Supplemental ultraviolet-B Radiation Research Articles

Effects of reduced, ambient, and enhanced UV-B radiation on pollen germination and pollen tube growth of six alpine meadow annual species

Six alpine meadow annual species Vicia angustifolia L. ex Reichard. (Leguminosae), Poa annua L. (Gramineae), Polygonum aviculare L. (Polygonaceae), Plantago depressa Willd. (Plantaginaceae), Elsholtzia densa Benth. (Labiatae), and Malva sinensis Cav. (Malvaceae) were grown in the field under reduced and ambient ultraviolet-B (UV-B) levels, and an ambient plus supplemental UV-B radiation, simulating a 9% ozone depletion over Gannan, China (102°53′E, 34°55′N, 2900 m above sea level). Pollen grains were collected at full florescence at 10:30–11:00 a.m. in fine day and then the pollen germination and pollen tube growth were measured. Species-specific effects were observed and distributed both positive and negative responses to the treatments. Enhanced UV-B radiation significantly ( p < 0.05) reduced pollen germination and pollen tube growth in V. angustifolia compared with ambient and reduced UV-B radiations. Pollen germination of P. annua and P. aviculare as well as pollen tube growth of P. annua and M. sinensis were significantly increased by enhanced UV-B radiation compared with the control. Meanwhile, no significant effect was found in pollen germination and pollen tube growth in P. depressa and E. densa by enhanced UV-B radiation compared with ambient control. Pollen germination in P. annua, P. aviculare, and E. densa and pollen tube growth in P. annua and E. densa were higher in pollen collected from the plant treated with reduced UV-B radiation than in pollen from plants grown in ambient conditions, and no significant negative effect was found in pollen germination and pollen tube growth by reduced UV-B radiation compared with the control in the tested species. Growth rate of pollen tube was much higher in V. angustifolia, P. annua, and P. aviculare in comparison to that in P. depressa, E. densa, and M. sinensis. Our results suggested that pollen of some species in Qinghai-Tibet Plateau were relatively tolerant to UV-B radiation in comparison to pollen of the species in lower altitude. In the future, the different responses among species to changed UV-B radiation may lead to a change in the competitive balance and species composition in natural communities on Qinghai-Tibet Plateau if UV-B radiation is increased.

Inheritance of UV-B tolerance in seven ecotypes of Arabidopsis thaliana L. Heynh. and their F1 hybrids.

We used a partial diallel mating design to examine morphologic response to supplementary ultraviolet-B (UV-B) radiation of seven ecotypes of Arabidopsis thaliana L. Heynh. from several geographic locations in Europe. We were particularly interested in the inheritance of UV-B tolerance by the F1 generation. Morphologic traits included plant height, rosette diameter, number of shoots (lateral branches from the rosette) and branches (lateral branches above the rosette), and reproductive and vegetative dry mass. To effect a large difference in UV treatments, plants under treatment received 11 kJ/m2/day of biologically effective UV-B radiation while control plants received no UV-B radiation. Genotype effects were observed for all traits (P < .0001), but a significant treatment effect and genotype x treatment interactions were detected only for plant height (P = .0001), rosette diameter (P = .0229), and vegetative (P = .0260) and reproductive dry mass (P = .0900). General combining ability was significant for plant height (P < .0001) and vegetative mass (P = .0563), whereas specific combining ability was significant for rosette diameter (P = .0220) and vegetative mass (P = .0506). These results suggest that both pure lines and hybrids of Arabidopsis can be developed for greater tolerance of UV-B radiation. Similar findings for crop species might lead to the development of UV tolerant varieties.

Population differences in Trifolium repens L. response to ultraviolet-B radiation: foliar chemistry and consequences for two lepidopteran herbivores.

