Both reduced and oxidized ascorbate (AA and DHA) are present in the aqueous phase of the extracellular space, the apoplast, of spinach (Spinacia oleracea L.) leaves. Fumigation with 0.3 [mu]L L-1 of ozone resulted in ozone uptake by the leaves close to 0.9 pmol cm-2 of leaf surface area s-1. Apoplastic AA was slowly oxidized by ozone. The initial decrease of apoplastic AA was <0.1 pmol cm-2 s-1. The apoplastic ratio of AA to (AA + DHA) decreased within 6 h of fumigation from 0.9 to 0.1. Initially, the concentration of (AA + DHA) did not change in the apoplast, but when fumigation was continued, DHA increased and AA remained at a very low constant level. After fumigation was discontinued, DHA decreased very slowly in the apoplast, reaching control level after 70 h. The data show that insufficient AA reached the apoplast from the cytosol to detoxify ozone in the apoplast when the ozone flux into the leaves was 0.9 pmol cm-2 s-1. The transport of DHA back into the cytosol was slower than AA transport into the apoplast. No dehydroascorbate reductase activity could be detected in the apoplast of spinach leaves. In contrast to its extracellular redox state, the intracellular redox state of AA did not change appreciably during a 24-h fumigation period. However, intracellular glutathi-one became slowly oxidized. At the beginning of fumigation, 90% of the total glutathione was reduced. Only 10% was reduced after 24-h exposure of the leaves to 0.3 [mu]L L-1 of ozone. Necrotic leaf damage started to become visible when fumigation was extended beyond a 24-h period. A close correlation between the extent of damage, on the one hand, and the AA content and the ascorbate redox state of whole leaves, on the other, was observed after 48 h of fumigation. Only the youngest leaves that contained high ascorbate concentrations did not exhibit necrotic leaf damage after 48 h.

Role of Ascorbate in Detoxifying Ozone in the Apoplast of Spinach (Spinacia oleracea L.) Leaves

Ascorbic acid (Asc) is the most abundant antioxidant in plants and serves as a major contributor to the cell redox state. Exposure to environmental ozone can cause significant damage to plants by imposing conditions of oxidative stress. We examined whether increasing the level of Asc through enhanced Asc recycling would limit the deleterious effects of environmental oxidative stress. Plants overexpressing dehydroascorbate reductase (DHAR), which results in an increase in the endogenous level of Asc, were exposed to acute or chronic levels of ozone. DHAR-overexpressing plants had a lower oxidative load, a lower level of oxidative-related enzyme activities, a higher level of chlorophyll, and a higher level of photosynthetic activity 24 h following an acute exposure (2 h) to 200 ppb ozone than control plants, despite exhibiting a larger stomatal area. Reducing the size of the Asc pool size through suppression of DHAR expression had the opposite effect. Following a chronic exposure (30 d) to 100 ppb ozone, plants with a larger Asc pool size maintained a larger stomatal area and a higher oxidative load, but retained a higher level of photosynthetic activity than control plants, whereas plants suppressed for DHAR had a substantially reduced stomatal area, but also a substantially lower level of photosynthetic activity. Together, these data indicate that, despite a reduced ability to respond to ozone through stomatal closure, increasing the level of Asc through enhanced Asc recycling provided greater protection against oxidative damage than reducing stomatal area.

Increasing tolerance to ozone by elevating foliar ascorbic acid confers greater protection against ozone than increasing avoidance.

