Biochemical and physiological processes associated with the differential ozone response in ozone-tolerant and sensitive soybean genotypes.

Biochemical and physiological traits of two soybean [Glycine max (L.) Merr.] genotypes differing in sensitivity to ozone (O3 ) were investigated to determine the possible basis for the differential response. Fiskeby III (O3 -tolerant) and Mandarin (Ottawa) (O3 -sensitive) were grown in a greenhouse with charcoal-filtered air for 4 weeks, then treated with O3 for 7 h·day(-1) in greenhouse chambers. Mandarin (Ottawa) showed significantly more leaf injury and hydrogen peroxide (H2 O2 ) and superoxide (O2 (-) ) production compared with Fiskeby III. Peroxidase activity in Mandarin (Ottawa) was 31% higher with O3 but was not significantly different in Fiskeby III. Ozone did not affect superoxide dismutase or glutathione reductase activities, or leaf concentrations of glutathione or ascorbic acid. Thus, variation in O3 response between Fiskeby III and Mandarin (Ottawa) was not explained by differences in antioxidant enzymes and metabolites tested. Ethylene emission from leaves declined in Fiskeby III following O3 exposure but not in Mandarin (Ottawa). Ozone exposure reduced quantum yield (ΦPSII ), electron transport rate (ETR) and photochemical quenching (qp ) in Mandarin (Ottawa) more than in Fiskeby III, indicating that efficiency of energy conversion of PSII and photosynthetic electron transport was altered differently in the two genotypes. Short-term exposure to O3 had minimal effects on net carbon exchange rates of both soybean cultivars. A trend toward higher stomatal conductance in Mandarin (Ottawa) suggested stomatal exclusion might contribute to differential O3 sensitivity of the two genotypes. Increased sensitivity of Mandarin (Ottawa) to O3 was associated with higher H2 O2 and O2 (-) production compared with Fiskeby III, possibly associated with genotype differences in stomatal function or regulation of ethylene during the initial phases of O3 response.

The challenge of making ozone risk assessment for forest trees more mechanistic.

Upcoming decades will experience increasing atmospheric CO2 and likely enhanced O3 exposure which represents a risk for the carbon sink strength of forests, so that the need for cause-effect related O3 risk assessment increases. Although assessment will gain in reliability on an O3 uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O3 uptake. Recent progress in research on the molecular and metabolic control of the effective O3 dose is reported along with advances in empirically assessing O3 uptake at the whole-tree and stand level. Knowledge on both O3 uptake and effective dose (measures of stress avoidance and tolerance, respectively) needs to be understood mechanistically and linked as a pre-requisite before practical use of process-based O3 risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O3 flux-based modelling tools.

Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles.

Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed to either ambient CO(2) concentration ([CO(2)]), ambient plus 175 or ambient plus 350 micromol CO(2) mol(-1) in branch chambers for 2 years. The CO(2) treatments were superimposed on a 2 x 2 factorial combination of irrigation and fertilization treatments. In addition, we compared the effects of branch chambers and open-top chambers on needle chemistry. Proanthocyanidin and N concentrations were measured in needles from branch chambers and from trees in open-top chambers exposed concurrently for two years to either ambient [CO(2)] or ambient plus 200 micromol CO(2) mol(-1) in combination with a fertilization treatment. In the branch chambers, concentrations of total soluble phenolics in needles generally increased with needle age. Concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated [CO(2)] treatments. There were no significant treatment effects on foliar lignin concentrations. Nitrogen concentrations were about 10% lower in needles from the elevated [CO(2)] treatments than in needles from the ambient [CO(2)] treatments. Soluble phenolic and PA concentrations were higher in the control and irrigated soil treatments in about half of the comparisons; otherwise, differences were not statistically significant. Needle N concentrations increased 23% in response to fertilization. Treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this part of the study N concentrations were not significantly affected by the CO(2) treatments in either the branch or open-top chambers. We conclude that elevated [CO(2)] and low N availability affected foliar chemical composition, which could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling.

Inheritance, gene expression, and lignin characterization in a mutant pine deficient in cinnamyl alcohol dehydrogenase.