White clover growing in New Zealand is experiencing increasing levels of ultraviolet-B (UV-B) radiation as a result of ozone depletion. We evaluated the effects of UV-B radiation on the foliar chemistry of two populations of white clover (Trifolium repens L.), 'Huia' and 'Tienshan,' and the consequences for the performance of armyworms (Spodoptera litura) and cutworms (Graphania mutans). Plants were grown in controlled environment rooms with and without supplemental UV-B radiation at a dose of 13.3 kJ m-2 day-1, corresponding to a 25% mid-summer ozone depletion above Palmerston North, New Zealand. In both white clover populations, UV-B radiation elicited changes in foliar chemistry, including slight increases in nitrogen concentrations and decreases in carbohydrate concentrations. In addition, the 'Huia' population showed decreases in fiber concentrations and marked increases in cyanogenic activity. No change in UV-absorbing compounds was detected in either population. Long- and short-term feeding trials were conducted to assess dietary effects on insect growth, consumption, and food utilization. Changes in the performance of both insect species were generally small. The most pronounced effect was a 36% reduction in weight of S. litura after 2 weeks of feeding on Huia grown at high UV, but larval development times were only slightly prolonged and pupal weights were unaffected. S. litura short-term performance was affected by differences in white clover population. The long-term performance of G. mutans was not affected and its short-term performance (stadium duration and consumption rate) was only marginally affected by the high-UV treatment. We conclude that the effects of elevated UV-B radiation on white clover plant chemistry can be specific to certain plant populations. The differences in sensitivity of the two generalist insect species suggest that effects may also be specific to certain plant-herbivore associations. These results indicate that future UV-B herbivory studies should examine genotypic effects in both plants and animals.

Influence of UV-B radiation and Cd2+ on chlorophyll fluorescence, growth and nutrient content in Brassica napus

The possible interaction of two stresses, UV-B radiation and cadmium, applied simultaneously, was investigated in Brassica napus L. cv. Paroll with respect to chlorophyll fluorescence, growth and uptake of selected elements. Plants were grown in nutrient solution containing CdCl 2 , (0, 0.5, 2 or 5 μM) and irradiated with photosynthetically active radiation (PAR, 400-700 nm, 800 μmol m -2 s -1 ) with or without supplemental ultraviolet-B radiation (UV-B, 280-320 nm, 15 kJ m -2 d -1 , weighted irradiance). After 14 d of treatment, the most pronounced effects were found at 2 and 5 μM CdCl 2 with and without supplemental UV-B radiation. Exposure to cadmium significantly increased the amount of Cd in both roots and shoots. In addition, increases occurred in the concentration of Fe, Zn, Cu, and P in roots, while K was reduced. In shoots the S content rose significantly both in the presence and absence of UV-B radiation, while significant increases in Mg, Ca, P, Cu, and K occurred only in plants exposed to Cd and UV-B radiation. Manganese decreased significantly under the combined exposure treatment. The rise in S content may have been due to stimulated glutathione and phytochelatin synthesis. Cadmium exposure significantly decreased root dry weight, leaf area, total chlorophyll content, carotenoid content, and the photochemical quantum yield of photosynthesis. As an estimation of energy dissipation processes in photosynthesis, non-photochemical quenching (q NPQ ) was measured using a pulse amplitude modulated fluorometer. The q NPQ increased with increasing Cd, while the combination of cadmium and UV-B reduced the q NPQ compared to that in plants exposed only to cadmium or UV-B radiation. The chlorophyll a:b ratio showed a reduction with UV-B at no or low Cd concentrations (0 μM, 0.5 μM CdCl 2 ), but not at the higher Cd concentrations used (2 μM, 5 μM CdCl 2 ). Thus in some instances there appeared to be a UV-B and Cd interaction, while in others plant response could be attributed to either treatment alone.

