Effects of enhanced UV-B radiation on pea (Pisum sativum L.) grown under field conditions in the UK

A new modulated lamp system is described. This system has successfully provided an ultraviolet-B (UV-B) supplement in proportion to ambient UV-B. The modulated system was used to simulate the UV-B environment resulting from an annual mean reduction of 15% in the stratospheric ozone under UK field conditions, but taking account of seasonal variation in depletion. The effects of this enhanced level of UV-B on the growth, physiology and yield of four cultivars of pea were assessed. Enhanced UV-B resulted in small reductions in the number of stems and total stem length per plant (respectively 4.7 and 8.7%). There were also significant decreases in the dry weight of peas (10.1%), pods (10.3%) and stems (7.8%) per plant. UV-B treatment had no effect on the number of peas per pod or average pea weight, but did significantly reduce (12.1%) the number of pods per plant. This decrease in pod number was partly due to enhanced abscission of pods during the final month of plant growth. UV-B treatment had no significant effect on chlorophyll fluorescence characteristics or CO₂assimilation rate per unit leaf area. These results are consistent with previous controlled environment experiments, and suggest that reduction in yield may be due to direct effects of UV-B on plant growth rather than a decrease in photosynthetic capacity per unit leaf area.     

Alterations in photosynthesis and pigment distributions in pea leaves following UV-B exposure

We compared photosynthetic and UV-B-absorbing pigment concentrations, gas-exchange rates and photosystem II (PSII) electron transport rates in leaves of pea (Pisum sativum mutant Argenteum) grown without UV-B or under an enhanced UV-B treatment (18 kJ m^?2 biologically effective daily dose) in a greenhouse. We also compared the distribution of chlorophyll by depth within leaves of each treatment by using image analysis of chlorophyll autofluorescence. Ultraviolet-B treatment elicited putative protective responses such as an 80% increase in UV-B-absorbing compound concentrations (leaf-area basis), and a slight increase in mesophyll thickness (178 in controls compared to 191 μm in UV-B-treated leaves). However, photosynthetic rates of UV-B-treated leaves were only 80% of those of controls. This was paralleled by reductions in leaf conductance to water vapor (50% of controls) and intercellular CO₂ concentrations, suggesting that stomatal limitations were at least partly responsible for lower photosynthetic rates under the UV-B treatment. Total chlorophyll concentrations (leaf-area basis) in UV-B-treated leaves were only 70% of controls, and there was a shift in the relative distribution of chlorophyll with depth in UV-B-treated leaves. In control leaves chlorophyll concentrations were highest near the adaxial surface of the upper palisade, dropped with depth and then increased slightly in the bottom of the spongy mesophyll nearest the abaxial surface. In contrast, in UV-B-treated leaves chlorophyll concentrations were lowest at the adaxial surface of the upper palisade and increased with depth through the leaf. The most notable treatment difference in chlorophyll concentrations was in the upper palisade near the adaxial surface of leaves, where we estimate that chlorophyll concentrations in each 1-μm-thick paradermal layer were about 50% lower in UV-B-treated leaves than in controls. We found reduced electron transport capacity in UV-B-treated leaves, based on lower maximum fluorescence (F_m), variable to maximum fluorescence ratios (F,/F_m) and quantum yield of PSII electron transport (Y). However, the above were assessed from fluorometer measurements on the adaxial leaf surface and may reflect the markedly lower chlorophyll concentrations in the upper palisade of UV-B-treated leaves.     

Changes in growth and pigment concentrations with leaf age in pea under modulated UV-B radiation field treatments