Human-induced and natural stress factors can affect fine roots and ectomycorrhizas. Therefore they have potential utility as indicators of environmental change. We evaluated, through meta-analysis, the magnitude of the effects of acidic deposition, nitrogen deposition, increased ozone levels, elevated atmospheric carbon dioxide, and drought on fine roots and ectomycorrhizal (ECM) characteristics. Ectomycorrhizal colonization was an unsuitable parameter for environmental change, but fine root length and biomass could be useful. Acidic deposition had a significantly negative impact on fine roots, root length being more sensitive than root biomass. There were no significant effects of nitrogen deposition or elevated tropospheric ozone on the quantitative root parameters. Elevated CO2 had a significant positive effect. Drought had a significantly negative effect on fine root biomass. The negative effect of acidic deposition and the positive effect of elevated CO2 increased over time, indicating that effects were persistent contrary the other factors. The meta-analysis also showed that experimental conditions, including both laboratory and field experiments, were a major source of variation. In addition to quantitative changes, environmental changes affect the species composition of the ectomycorrhizal fungal community.

https://www.tandfonline.com/doi/pdf/10.1080/11263500701626028

Fine roots and ectomycorrhizas as indicators of environmental change

Effects of ozone exposure in open-top chambers on poplar (Populus nigra) and beech (Fagus sylvatica): a comparison

summary
Nitrogen (N) resorbed from foliage before leaf abscission is a significant source of N for new leaves. The widely distributed photochemical air pollutant ozone (O3) accelerates foliar senescence and leaf abscission and therefore could interfere with the process of N resorption. The objective of this study was to determine the effects of O. and drought stress on the N economy of ponderosa pine (Pinus ponderosa Laws.), a species highly susceptible to O Potted ponderosa pine trees (2-yr-old) were exposed for 3 yr to three concentrations of O3 and two soil water regimes in open-top chambers in the Sierra Nevada mountains of California. One-third of the trees was harvested at the end of each exposure season and each age class of needles, branches, stems, coarse roots, fine roots, and leaf litter was analysed for N. Exposure to a seasonal O3 mean of 88 ppb increased N concentrations in pine foliage, particularly in current-year needles. Current-year foliage from well-watered (WW) trees exposed to elevated O averaged 34% higher N than that from trees in charcoal-filtered (CF) chambers. Drought-stressed (DS) trees from all O3 treatments averaged 10% higher N in current-year needles than WW seedlings. Resorbed N from older age classes of needles from WW trees increased from 32% of current-year needle N in CF seedlings to 51% of current-year N in the high O3 treatment. Nitrogen resorbed from older needles of DS trees increased from 15% in CF to 38% of current-year N in the high O3 treatment. Thus O3-injured ponderosa pine seedlings increased resorption of N from older needles and increased partitioning of N to current-year foliage. The increased N in current-year needles facilitated increased rates of photosynthesis in these needles, thus partly compensating for the O3-induced loss of older leaves.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1995.tb01819.x

Nitrogen allocation in ponderosa pine seedlings exposed to interacting ozone and drought stresses

Abstract Young poplar cuttings (Populus nigra L. cv. Loenen and P. maximowiczii Henry x P. nigra L. cv. Rochester) were exposed for six weeks in open-top chambers to realistic concentrations of pollutant mixtures: 1) control; 2) SO2/NOx; 3)O3/ NOx and 4)SO2/O3/NOx. In this sequence of fumigation variants, the degree of influence of the various parameters of the nitrogen metabolism and of premature leaf drop increased very frequently compared to the control plants, P. nigra L. proving to be the more sensitive species. The elevated Kjeldahl nitrogen content of the fumigated leaves was accompanied by either an increase in free amino acids or in total protein or, in the case of particularly large rises (SO2/O3/NOx variants), by increases in both substance groups. Proteolytic processes as a cause of the elevated content of free amino acids could be excluded to a large extent. A diminished de novo synthesis of proteins obviously led to a shift in the amino acid/protein relationship. In the younger fumigated leaves, the total concentration of free amino acids exceeded the values of the older leaves. The elevated amino acid content of the fumigated leaves was produced to a high degree by the glycolate pathway and the Krebs cycle. The increased turnover of the carbon skeletons was connected with a drastic starch degradation, especially in the older leaves. The interaction of the amino acid and carbohydrate metabolisms is probably an important regulator in the promotion of rapid growth of young leaves in order to compensate premature leaf loss.

Simone Oppenheimer

Papers

Reactions of Cloned Poplars to Air Pollution: Premature Leaf Loss and Investigations of the Nitrogen Metabolism