Influence of UV-B radiation and Cd2+ on chlorophyll fluorescence, growth and nutrient content in Brassica napus

The possible interaction of two stresses, UV-B radiation and cadmium, applied simultaneously, was investigated in Brassica napus L. cv. Paroll with respect to chlorophyll fluorescence, growth and uptake of selected elements. Plants were grown in nutrient solution containing CdCl2, (0, 0.5, 2 or 5 μM) and irradiated with photosynthetically active radiation (PAR, 400–700 nm, 800 μmol m−2 s−1) with or without supplemental ultraviolet-B radiation (UV-B, 280–320 nm, 15 kJ m−2 d−1, weighted irradiance). After 14 d of treatment, the most pronounced effects were found at 2 and 5 μM CdCl2 with and without supplemental UV-B radiation. Exposure to cadmium significantly increased the amount of Cd in both roots and shoots. In addition, increases occurred in the concentration of Fe, Zn, Cu, and P in roots, while K was reduced. In shoots the S content rose significantly both in the presence and absence of UV-B radiation, while significant increases in Mg, Ca, P, Cu, and K occurred only in plants exposed to Cd and UV-B radiation. Manganese decreased significantly under the combined exposure treatment. The rise in S content may have been due to stimulated glutathione and phytochelatin synthesis. Cadmium exposure significantly decreased root dry weight, leaf area, total chlorophyll content, carotenoid content, and the photochemical quantum yield of photosynthesis. As an estimation of energy dissipation processes in photosynthesis, non photochemical quenching (qNPQ) was measured using a pulse amplitude modulated fluorometer. The qNPQ increased with increasing Cd, while the combination of cadmium and UV-B reduced the qNPQ compared to that in plants exposed only to cadmium or UV-B radiation. The chlorophyll a : b ratio showed a reduction with UV-B at no or low Cd concentrations (0 μM, 0.5 μM CdCl2), but not at the higher Cd concentrations used (2 μM, 5 μM CdCl2). Thus in some instances there appeared to be a UV-B and Cd interaction, while in others plant response could be attributed to either treatment alone.

Photochemical and biochemical changes in wheat seedlings exposed to supplementary ultraviolet-B radiation

The effect of UV-B radiation (312 nm; 1 mW cm−2) was studied on net photosynthesis, chlorophyll fluorescence and changes in flavonoid and carotenoid contents in wheat seedlings. Control plants (without UV-B treatment but identical light and temperature regime as for UV-B treatment) and UV-B treated plants were grown in two separate growth chambers for 15 days. The supplementary UV-B radiation caused a significant decrease in net photosynthesis which was much greater than could be explained by limitation of stomatal conductance. Initial fluorescence (Fo), Fv/Fm and photochemical quenching (qP) and non-photochemical quenching (qN) of chlorophyll fluorescence did not change due to 5 days of UV-B treatment, but longer treatment (up to 15 days) increased Fo while decreasing the Fv/Fm ratio. qP and qN also decreased after 15 days of UV-B exposure. Changes in UV-B absorbing phenolic compounds such as flavonol (kaempferol), caumarin and anthocyanin were observed, whereas cinnamic acid was only synthesised after 4 days of UV-B treatment. No zeaxanthin formation was observed while neoxanthin showed a decrease in longer UV-B treatment. Violaxanthin showed an increase in early treatment but decreased after 15 days of UV-B exposure. Our results suggest two types of damage, one within 4–5 days of UV-B exposure and another with longer than 5 days exposure.

Effects of Supplementary Ultraviolet-B Radiation on Photosynthetic Transcripts at Different Stages of Leaf Development and Light Levels in Pea (Pisum sativum L.): Role of Active Oxygen Species and Antioxidant Enzymes.

Effects of Supplementary Ultraviolet-B Radiation on Photosynthetic Transcripts at Different Stages of Leaf Development and Light Levels in Pea (Pisum sativum L.): Role of Active Oxygen Species and Antioxidant Enzymes.

Ultraviolet-B-radiation-induced changes in nicotinamide and glutathione metabolism and gene expression in plants.