We assessed whether growth of garden pea (Pisum sativum mutant Argenteum) was reduced under ecologically relevant enhancements of ultraviolet-B radiation (UV-B, 280–320 nm) by employing modulated field lampbanks which simulated 0, 16 or 24% ozone depiction. In addition, we determined whether enhanced UV-B altered the concentration and distribution of chlorophyll and UV-B-absorbing compounds in leaves, and whether this was dependent on leaf age. There were no significant UV-B effects on the four whole-plant parameters we examined (height, above-ground biomass, total leaflet area or average leaflet area). Of the 12 leaf-level parameters we examined, UV-B had a significant effect (P < 0.05) on only one parameter: the ratio of UV-B-absorbing compounds to chlorophyll, which was greatest at the highest UV-B level. Total chlorophyll concentrations tended to be lower under enhanced UV-B (P= 0.11), while the proportion of UV-B-absorbing compounds in the adaxial epidermis tended to be higher (P= 0.11). Total leaf concentrations of UV-B-absorbing compounds were unaffected by UV-B level. Cooler, suboptimal growing conditions during this late summer/early autumn experiment may have masked some potential UV-B effects. In contrast to the UV-B effects, we found strong leaf-age effects on nearly all parameters that we assessed. On an area basis, concentrations of total chlorophyll and UV-B-absorbing compounds increased with leaf age, while Chlorophyll a/b) ratios decreased. One of the few parameters unaffected by leaf age was the ratio of UV-B-absorbing compounds to total chlorophyll, which remained constant within a given UV-B treatment. Pea was much less sensitive to enhanced UV-B than in previous growth-chamber and greenhouse studies, and in nearly all cases UV-B treatment effects were overshadowed by leaf-age effects. In view of the large effect leaf age had on concentrations of UV-B-absorbing compounds, as well as their distribution within leaves, researchers may need to consider leaf age in UV-B experimental designs.     

Beneficial interaction between photosynthesis and respiration in mesophyll protoplasts of pea during short light-dark cycles

The respiratory uptake or photosynthetic evolution of oxygen by mesophyll protoplasts of pea ( Pisum sativum L. cv. Arkel) were monitored during successive short. (3–5 min) cycles of darkness and illumination. The rate of respiration was nearly doubled after 3–4 short periods of illumination while there was a 15–20% enhancement in photosynthesis with cycles of illumination and darkness preceding illumination. Such interaction between photosynthesis and respiration was statistically significant when bicarbonate was present in the reaction medium. The inhibitors of photosynthesis [3(3,4–dichlorophenyl)-l,l-dimethylurea (DCMU), glyceraldehyde] decreased respiration after periods of illumination, whereas inhibitors of respiratory electron transport (Rotenone, antimycin A, NaN₃) suppressed photosynthesis, as well. We suggest that a rapid beneficial interaction exists between photosynthesis and respiration in protoplasts, even during short cycles of light and darkness.     

增强UV-B辐射和干旱对春小麦光合作用及其生长的影响

在室外盆栽条件下研究了UV-B辐射和土壤干旱对春小麦 '和尚头'生长和光合作用的影响.结果显示:(1)干旱、UV-B辐射、干旱+UV-B(复合)处理均可使叶片类黄酮含量增加,且干旱+UV-B处理增加显著高于其他处理(P<0.05).UV-B辐射和干旱单独处理均能显著降低叶片光合色素含量,但UV-B辐射的副作用大于干旱,复合处理对光合色素的影响介于UV-B和干旱之间.(2)各处理间的光合速率日均值大小次序为:对照>UV-B+干旱>UV-B>干旱;增强UV-B对净光合速率的抑制作用大于干旱,而UV-B+干旱处理的抑制作用较二者单独处理有所减轻.(3)UV-B辐射和干旱单独处理后总生物量比对照减少15%,且抑制作用为:干旱>UV-B>复合处理; UV-B辐射和干旱胁迫不但影响春小麦的生物量,而且影响小穗特征和产量.研究表明,UV-B辐射和干旱之间存在交互作用,说明一种胁迫可以减缓(轻)另外一种胁迫对春小麦的抑制作用.     

