The changing solar ultraviolet climate and the ecological consequences for higher plants.

There is compelling evidence that a general erosion of the global ozone layer is occurring. Since ozone in the stratosphere absorbs much of the shortwave solar ultraviolet radiation (UV-B), diminished ozone means that more UV-B of a very specific wavelength composition will be received at the earth's surface. Evaluating the implications for vegetation involves consideration of the wavelength specificity of biological photochemical reactions and their sensitivity to the extant and future solar spectrum. Recent research suggests the occurrence of direct damaging reactions and of indirect morphological and chemical responses with implications at the community and ecosystem levels.

Action of ultraviolet radiation (UV-B) upon cuticular waxes in some crop plants.

The surface structure and composition of surface lipids were examined in leaves of barley, bean, and cucumber seedlings grown in a growth chamber under white light and low levels of ultraviolet (UV-B; 280-320 nm) radiation. The cuticular wax of cucumber cotyledons and bean leaves appeared as a thin homogeneous layer, whereas on barley leaves crystal-like structures could be observed under these irradiation conditions. Principally, the amount of cuticular wax found in barley leaves was five times greater than in bean or cucumber leaves. The prediominant wax components were primary alcohols in barley, primary alcohols and monoesters in bean, and alkanes in cucumber cotyledons. Irradiation with enhanced UV-B levels caused an increase of total wax by about 25% in all plant species investigated. Aldehydes, detected as a minor constituent of cucumber and barley wax, increased twofold. Distribution patterns of the homologs within some wax classes were different at low and enhanced UV-B levels. In general, the distribution of the homologs was shifted to shorter acyl chain lengths in wax of leaves exposed to enhanced UV-B levels. This was most apparent in cucumber wax, less in bean or barley wax. The UV-B-caused effects upon cucumber wax were mainly due to a response by the adaxial surface of the leaf.

Composition and function of plastoglobuli : II. Lipid composition of leaves and plastoglobuli during beech leaf senescence.

The lipid composition of whole leaves and isolated plastoglobul of beech (Fagus sylvatica) has been studied during four natural autumnal senescence stages. Chlorophylls, glycolipids, and phospholipids were extensively degraded in leaves. About 20% of the glycolipids found in leaves during summer, however, remained in the last stage of leaf senescence. Triacylglycerols, also detected in large amounts in summer leaves, were hydrolyzed during senescence. The content of free fatty acids derived from degradation of glycerolipids therefore increased. The total carotenoid and prenyl quinone content was largely unchanged during senescence, except during the last stage investigated, but the reduced forms of prenyl quinones decreased while the oxidized prenyl quinones increased. Plastoglobuli isolated from summer leaves mainly contained triacylglycerols, plastohydroquinone, and α-tocopherol. The triacylglycerol content declined in plastoglobuli during senescence. Most of the triacylglycerols must be located outside the plastoglobuli throughout the stages investigated. Carotenoids liberated from thylakoids were esterified and increasingly deposited in plastoglobuli during senescence. In the last senescence stage, carotenoid esters were the main component of plastoglobuli. Prenyl quinones were also transferred into plastoglobuli. Reduced prenyl quinones were sucessively oxidized during senescence and plastoquinone (oxidized) was the predominant prenyl quinone in plastoglobuli isolated from the last senescence stage. The carotenoid and prenyl quinone distribution was identical in leaves and plastoglobuli during late senescence. The main constituents of thylakoids, glycolipids and proteins, were not deposited in plastoglobuli and therefore did not play an important role in plastoglobuli metabolism.

Composition and function of plastoglobuli : I. Isolation and purification from chloroplasts and chromoplasts.