Pea (Pisum sativum L. cv. Greenfeast) plants were exposed to supplementary ultraviolet-B (UV-B) radiation (biologically effective dose rates normalised to 300 nm, UV-B[BE,300]: 0.18, 0.32 or 1.4 W m[-2]). Leaf nicotinamide, trigonelline, GSHtot (total glutathione) and GSSG (oxidised glutathione) levels remained unchanged after exposure to the lowest dose rates. 1.4 W m(-2) UV-B(BE,300) gave rise to 60-fold and 4.5-fold increases in GSSG and GSHtot, respectively. 3.5-fold and 9.5-fold increases were found in nicotinamide and trigonelline, respectively. cab (Chlorophyll-a/b-binding protein) transcript levels decreased and CHS (chalcone synthase) and PAL (phenylalanine ammonia-lyase) mRNA increased after shorter UV-B exposures (hours) to the higher dose rate of UV-B, and after exposure to the intermediate dose rate. CHS and PAL mRNAs also increased after prolonged exposure to the lowest dose rate. cab transcripts completely disappeared, whereas CHS and PAL mRNA levels rose by 60-fold and 17-fold, respectively, after 12 h exposure at the highest dose rate and 12 h of development. Our results indicate that nicotinamide or trigonelline do not function as signalling compounds for CHS and PAL gene expression. Elevated nicotinamide and trigonelline levels occur in response to UV-B, but only at UV-B doses high enough to cause oxidative stress.

Influence of supplemental UV-B radiation on photosynthetic characteristics of rice plants

In a field experiment with rice (Oryza sativa L. cv. Saket 4) grown under ambient and supplemental ultraviolet-B (UV-B) radiation at 20 % ozone depletion, differences in gas exchange, concentrations of photosynthetic pigments, anthocyanins and flavonoids, biomass accumulation, catalase and peroxidase activities, and contents of ascorbic acid and phenol were determined. Decline in photosynthesis was associated with reductions in stomatal conductance and concentrations of photosynthetic pigments. Enhanced UV-B radiation (eUV-B) increased the contents of flavonoid and phenolic compounds in leaves. Peroxidase activity increased and catalase activity was always lower at eUV-B. The total plant biomass decreased at eUV-B.

The effect of supplementary ultraviolet-B radiation on mRNA transcripts, translation and stability of chloroplast proteins and pigment formation in Pisum sativum L.

The effect of supplemental UV-B radiation on gene expression for three photosynthetic proteins (chlorophyll a/b-binding protein, D1 polypeptide of PS II and RUBISCO) and on flavonoid composition has been investigated in expanded leaves and in apical buds of pea seedlings. In the expanded third leaves, UV-B caused a decrease in nuclear-encoded Lhcb and RbcS transcript levels within 2 d of UV-B treatment and a later decline in the SSU and LHCB polypeptides. The effect of UV-B on the chloroplast-encoded rbcL and psbA genes was more complex. Levels of LSU and D1 initially appeared to be regulated at the translational and/or post-translational level and only later in the UV-B treatment, at the level of mRNA abundance. UV-B also accelerated the degradation of the D1 polypeptide. For all genes studied the UV-B-induced inhibition of transcripts was greater in leaves than in the apical buds, indicating that the bud is less sensitive to UV-B damage. The mRNA transcript levels for chalcone synthase (chs) were also measured. In leaves, chs RNA transcripts were initially present in low amounts and exposure to supplemental UV-B resulted in a transient increase in the level of these transcripts. In contrast, chs transcripts in the apical buds were initially present at high levels, with UV-B exposure resulting in a slow progressive decline in transcript levels. These results indicate that the response to UV-B is complex and is dependent on the organ studied.

Short-term treatment of pea leaves with supplementary UV-B at different oxygen concentrations: impacts on chloroplast and plasma membrane bound processes