Non-photosynthetic mechanisms of growth reduction in pea (Pisum sativum L.) exposed to UV-B radiation

Pisum sativum cv. Guido grown under controlled environment conditions was exposed to either low or high UV-B radiation (2·2 or 9·9 kJ m^–2 d^–1 plant-weighted UV-B, respectively). Low or high UV-B was maintained throughout growth (LL and HH treatments, respectively) or plants were transferred between treatments when 22 d old (giving LH and HL treatments). High UV-B significantly reduced plant dry weight and significantly altered plant morphology. The growth and morphology of plants transferred from low to high UV-B were little affected, when compared with those of LL plants. By contrast, plants moved from high to low UV-B showed marked increases in growth when compared with HH plants. This contrast between HL and LH appeared to be related to the effect of UV-B on plant development. Exposure to high UV-B throughout development consistently reduced leaf areas. In fully expanded leaves there was no significant UV-B effect on cell area and reduced leaf area could be attributed to reduced cell number, suggesting effects on leaf primordia. Further reductions in the leaf area of younger leaves were the result of the slower development rate of plants grown at high UV-B, which also resulted in significant reductions in leaf number.     

Phloem exudate metabolic content reflects the response to water-deficit stress in pea plants (Pisum sativum L.)

Drought stress impacts the quality and yield of Pisum sativum. Here, we show how short periods of limited water availability during the vegetative stage of pea alters phloem sap content and how these changes are connected to strategies used by plants to cope with water deficit. We have investigated the metabolic content of phloem sap exudates and explored how this reflects P. sativum physiological and developmental responses to drought. Our data show that drought is accompanied by phloem-mediated redirection of the components that are necessary for cellular respiration and the proper maintenance of carbon/nitrogen balance during stress. The metabolic content of phloem sap reveals a shift from anabolic to catabolic processes as well as the developmental plasticity of P. sativum plants subjected to drought. Our study underlines the importance of phloem-mediated transport for plant adaptation to unfavourable environmental conditions. We also show that phloem exudate analysis can be used as a useful proxy to study stress responses in plants. We propose that the decrease in oleic acid content within phloem sap could be considered as a potential marker of early signalling events mediating drought response.     

Proton cellular influx as a probable mechanism of variation potential influence on photosynthesis in pea

Electrical signals (action potential and variation potential, VP) caused by environmental stimuli are known to induce various physiological responses in plants, including changes in photosynthesis; however, their functional mechanisms remain unclear. In this study, the influence of VP on photosynthesis in pea (Pisum sativum L.) was investigated and the proton participation in this process analysed. VP, induced by local heating, inactivated photosynthesis and activated respiration, with the initiation of the photosynthetic response connected with inactivation of the photosynthetic dark stage; however, direct VP influence on the light stage was also probable. VP generation was accompanied with pH increases in apoplasts (0.17–0.30 pH unit) and decreases in cytoplasm (0.18–0.60 pH unit), which probably reflected H⁺‐ATPase inactivation and H⁺ influx during this electrical event. Imitation of H⁺ influx using the protonophore carbonyl cyanide m‐chlorophenylhydrazone (CCCP) induced a photosynthetic response that was similar with a VP‐induced response. Experiments on chloroplast suspensions showed that decreased external pH also induced an analogous response and that its magnitude depended on the magnitude of pH change. Thus, the present results showed that proton cellular influx was the probable mechanism of VP's influence on photosynthesis in pea. Potential means of action for this influence are discussed.     

Subcellular aspects of the protective effect of spermine against atrazine in pea plants

The action of the exogenously applied tetraamine spermine in reversing the effect of atrazine stress on intact pea plants (Pisum sativum L., cv. Koray) was investigated at the ultrastructural level. The results indicate that atrazine increases cell senescence by lipid peroxidation and loss of unsaturated fatty acids from thylakoid membranes of pea plant chloroplasts. Spermine acts as a polyfunctional effector. It stabilises the molecular composition of the membranes by preventing lipid peroxidation and as a consequence protects the conformation of the thylakoid system and structural integrity of the chloroplasts. Spermine treatment also contributes to the process of neutralisation of the free radicals by peroxisomes.     

