plants at physiological and biochemical level, with particular emphasis on protective structures and mechanisms.

Flavonoid gene expression and UV photoprotection in transgenic and mutant Petunia leaves

The effects of UVB radiation on plant growth rate, gene expression and flavonoid content in wild-type, and in transgenic and mutant F3'H deficient Petunia lines have been studied for the first time. In wild-type Petunia, UVB induced an increase in total levels of flavonols and this was due to an up-regulation of several genes in the phenylpropanoid pathway. Furthermore, UVB induced a higher rate of production of dihydroxylated flavonols than mono-hydroxylated equivalents. Thus, the ratio of quercetin (ortho-dihydroxylated) to kaempferol (monohydroxylated) increased. In the F3H deficient mutant line, increasing UVB resulted in up-regulation of all of the basic flavonoid biosynthetic genes. Total flavonoids increased to levels significantly higher than in control plants, and the predominant flavonoid was kaempferol. The leaves of these plants grew at a significantly slower rate than comparably treated wild-type plants under ambient or enhanced UVB radiation. This suggests that the predominance of quercetin in the wild-type confers a protective advantage that is not matched in the mutant, even with higher overall flavonoid levels. In contrast, the antisense F3H construct produced an unexpected down-regulation of C4H, CHS and CHI transcription. This resulted in lower total flavonoid production in these plants. The growth rate of these plants was not impaired in UVB to a statistically significant extent, and the Q:K ratio did not change with increasing UVB radiation. This investigation has established a likely correlation between the effect of UVB on plant growth rate, the level of activity of the F3'H gene, and the proposed photoprotection afforded by an increased Q:K ratio.

Effects of ultraviolet-A exposure on ultraviolet-B-induced accumulation of specific flavonoids in Brassica napus

Many plant species are able to acclimate to changes in ultraviolet-B radiation (UVB) (290-320 nm) exposure. Due to the wide range of targets of UVB, plants have evolved diverse repair and protection mechanisms. These include increased biosynthesis of UVB screening compounds, elevated antioxidant activity and increased rates of DNA repair. We have shown previously that Brassica napus L. cv Topas plants can acclimate quite effectively to environmentally relevant increases in UVB through the accumulation of specific flavonoids in the leaf epidermis. However, B. napus was found to lose other flavonoids when plants are exposed to ultraviolet-A radiation (UVA) (320-400 nm) and/or UVB (Wilson et al. [1998] Photochem. Photobiol. 67, 547-553). In this study we demonstrate that the levels of all the extractable flavonoids in the leaves of B. napus plants are decreased in a dose-dependent manner in response to UVA exposure. Additionally, the accumulation of the extractable flavonoids was examined following a shift from photosynthetically active radiation (PAR) + UVA to PAR + UVB to assess if preexposure to UVA affected UVB-induced flavonoid accumulation. UVA preexposures were found to impede UVB-induced accumulation of some flavonoids. This down regulation was particularly evident for quercetin-3-O-sophoroside and quercetin-3-O-sophoroside-7-O-glucoside, which is interesting because quercetins have been demonstrated to be induced by UVB and correlated with UVB tolerance in some plant species. The photobiological nature of these UVA-mediated effects on flavonoid accumulation implies complex interactions between UVA and UVB responses.

Ultraviolet-B radiation impacts light-mediated turnover of the photosystem II reaction center heterodimer in Arabidopsis mutants altered in phenolic metabolism

Ultraviolet-B (UV-B) radiation can have a negative impact on the growth and development of plants. Plants tolerant to UV-B alleviate these effects using UV-screening pigments that reduce the penetration of UV-B into mesophyll tissue. Little is known about the relative contribution of specific phenolic compounds to the screening capacity of leaves. The D1 and D2 proteins constituting the photosystem (PS) II reaction center heterodimer are targets of UV-B radiation and can be used as an in situ sensor for UV penetration into photosynthetic tissue. Degradation of these proteins occurs under very low fluences of UV-B, and is strongly accelerated in the presence of visible light. Using the D1-D2 degradation assay, we characterized UV-B sensitivity of Arabidopsis mutants (tt4, tt5, and fah1) that are genetically altered in their composition of phenolic compounds. We found that changes in phenol metabolism result in altered rates of PSII reaction center heterodimer degradation under mixtures of photosynthetically active radiation and UV-B. A comparison of D2 degradation kinetics showed increased UV sensitivity of the Landsberg (Landsberg erecta) tt5 mutant relative to the Landsberg tt4 mutant and the Landsberg wild type. Despite a lack of flavonoid accumulation, the tt4 mutant is not particularly UV sensitive. However, the tolerance of this mutant to UV-B may reflect the increased accumulation of sinapate esters that strongly absorb in the UV range, and may thus protect the plant against environmentally relevant UV-B radiation. This sinapate-mediated protection is less obvious for the tt4 mutant of Columbia ecotype, indicating that the relative contribution of particular phenolics to the total screening capacity varies with the genetic background. The role of sinapate esters in UV screening is further substantiated by the results with the fah1 mutant where absence of most of the sinapate esters results in a significantly accelerated degradation of D2 under mixed light conditions. Because the latter mutant is not expected to be deficient in flavonoids, the relative contribution of flavonoids as protectants of PSII reaction center heterodimer against UV-B damage in Arabidopsis needs to be re-evaluated vis-a-vis screening by simple phenolics like sinapate esters.