The effect of short-term supplementary ultraviolet-B (UV-B, biologically effective irradiance 1.2 W m −2) radiation on membrane function of detached leaves of pea ( Pisum sativum L.) was studied at low (2%), normal (22%) and high (60%) oxygen concentration to evaluate if photo-oxidative processes are a primary cause of UV-B-induced damage. The effect of supplementary UV-B on thylakoids was investigated by measuring the maximal photochemical efficiency of photosystem II ( Fv Fm ), the relaxation half-time ( t 1 2 ) of the rapid flash-induced decrease in light transmission at 515 nm and polyunsaturated lipid peroxidation, determined as malondialdehyde (MDA) formation. Damage to the plasma membrane was assessed by measuring electrolyte leakage from leaf disks. Fv Fm decreased linearly during exposure to supplementary UV-B due to a concomitant increase of initial fluorescence, Fo and decrease of maximal fluorescence, Fm. The decrease of Fv Fm in 60% oxygen was similar to that in air, but was enhanced in 2% oxygen compared with air. The relaxation half-time ( t 1 2 ) decreased both in treated and control leaves, but the decrease was about 2 times faster in UV-B exposed leaves. The UV-B-induced decrease of t 1 2 was not influenced by high or low oxygen concentration. Supplementary UV-B induced an increase of lipid peroxidation as a delayed response after exposure, indicating that UV-B-induced lipid peroxidation was a secondary effect. Lipid peroxidation was not differentially influenced by high or low oxygen concentration during exposure to UV-B. Exposing leaf disks to supplementary UV-B for 8 h caused a 2.5-fold higher electrolyte leakage than in controls; leakage of 8 h-exposed disks further increased during storage in the dark following UV-B exposure, while leakage of control disks slightly decreased during dark storage. Low oxygen concentration (2%) did not protect against UV-B-induced damage of the plasma membrane. These results do not support a direct involvement of photo-oxidative processes as a primary cause of UV-B-induced membrane damage in pea leaves.

Changes in the Relaxation of Electrochromic Shifts of Photosynthetic Pigments and in the Levels of mRNA Transcripts in Leaves of Pisum sativum as a Result of Exposure to Supplementary UV-B Radiation. The Dependency on the Intensity of the Photosynthetically Active Radiation

Supplementary ultraviolet-B radiation (UV-B) caused decreases in mRNA levels for photosynthetic genes and for the chloroplastic defensive enzyme Cu/Zn-superoxide dismutase. The magnitude of these decreases depended on the intensity of the photosynthetically active radiation (PAR): the mRNA levels being considerably higher under high PAR (400–600 μE m-2 s-1) than under low (150 μE m-2 s-1). Transcript levels for the non-chloroplastic defence protein chalcone synthase also were PAR-dependent, since an increase of mRNA transcripts occurred under low PAR only. UV-B also caused a PAR-independent increase in the rates of relaxation of the single turnover flash-induced electrochromic shift of thylakoid pigments. The UV-B doses needed were about 10-fold lower than the doses needed to affect photosynthetic components. Chlorophyll content and chlorophyll fluorescence of PSII were unaltered by these doses. We suggest that the lowered sensitivity of mRNA levels under high PAR may be caused by a protective mechanism at the level of DNA or transcription. This mechanism would not be involved in repair of the thylakoid membrane or its lipids. These results also highlight the importance of PAR intensity for the assessing of UV-B effects on plants.

Modification of the Cellular Heat Sensitivity of Cucumber by Growth under Supplemental Ultraviolet-B Radiation.

The effect of ultraviolet B (UV-B) radiation on the thermal sensitivity of cucumber (Cucumis sativus L.) was studied using UV-B-sensitive cv Poinsett 76 and UV-B-resistant cv Ashley grown under control and elevated (300 mW m-2) UV-B radiation levels. Using both cotyledon and leaf discs, the ability of the tissue to reduce triphenyl tetrazolium chloride (TTC) was determined after treatment at 50[deg]C for various times. Semilogarithmic plots of TTC reduction as a function of time at 50[deg]C were curvilinear. They were monophasic for the control cucumber and biphasic for cucumber grown in the presence of elevated UV-B. Treatment of cucumber plants at 37[deg]C for 24 h or of tissue discs at acute UV-B levels for 1 h further modified their response to elevated temperature. These results suggest that growth of cucumber under enhanced UV-B radiation levels increased its ability to withstand elevated temperatures.