Activated oxygen and antioxidant defences in iron-deficient pea plants

Iron (Fe) deficiency in pea leaves caused a large decrease (44–62&) in chlorophyll a, chlorophyll b and carotenoids, and smaller decreases in soluble protein (18&) and net photosynthesis (28&). Catalase, non-specific peroxidase and ascorbate peroxidase activities declined by 51& in young Fe-deficient leaves, whereas monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase activities remained unaffected. Ascorbate peroxidase activity was highly correlated (r²= 0. 99, P < 0. 001) with the Fe content of leaves, which allows its use as an indicator of the Fe nutritional status of the plant. Fe deficiency resulted in an increase of CuZn-superoxide dismutase but not of Mn-superoxide dismutase. The content of ascorbate decreased by only 24& and those of reduced and oxidized glutathione and vitamin E did not vary. The low-molecular-mass fraction of Fe-sufficient leaves contained 30–65 μg (g dry weight)^?1 Mn. This concentration was 15–60 times greater than that of Fe and Cu in the same fraction, and was further enhanced (1. 5- to 2. 5-fold) by Fe deficiency without causing Mn toxicity. The concentration of catalytic Fe, that is, of Fe active for free radical generation, was virtually zero and that of catalytic Cu did not change with severe Fe deficiency. Because catalytic metals mediate lipid and protein oxidation in vivo, the above findings would explain why oxidatively damaged lipids and proteins do not accumulate in Fe-deficient leaves.     

Effect of apex excision and replacement by 1-naphthylacetic acid on cytokinin concentration and apical dominance in pea plants

As known from literature lateral buds from pea ( Pisum sativum ) plants are released from apical dominance when repeatedly treated with exogenous cytokinins. Little is known, however, about the endogenous role of cytokinins in this process and whether they interact with basipolar transported IAA, generally regarded as the main signal controlling apical dominance. This paper presents evidence that such an interaction exists.
The excision of the apex of pea plants resulted in the release of inhibited lateral buds from apical dominance (AD). This could be entirely prevented by applying 1-naphthylacetic acid (NAA) to the cut end of the shoot. Removal of the apex also resulted in a rapid and rather large increase in the endogenous concentrations of zeatin riboside (ZR), isopentenyladenosine (iAdo) and an as yet unidentified polar zeatin derivative in the node and internode below the point of decapitation. This accumulation of ZR and iAdo, was strongly reduced by the application of NAA. The observed increase in cytokinin concentration preceded the elongation of the lateral buds, suggesting that endogenous cytokinins play a significant role in the release of lateral buds from AD. However, the effect of NAA on the concentration of cytokinins clearly demonstrated the dominant role of the polar basipetally transported auxin in AD. The results suggest a mutual interaction between the basipolar IAA transport system and cytokinins obviously produced in the roots and transported via the xylem into the stem of the pea plants.     

Inhibition of photosynthesis by osmotic stress in pea (Pisum sativum) mesophyll protoplasts is intensified by chilling or photoinhibitory light; intriguing responses of respiration

The effects of reduced osmotic potential on photosynthesis and respiration were studied in mesophyll protoplasts of pea (Pisum sativum). Osmotic stress was induced by increasing the sorbitol concentration in the medium from 0·4 kmol m⁻³ (-1·3 MPa) to 1·0 kmol m⁻³ (-3·1 MPa). Protoplasts lost up to 35% of the maximum capacity of photo-synthetic carbon assimilation (but not PS II mediated activity) soon after exposure to 1·0 kmol m⁻³ sorbitol. The response of protoplast respiration to osmotic stress was intriguing. Respiration was stimulated if stress was induced at 25°C, but was inhibited when protoplasts were subjected to osmotic stress at 0°C. Photosynthesis was also much more sensitive to osmotic stress at 0°C than at 25°C. The inhibitory effects of osmotic stress on photosynthesis as well as respiration were amplified by not only chilling but also photoinhibitory light. The photosynthetic or respiratory activities of protoplasts recovered remarkably when they were transferred from hyperosmotic (1·0 kmol m⁻³ sorbitol) back to iso-osmotic medium (0·4 kmol m⁻³ sorbitol), demonstrating the reversibility of osmotic-stress-induced changes in protoplasts. Respiration was more resistant to osmotic stress and was quicker to recover than photosynthesis. We suggest that the experimental system of protoplasts can be useful in studying the effects of osmotic stress on plant tissues.     

