Effects of Atmospheric NO2 on Azolla-Anabaena Symbiosis.

Cultures of the water fern Azolla pinnata R, Br. exposed for 1 week to atmospheric NO2 (50, 100 or 200 nl l-1) induced additional levels of nitrate reductase (NaR) protein and nitrite reductase (NiR) activity. At low concentrations of NO2 (50 nl l-1), nitrate derived from NO2 provides an alternative N source for Azolla but does not affect rates of acetylene reduction. However, the symbiotic relationship between Azolla and its endosymbiont, Anabaena azollae is only affected adversely by high concentrations (100 and 200 nl l-1) of atmospheric NO2. The resultant decreases in rate of growth, nitrogen fixation, heterocyst formation, and overall nitrogen cycling are probably due to the additional accumulation of N products derived from higher levels of atmospheric NO2. Parallel increases in levels of polyamines suggest that Azolla partially alleviates these harmful effects by incorporating some of the extra NO2-induced N into polyamines

Effects of Atmospheric O3 on Azolla-Anabaena Symbiosis.

Cultures of water fern Azolla pinnata R. Br. exposed for 1 week to either 30, 50 or 80 nl l-1 O3 showed significant reductions in rates of growth and N2 fixation, and had fewer heterocysts. Although the levels of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activity were decreased by low concentrations of O3 exposures (30 or 50 nl l-1), significant increases in levels of the same enzymes were caused by higher concentrations of O3 (80 nl l-1). Increased levels of total protein, polyamines (putrescine and spermidine), and the xanthophyll-cycle precursor of abscisic acid (ABA), violaxanthin, were also found with higher levels of O3 (80 nl l-1). Levels of ABA itself were significantly increased by low level O3 fumigation (30 nl l-1) but significantly decreased by exposure to 80 nl l-1 O3. This may indicate that higher levels of atmospheric O3 inhibit the final stages of ABA biosynthesis from violaxanthin

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

Tolerance to atmospheric ozone in transgenic tobacco over-expressing glutathione synthetase in plastids.

A cross between transgenic tobacco (Nicotiana tabacum L.) plants which over‐expressed either γ‐glutamylcysteine synthetase (cpGSHI) or glutathione synthetase (cpGSHII) in their chloroplasts was used to compare the consequences of over‐expression of components of the glutathione synthetic pathway on tolerance to atmospheric O3 or paraquat. A high proportion (50%) of those progeny which carried the cpGSHII transgene alone showed tolerance to atmospheric O3 but not to paraquat. Progeny of an additional two, independent, self‐pollinated primary transgenic lines, which segregated in a Mendelian fashion for the presence of the cpGSHII transgene and therefore included both transformed and non‐transformed (recessive, wild‐type) plants, were also challenged by fumigation with O3. Again, in both cases, about 50% of the plants expressing the epGSHII transgene were found to be O3‐tolerant on the basis of reduced ethylene emissions and increased or unchanged total pigment concentrations. However, this tolerance was not due to specific changes in stomatal densities

Elevated glutathione biosynthetic capacity in the chloroplasts of transgenic tobacco plants paradoxically causes increased oxidative stress.

Glutathione (GSH), a major antioxidant in most aerobic organisms, is perceived to be particularly important in plant chloroplasts because it helps to protect the photosynthetic apparatus from oxidative damage. In transgenic tobacco plants overexpressing a chloroplast-targeted -glutamylcysteine synthetase (-ECS), foliar levels of GSH were raised threefold. Paradoxically, increased GSH biosynthetic capacity in the chloroplast resulted in greatly enhanced oxidative stress, which was manifested as light intensity–dependent chlorosis or necrosis. This phenotype was associated with foliar pools of both GSH and -glutamylcysteine (the immediate precursor to GSH) being in a more oxidized state. Further manipulations of both the content and redox state of the foliar thiol pools were achieved using hybrid transgenic plants with enhanced glutathione synthetase or glutathione reductase activity in addition to elevated levels of -ECS. Given the results of these experiments, we suggest that -ECS–transformed plants suffered continuous oxidative damage caused by a failure of the redox-sensing process in the chloroplast

The influence of UV-B radiation on the physicochemical nature of tobacco (Nicotiana tabacum L.) leaf surfaces.

Relationships between leaf wettability and surface physicochemical characteristics were examined in two genotypes of tobacco (Nicotiana tabacum L. cv. Samsun) grown under controlled conditions at three different levels of biologically effective ultraviolet-B (UV-BBE; 280–320 nm) radiation; 0 (control), 4.54 and 5.66 kJ m–2d–1. Leaf wettability, assessed by measuring leaf-water droplet contact angles, was positively correlated with epicuticular wax chemical composition and trichome density, but not the amount of wax on the surface of leaves. Tobacco wax comprised a mixture of C19–C33 n-alkanes (59%) with homologues containing an odd number of carbon atoms predominating, C28–C32 br-alkanes (38%), and a small quantity (3%) of free Cl6–C18 fatty acids. Significant effects of UV-B radiation upon wax production and chemical composition were restricted to the adaxial surface of leaves. Enhanced UV-B radiation reduced the quantity of epicuticular wax in the more sensitive genotype [GR32-3], assessed from effects on dry matter accumulation, partitioning and changes in leaf morphology, and resulted in marked changes in wax composition and homologue distributions in both genotypes. UV-B-induced increases in branching, and shifts toward the synthesis of shorter-chain homologues provided evidence for a fundamental effect of UV-B radiation on wax biosynthesis, with the observed effects consistent with a highly specific and direct effect of UV-B radiation on microsomal-based elongases in the epidermis. UV-B radiation also reduced the density of trichomes on the adaxial leaf surface, whilst increasing the number of trichomes on the abaxial leaf surface. Changes in wax composition and trichome density induced by UV-B radiation were associated with increases in leaf surface wettability which were particularly pronounced on the adaxial surface. The subtle, though possibly far-reaching, physiological consequences of such UV-B-induced changes in surface wettability are discussed in the light of other recent findings