Alteration in Expression of Defence Genes in &lt;italic&gt;Pisum sativum&lt;/italic&gt; after Exposure to Supplementary Ultraviolet-B Radiation

Alteration in Expression of Defence Genes in &lt;italic&gt;Pisum sativum&lt;/italic&gt; after Exposure to Supplementary Ultraviolet-B Radiation

Responses of Detached Rice Leaves (Oryza sativa L.) To Moderate Supplementary Ultraviolet-B Radiation Allow Early Screening for Relative Sensitivity to Ultraviolet-B Irradiation

Responses to short-term supplementary ultraviolet-B (UV-B) radiation were studied in detached leaves of two indica rice cultivars (Er Bai Ai and Lemont) to evaluate whether this might be an initial method for screening for UV-B susceptibility. Leaf tissue from plants grown in a greenhouse (28�C day/ 20�C night, with a maximum irradiance of 800-1000 μmol photons m-2 s-1) was placed under moderate supplementary UV-B radiation for 20 h. The effects of this short-term treatment were measured by determining the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm), quantum yield of photosynthetic O2 exchange, chlorophyll content, maximum Rubisco activity as well as the concentrations of total soluble protein and Rubisco protein. All the above parameters showed considerable declines, which were always greater in cv. Er Bai Ai than in cv. Lemont. The in vivo activation of Rubisco was markedly increased in detached leaves treated with supplementary UV-B compared with control leaves; the increase was greater in cv. Er Bai Ai than in cv. Lemont. The photosynthetic responses invoked in the detached rice leaves are remarkably similar to those observed previously [He et al. (1993). Aust. J. Plant Physiol. 20, 129-42] in intact rice plants which had a longer-term supplementary UV-B exposure of comparable cumulative biologically effective UV-B dosage. We conclude that rapid short-term responses of detached leaves allow early screening of relative sensitivity of rice cultivars to UV-B.

Effects of Supplementary Ultraviolet-B Radiation on Rice and Pea Plants

To compare the effects of supplementary ultraviolet-B (UV-B) radiation on a tropical/subtropical and a temperate plant, two indica rice cultivars (Er Bai Ai and Lemont) and peas were exposed to supplementary UV-B radiation for 8 days (biologically effective irradiance of 0.68 W m-2, 12 h per day). Marked decreases occurred in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution over the 8 day treatment period. The greatest decline always occurred in pea leaves, while in rice, cv. Er Bai Ai was more susceptible to UV-B radiation than cv. Lemont. Compared with control plants, the content of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein decreased significantly after the UV-B treatment; the greatest decrease occurred in pea leaves, while the rate of decrease in rice was greater in cv. Er Bai Ai than cv. Lemont. Over the 8 day UV-B treatment period, the increase of UV-B-absorbing compounds was greater in rice leaves than in pea leaves, and greater in cv. Er Bai Ai than in cv. Lemont, although cv. Lemont was more UV-B tolerant. We suggest that the increase in these compounds is not the only indicator of resistance to UV-B damage; other factors, yet to be identified, may also confer UV-B tolerance. While leaf orientation may be related to the severity of UV-B damage, naturally near-horizontal pea leaves were still much more susceptible to UV-B damage compared with rice leaves which were held horizontally over the 8 day treatment period. Brown spots appeared on the upper surface of leaves of cv. Er Bai Ai after 2 days of UV-B treatment and showed a cumulative increase with the duration of exposure. Our study confirms the multiplicity of photosynthetic responses and of different protective strategies that may account for the differential sensitivity of plants to supplementary UV-B radiation.

Reduction in cab and psb A RNA transcripts in response to supplementary ultraviolet-B radiation

The cab and psb A RNA transcript levels have been determined in Pisum sativum leaves exposed to supplementary ultraviolet-B radiation. The nuclear-encoded cab transcripts are reduced to low levels after only 4 h of UV-B treatment and are undetectable after 3 days exposure. In contrast, the chloroplast-encoded psb A transcript levels, although reduced, are present for at least 3 days. After short periods of UV-B exposure (4 h or 8 h), followed by recovery under control conditions, cab RNA transcript levels had not recovered after 1 day, but were re-established to ca. 60% of control levels after 2 more days. Increased irradiance during exposure to UV-B reduced the effect upon cab transcripts, although the decrease was still substantial. These results indicate rapid changes in the cellular regulation of gene expression in response to supplementary UV-B and suggest increased UV-B radiation may have profound consequences for future productivity of sensitive crop species.