Effects of UV-B radiation on growth, photosynthesis, UV-B-absorbing compounds and NADP-malic enzyme in bean (Phaseolus vulgaris L.) grown under different nitrogen conditions

The effects of UV-B radiation on growth, photosynthesis, UV-B-absorbing compounds and NADP-malic enzyme have been examined in different cultivars of Phaseolous vulgaris L. grown under 1 and 12 mM nitrogen. Low nitrogen nutrition reduces chlorophyll and soluble protein contents in the leaves and thus the photosynthesis rate and dry-matter accumulation. Chlorophyll, soluble protein and Rubisco contents and photosynthesis rate are not significantly altered by ambient levels of UV-B radiation (17 microW m-2, 290-320 nm, 4 h/day for one week). Comparative studies show that under high nitrogen, UV-B radiation slightly enhances leaf expansion and dry-matter accumulation in cultivar Pinto, but inhibits these parameters in Vilmorin. These results suggest that the UV-B effect on growth is mediated through leaf expansion, which is particularly sensitive to UV-B, and that Pinto is more tolerant than Vilmorin. The effect of UV-B radiation on UV-B-absorbing compounds and on NADP-malic enzyme (NADP-ME) activity is also examined. Both UV-B radiation and low-nitrogen nutrition enhance the content of UV-B-absorbing compounds, and among the three cultivars used, Pinto exhibits the highest increases and Arroz the lowest. The same trend is observed for the specific activity and content of NADP-ME. On a leaf-area basis, the amount of UV-B-absorbing compounds is highly correlated with the enzyme activity (r2 = 0.83), suggesting that NADP-ME plays a key role in biosynthesis of these compounds. Furthermore, the higher sensitivity of Vilmorin than Pinto to UV-B radiation appears to be related to the activity of NADP-ME and the capacity of the plants to accumulate UV-B-absorbing compounds.

Low-temperature stress and photoperiod affect an increased tolerance to photoinhibition in Pinus banksiana seedlings

The capacity to develop tolerance to photoinhibition of photosynthesis was assessed in jack pine seedlings (Pinus banksiana Lamb.). Photoinhibition induced at 5 °C in control jack pine seedlings grown at 20 °C was saturated above an irradiance of 1000 μmol ∙ m−2 ∙ s−1 but was detectable at an irradiance as low as 25 μmol ∙ m−2 ∙ s−1. However, 20 °C seedlings shifted to 5 °C were 2-fold more tolerant to photoinhibition than 20 °C unshifted control seedlings, as detected by either the light-dependent decrease in photochemical efficiency or the apparent quantum yield of O2 evolution. The extent of this tolerance of photoinhibition was dependent upon time, photoperiod, and irradiance during exposure to the low-temperature shift. Furthermore, the tolerance of photoinhibition was correlated with anthocyanin accumulation in 20 °C grown seedlings shifted to 5 °C. In addition, seedlings shifted to 5 °C and an 8-h photoperiod exhibited a 2-fold higher yield of photosystem II electron transport, which was associated with an increased capacity to keep QA, the first stable quinone electron acceptor of photosystem II, oxidized at high irradiance. This was consistent with a 2-fold higher rate of photosynthesis on a chlorophyll basis. We propose that the combination of light attenuation by anthocyanin in the epidermis and enhanced rates of photosynthesis may, in part, account for the reduced sensitivity of jack pine to photoinhibition at low temperature. Key words: anthocyanin, light attenuation, low temperature, Pinus banksiana Lamb, (jack pine), photosynthesis, photoinhibition, photoperiod.