The effects of O3 fumigation during leaf development on photosynthesis of wheat and pea: An in vivo analysis

Previous studies have shown that short exposure of plants to high doses of ozone decreases subsequent photosynthesis; initially by reducing carboxylation capacity. This study tests the hypothesis that this is also the primary cause of loss of photosynthetic capacity in leaves affected by development under a low level of ozone. Triticum aestivum and Pisum sativum plants were exposed from germination to ozone in air (80 nmol mol^-1 for 7 hours per day, for 18 days. Leaves that had completed lamina expansion at this time were free of visible injury and light absorptance was unaffected. However, some significant changes in photosynthetic gas exchange were evident. Photosynthetic CO2 uptake at light saturation was decreased significantly by 35% in T. aestivum but was unchanged in P. sativum. The reduction in photosynthesis of T. aestivum was accompanied by a 31% decline in the maximum velocity of carboxylation measured in vivo. Decreased stomatal conductance did not contribute to this reduction of photosynthesis because there was no significant change in the stomatal limitation to CO2. Processes directly dependent upon photochemical reactions; that is, the quantum yield of CO2 uptake and capacity for regeneration of ribulose 1,5-bisphosphate were not affected by O3 fumigation in either species. This suggests that for wheat, the quantitative cause of decreased photosynthetic rate in vivo is a decrease in the quantity of active ribulose-1,5- bisphosphate carboxylase-oxygenase.     

Effect of leaf surface wetness and wettability on photosynthesis in bean and pea

Leaf surface wetness that occurs frequently in natural environments has a significant impact on leaf photosynthesis. However, the physiological mechanisms for the photosynthetic responses to wetness are not well understood. The responses of leaf CO₂ assimilation rate (A) to 72 h of artificial mist of a wettable (bean; Phaseolus vulgaris) and a non‐wettable species (pea; Pisum sativum) were compared. Stomatal and non‐stomatal limitations to A were investigated. A 28% inhibition of A was observed in the bean leaves as a result of a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco. The decrease of Rubisco was mainly due to its partial degradation. In contrast to the bean leaves, a 22% stimulation of A was obtained in the 72 h mist‐treated pea leaves. Mist treatment increased stomatal conductance by 12.5% and had no effect on the amount of Rubisco. These results indicated that a positive photosynthetic response to wetness occurred only in non‐wettable species and is due to the change in stomatal regulation.     

Effects of UV-B radiation on photosynthesis and growth of terrestrial plants

The photosynthetic apparatus of some plant species appears to be well-protected from direct damage from UV-B radiation. Leaf optical properties of these species apparently minimizes exposure of sensitive targets to UV-B radiation. However, damage by UV-B radiation to Photosystem II and Rubisco has also been reported. Secondary effects of this damage may include reductions in photosynthetic capacity, RuBP regeneration and quantum yield. Furthermore, UV-B radiation may decrease the penetration of PAR, reduce photosynthetic and accessory pigments, impair stomatal function and alter canopy morphology, and thus indirectly retard photosynthetic carbon assimilation. Subsequently, UV-B radiation may limit productivity in many plant species. In addition to variability in sensitivity to UV-B radiation, the effects of UV-B radiation are further confounded by other environmental factors such as CO₂, temperature, light and water or nutrient availability. Therefore, we need a better understanding of the mechanisms of tolerance to UV-B radiation and of the interaction between UV-B and other environmental factors in order to adequately assess the probable consequences of a change in solar radiation.Abbreviations A_max light and CO₂ saturated rate of oxygen evolution - Ci internal CO₂ concentration - F_v/F_m ratio of variable to total fluorescence yield - PAR photosynthetically active radiation (400–700 nm) - PS II Photosystem II - _app apparent quantum yield of photosynthesis - SLW specific leaf weight - UV-B ultraviolet-B radiation between 290–320 nm     

Correlation between the inhibition of photosynthesis and the decrease in area of detached leaf discs or volume/absorbance of protoplasts under osmotic stress in pea (Pisum sativum)