Plastoglobuli have been isolated and purified from chloroplasts of beech and spinach leaves and from broom flower chromoplasts by a repeated floating-gradient technique. The main components in plastoglobuli isolated from chloroplasts were triacylglycerols and lipophilic prenyl quinones, mainly plastohydroquinone and α-tocopherol. The corresponding oxidized prenyl quinones, plastoquinone (ox), α-tocoquinone, and the phylloquinone vitamin K1, were detected in trace amounts. Plastoglobuli isolated from chromoplasts contained large amounts of carotenoid esters. Triacylglycerols constituted two-thirds of the content of these plastoglobuli. The total prenyl quinone content was low in chromoplast plastoglobuli. Plastoquinone (ox) was the major prenyl quinone constituent. Plastoglobuli contained small amounts of chlorophylls, carotenoids (with the exception of chromoplast plastoglobuli), glycolipids, and proteins due to adsorption phenomena during the isolation process; however, increasing purification of the plastoglobuli fractions resulted in an exponential decline of these components. Adsorption of thylakoid lipids onto the plastoglobuli during the isolation process was demonstrated using an artificial globuli system. Therefore, pigments, glyco- and phospholipids, and proteins were regarded as thylakoid contaminations and not as actual constituents of plastoglobuli.

Some effects of enhanced UV-B irradiation on the growth and composition of plants.

Barley (Hordeum vulgare), corn (Zea mays), bean (Phaseolus vulgaris), and radish (Raphanus sativus) seedlings were continuously irradiated under a lighting device for 5-10 d at an increased ultraviolet (UV)-B fluence rate. In their growth parameters, composition, and leaf surface, these four species responded differently to the increased UV-B exposure. Bean seedlings suffered the most serious effects, radish and barley less, and corn was hardly influenced at all. In all plant species, the fresh weight, the leaf area, the amounts of chlorophylls, carotenoids and the galactolipids of the chloroplasts were reduced. The lipid content of the corn and bean seedlings also diminished. But all the irradiated plants showed a rise in their protein content compared to the control plants. The content of flavonoids increased in barley and radish seedlings by about 50%. The effects on growth parameters and composition were more extensive with increasing UV-B fluence rates, at least as shown in the case of barley seedlings. The fresh weights fell proportionally with the chlorophylls and carotenoids. In contrast, the flavonoid content of barley leaves rose parallel to the increasing UV-B fluence rates and reached 180% of the value in the control plants with the highest UV-B fluence rate. Scorching appeared regularly in the form of bronze leaf discoloration at the highest UV-B fluence rates. Scanning electron micrographs of the leaf surface of UV-B irradiated plants showed deformed epidermal structures.

[Changes of glyco- and phospholipid contents during leaf senescence]. / Veränderungen der Glyko- und Phospholipidgehalte während der Blattvergilbung.

The glyco- and phospholipid levels in green, green-yellow and yellow leaves of Impatients balsamina L., Daucus carota L. and Cucurbita pepo L. were determined on a leaf area basis. In Fagus silvatica L. leaves the kinetics of lipid breakdown during autumnal senescence was analysed. It was shown that yellow tissues still contain 40% to 50% of galactolipids and 50% to 60% of phospholipids present in green tissues. In the early stages of senescence of autumnal Fagus leaves we found a more rapid decline in the levels of the galactolipids and of the phospholipids phosphatidylglycerol and phosphatidylcholine, whereas in the later stages the breakdown of these lipids is slower than that of chlorophyll. The last yellow stage still contains 30% of the galactolipids and about 23% of the phospholipids present in fully greened leaves. Phosphatidylinositol shows a different behavior during senescence.

[The effect of phospholipase D on photochemical activity and lipid composition of isolated spinach chloroplasts]. / Effekte der Phospholipase D auf den Elektronentransport und die Lipidzusammensetzung isolierter Spinatchloroplasten.

Phospholipase D shows short and longtime effects on photochemical activity of isolated spinach chloroplasts. After very short incubations with Phospholipase D (Pl D) the Ferricyanide reduction and Dichlorphenol-idenophenol reduction are 70% to 90% higher than in control chloroplasts. In uncoupled chloroplasts the reduction rates are about 20% higher than in the controls. After one h of incubation time with Phospholipase D the photochemical activity is inhibited and now shows only 40% of the control activity. The effect of Phospholipase D on uncoupled chloroplasts is somewhat lower. After two h of incubation time the control activity decreases to about 50% whereas the PLD-effected activity is reduced to 10% of the initial rates. Cyclic phosphorylation is inhibited by Phospholipase D, presumably because Phospholipase D exerts an uncoupling effect.