Inhibition of PS II photochemistry by PAR and UV radiation in natural phytoplankton communities

The effects of PAR and UV radiation on PS II photochemistry were examined in natural phytoplankton communities from coastal waters off Rhode Island (USA) and the subtropical Pacific. The photochemical energy conversion efficiency, the functional absorption cross section and the kinetics of electron transfer on the acceptor side of PS II were derived from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phytoplankton communities displayed marked decreases in PS II photochemical energy conversion efficiency that were correlated with increased PAR. In the coastal waters, the changes in photochemical energy conversion efficiency were not statistically different for samples treated with supplementary UV-B radiation or screened to exclude ambient UV-B. Moreover, no significant light-dependent changes in the functional absorption cross section of PS II were observed. The rate of electron transfer between QA (-) and QB was, however, slightly reduced in photodamaged cells, indicative of damage on the acceptor side. In the subtropical Pacific, the decrease in photochemical energy conversion efficiency was significantly greater for samples exposed to natural levels of UV-A and/or UV-B compared with those exposed to PAR alone. The cells displayed large diurnal changes in the functional absorption cross section of PS II, indicative of non-photochemical quenching in the antenna. The changes in the functional absorption cross section were highly correlated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited reaction centers rapidly recover (within an hour or two) to their preillumination values. Thus, while we found definitive evidence for photoinhibition of PS II photochemistry in both coastal and open ocean phytoplankton communities, we did not find any effect of UV-B on the former, but a clear effect on the latter. The results of this study indicate that the effects of UV-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body and the mixing rate, as on the wavelength and energy distribution of the radiation and the absorption cross sections of the biophysical targets.

UV-B damage and protection at the molecular level in plants

Influx of solar UV-B radiation (280-320 nm) will probably increase in the future due to depletion of stratospheric ozone. In plants, there are several targets for the deleterious UV-B radiation, especially the chloroplast. This review summarizes the early effects and responses of low doses of UV-B at the molecular level. The DNA molecules of the plant cells are damaged by UV due to the formation of different photoproducts, such as pyrimidine dimers, which in turn can be combatted by specialized photoreactivating enzyme systems. In the chloroplast, the integrity of the thylakoid membrane seems to be much more sensitive than the activities of the photosynthetic components bound within. However, the decrease of mRNA transcripts for the photosynthetic complexes and other chloroplast proteins are among very early events of UV-B damage, as well as protein synthesis. Other genes, encoding defence-related enzymes, e.g., of the flavonoid biosynthetic pathway, are rapidly up-regulated after commencement of UV-B exposure. Some of the cis-acting nucleotide elements and trans-acting protein factors needed to regulate the UV-induced expression of the parsley chalcone synthase gene are known.

Ultraviolet radiation, ozone depletion, and marine photosynthesis

Concerns about stratospheric ozone depletion have stimulated interest in the effects of UVB radiation (280-320 nm) on marine phytoplankton. Research has shown that phytoplankton photosynthesis can be severely inhibited by surface irradiance and that much of the effect is due to UV radiation. Quantitative generalization of these results requires a biological weighting function (BWF) to quantify UV exposure appropriately. Different methods have been employed to infer the general shape of the BWF for photoinhibition in natural phytoplankton, and recently, detailed BWFs have been determined for phytoplankton cultures and natural samples. Results show that although UVB photons are more damaging than UVA (320-400 nm), the greater fluxes of UVA in the ocean cause more UV inhibition. Models can be used to analyze the sensitivity of water column productivity to UVB and ozone depletion. Assumptions about linearity and time-dependence strongly influence the extrapolation of results. Laboratory measurements suggest that UV inhibition can reach a steady-state consistent with a balance between damage and recovery processes, leading to a non-linear relationship between weighted fluence rate and inhibition. More testing for natural phytoplankton is required, however. The relationship between photoinhibition of photosynthesis and decreases in growth rate is poorly understood, so long-term effects of ozone depletion are hard to predict. However, the wide variety of sensitivities between species suggests that some changes in species composition are likely. Predicted effects of ozone depletion on marine photosynthesis cannot be equated to changes in carbon flux between the atmosphere and ocean. Nonetheless, properly designed studies on the effects of UVB can help identify which physiological and ecological processes are most likely to dominate the responses of marine ecosystems to ozone depletion.