Exposure to osmotic stress reduces leaf area and protoplast volume while decreasing photosynthesis. But the measurement of protoplast volume is tedious, while rapid determinations of leaf area in the field are difficult. We evaluated the quantitative relationship between the extent of decrease in area of detached leaf discs or the volume of protoplast of pea ( Pisum sativum ) and reduction in their photosynthetic capacity under osmotic stress. Osmotic stress was induced by increasing sorbitol concentration in the surrounding medium of the leaf discs from zero to 1.0 M (-3.1 MPa), and in case of protoplasts from 0.4 M (-1.3 MPa, isotonicity) to 1.0 M (-3.1 MPa, hypertonicity). There was a high degree of positive correlation between the extent of reduction in the area of detached leaf discs or the volume of protoplasts (indicated by diameter or absorption at 440 nm) and the decrease in photosynthesis. The correlation coefficients between inhibition of photosynthesis and the decrease in leaf disc area or protoplast volume were 0.96 and 0.99, respectively. We therefore suggest that the decrease in absorbance at 440 nm (corrected for turbidity at 750 nm) can be used as a simple measure to predict the inhibition due to osmotic stress of photosynthesis in mesophyll protoplasts. Similarly, the reduction in area of detached leaf discs could also be a very simple and useful criterion to assess osmotic tolerance of photosynthesis.     

The rates of shoot and root growth in intact plants of pea mutants in leaf morphology

Isogenic lines of pea (Pisum sativum L.) with the genetically determined changes in leaf morphology, afila (af) and tendril-less (tl), were used to study the relationship between shoot and root growth rates. The time-course of shoot and root growth was followed during the pre-floral period in the intact plants grown under similar conditions. The af mutation produced afila leaves without leaflets, whereas in the case of the tl mutations, tendrils were substituted with leaflets, and acacia-like leaves were developed. Due to the changes in leaf morphology caused by these mutations, pea genotypes differed in leaf area: starting from day 7, the leaf area was lower in the af plants and larger in the tl plants as compared to the wild-type plants. Such divergence was amplified in the course of plant development and reached its maximum immediately before the transition to flowering. Plants of isogenic lines did not notably differ in stem surface areas. In spite of significant difference in total leaf area, the wild type and tl plants did not differ in leaf dry weight. Starting from leaf 9, the af plants lagged behind two leaflet-bearing genotypes (wild type and tl) in leaf dry weight, whereas stem dry weight was similar in the wild type and tl forms and slightly lower in the af plants. Root dry weights were practically similar in the wild type and tl plants until flowering. The reduction of leaf area in the af plants drastically reduced root dry weight. In other words, the latter index was related to the total weight and total area of leaves and stems. The correlation analysis demonstrated an extremely low relationship between leaf and stem area and dry weight and those of roots early in plant development (when plants develop five to seven leaves). Later, immediately before flowering (nine to eleven leaves), root weight was positively related to leaf weight and area; however, stem area and root weight did not correlate. Thus, in three genotypes (wild type, af, and tl), at the end of their vegetative growth phase, leaf and root biomass accumulated in proportion, independently of leaf area expansion.     

Ascorbic acid effect on the onset of cell proliferation in pea root

The ability of ascorbic acid to induce cell proliferation of non-cycling cells was investigated in quiescent embryo root of Pisum sativum L. cv. Lincoln, as well as in the active plantlet root meristem, where a minor portion of the cells is non-proliferating. Quiescent embryo cells speeded up the G₀–G₁ transition during germination in the presence of ascorbic acid. In addition, proliferating cells present in the root tip of 3-day-old plantlets, arrested at the G₁/S boundary by hydroxyurea, resumed the cycle earlier than the control, when treated with ascorbic acid. In contrast, ascorbic acid was unable to induce the proliferation of non-cycling cells present in the active meristem. Therefore, these data suggest that the ability of ascorbic acid lo induce cell proliferation depends on the physiological status of the cell. In particular the data indicate that ascorbic acid is involved in cell proliferation as a factor necessary to enable already competent cells to progress through the cell cycle phases, but not as a factor able to induce non-competent cells to overcome proliferation arrest.