Effect of UV-B Radiation on Leaf Optical Properties Measured with Fibre Optics

Changes in the internal light microenvironment in leaves of plants of Brassica campestris L. cv. Emma, B. carinata L., and Medicago saliva L. cv. Armour in response to exposure to UV-B (UV-B, 280–320 nm) radiation were measured using a fibreoptic microprobe. Plants were exposed for 2 weeks either to high visible light or to supplemental ultraviolet-B radiation. The spectral regime (400–700 nm; PAR) was measured either midway through the leaf palisade or the spongy mesophyll. After exposure to UV-B radiation leaves of Brassica campesiris attenuated transmitted light more than the controls. At the same time both forward and back scattered light increased in the palisade and spongy mesophylls. In contrast, UV-treatment of Medicago saliva leaves increased light transmission into the palisade, while the back scattered component showed little change. Leaves of cariiwla showed little change in response to UV. Other responses to UV-B radiation included increases in leaf thickness, decreased total chlorophyll content, and changes in UV-B screening pigments and chlorophyll fluorescence induction kinetics. Brassica campestris was most sensitive to exposure to enhanced levels of UV-B radiation, whereas leaves of B. carinata were the least sensitive. Our data indicate that exposure to UV-B radiation altered the light microenvironment within leaves of the species different ways. These changes appeared to be caused by alterations in pigment content and leaf anatomy. In turn, the altered distribution of PAR within the leaf could influence photosynthesis.

Effects of supplementary ultraviolet-B radiation on photosynthesis in Pisum sativum

Pea plants ( Pisum sativum L., cv. Greenfeast) were exposed to supplementary UV-B light (up to 8 days) starting on the 17th day after sowing. The effects of this exposure on photosynthesis and the content and activities of some chloroplast components of the mature leaves of these plants were studied. (i) The total chorophyll content of pea leaves was approximately 40% of that in the control leaves on the 8th day of UV-B exposure. Chlorophyll a levels decreased to a greater extent than the content of chlorophyll b. The decrease in carotenoids paralleled the decrease in chlorophyll b. (ii) On a chlorophyll basis, the contents of Photosystem I and cytochrome f were stable, whereas Photosystem II, ATP hydrolysis by the ATP synthase and the maximum ribulose-1,5-bisphosphate carboxylase (Rubisco) activity decreased by 55, 47 and 80%, respectively, when compared with the controls at the end of the 8-day illumination period. (iii) On a leaf-area basis, Photosystem I and cytochrome f content decreased by 58%, Photosystem II by 80%, ATP hydrolysis by 80%, and Rubisco activity by 90%, when compared with the controls. The in vivo activation of Rubisco was markedly increased in UV-B-treated pea leaves. The underlying mechanisms for these results are discussed.

Field Study of the Interaction between Solar Ultraviolet-B Radiation and Drought on Photosynthesis and Growth in Soybean

Soybean, Glycine max (L.) Merr. cv Essex, plants were grown in the field in a 2 x 2 factorial design, under ambient and supplemental levels of ultraviolet-B (UV-B) radiation (supplemental daily dose of 5.1 effective kilojoules per square meter) and were either well-watered or subjected to drought. Soil water potentials were reduced to -2.0 megapascals by the exclusion of natural precipitation in the drought plots and were maintained at approximately -0.5 megapascal by supplemental irrigation in well-watered plots. Plant growth and gas exchange characteristics were affected under both drought and supplemental UV-B radiation. Whole-leaf gas exchange analysis indicated that stomatal limitations on photosynthesis were only significantly affected by the combination of UV-B radiation and drought but substrate (ribulose bisphosphate) regeneration limitations were observed under either stress. The combined effect of both drought and UV-B radiation on photosynthetic gas exchange was a reduction in apparent quantum efficiency and the rapid appearance of biochemical limitations to photosynthesis concomitant with reduced diffusional limitations. However, the combination of stresses did not result in additive effects on total plant growth or seed yield compared to reductions under either stress independently.