We have discovered a mutant loblolly pine (Pinus taeda L.) in which expression of the gene encoding cinnamyl alcohol dehydrogenase (CAD; EC 1.1.1.195) is severely reduced. The products of CAD, cinnamyl alcohols, are the precursors of lignin, a major cell wall polymer of plant vascular tissues. Lignin composition in this mutant shows dramatic modifications, including increased incorporation of the substrate of CAD (coniferaldehyde), indicating that CAD may modulate lignin composition in pine. The recessive cad-n1 allele, which causes this phenotype, was discovered in a tree heterozygous for this mutant allele. It is inherited as a simple Mendelian locus that maps to the same genomic region as the cad locus. In mutant plants, CAD activity and abundance of cad RNA transcript are low, and free CAD substrate accumulates to a high level. The wood of the mutant is brown, whereas the wood in wild types is nearly white. The wood phenotype resembles that of brown midrib (bm) mutants and some transgenic plants in which xylem is red-brown due to a reduction in CAD activity. However, unlike transgenics with reduced CAD, the pine mutant has decreased lignin content. Wood in which the composition of lignin varies beyond previous expectations still provides vascular function and mechanical support.

Catechin, proanthocyanidin and lignin contents of loblolly pine (Pinus taeda) needles after chronic exposure to ozone.

Concentrations of soluble and bound phenolic compounds were measured in needles of 3-yr-old loblolly pine (Pinus taeda L.) trees exposed from May to November 1993 to a range of ozone (O3 ) concentrations in open-top field chambers, The treatments were charcoal-filtered air (CF). non-filtered air (NF), and NF air with O2 added at 1.5 times (NF 1.5) and 2(1 times (NF 2.0) the ambient O., concentration for 12 h daily. Average daily (0800-2000 hours) O3 concentrations in the CF. NF. NF 1.5 and NF 20 treatments were. 29. 47, 76 and 98 nl 1(-1) . respectively, for the 140 d treatment period. At the end of the treatment period, total phenolic and proanthocyanidin concentrations in the previous year's needles were 25-29% higher in the NF 2.0 treatment compared with the lower O3 treatments. Catechin concentration increased in the previous year's needles by as much as 81 % between the NF 2.0 treatment and the lower O3 treatments. Catechin is an effective antioxidant, and elevated levels might confer some protection against O3 injury. No significant differences in total phenolics and proanthocyanidins in the previous year's needles were detected among the remaining treatments, or among any O3 treatment for the current year's needles. Lignin content in needles of both years was not significantly affected by O3 exposure. Chances in the phenolic content of older needles in response to elevated O3 could alter plant-pathogen interactions and slow down microbiol decomposition, which could contribute to a decline in site soil quality.

Is increased UV-B a threat to crop photosynthesis and productivity?

It has been suggested that increases in ground-level UV-B, as a result of stratospheric ozone depletion, may have major deleterious effects on crop photosynthesis and productivity. The direct consequences of such effects have been projected by some as a world-wide decrease in crop yields of 20-25%. Further losses, or unrealized gains, have also been suggested as a result of increased UV-B counteracting the beneficial effects of elevated atmospheric CO2. Deleterious UV-B effects may be largely partitioned between damage to the plant genome and damage to the photosynthetic machinery. Direct damage to DNA is a common result of absorption of high energy UV-B photons. However, most plants possess repair mechanisms adequate to deal with the levels of damage expected from projected increases in ground-level UV-B. In addition, most plants have the ability to increase production of UV-absorbing compounds in their leaves as a result of exposure to UV-B, UV-A and visible radiation. These compounds contribute substantially to reducing UV-B damage in situ. It has also been shown that in some plants, under the proper conditions, almost every facet of the photosynthetic machinery can be damaged directly by very high UV-B exposures. However, electron transport, mediated by Photosystem II (PS II) appears to be the most sensitive part of the system. Various laboratories have reported damage to virtually all parts of the PS II complex from the Mn binding site to the plastoquinone acceptor sites on the opposite surface of the thylakoid membrane. However, a critical review of the literature with emphasis on exposure protocols and characterization of the radiation environment, revealed that most growth chamber and greenhouse experiments and very many field experiments have been conducted at unrealistic or indeterminate UV-B exposure levels, especially with regard to the spectral balance of their normal radiation environment. Thus, these experiments have led directly to large overestimates of the potential for damage to crop photosynthesis and yield within the context of 100 year projections for stratospheric ozone depletion. Indeed, given the massive UV-B exposures necessary to produce many of these effects, we suggest it is unlikely that they would occur in a natural setting and urge reconsideration of the purported impacts of projected increases of UV-B on crop productivity.

Technetium-99m-tetrofosmin as a new radiopharmaceutical for myocardial perfusion imaging.

A new cationic complex, [99mTc(tetrofosmin)2O2]+, where tetrofosmin is the ether functionalized diphosphine ligand 1,2-bis[bis(2-ethoxyethyl)phosphino]ethane, has been synthesized and evaluated for potential use in myocardial perfusion imaging. The structure of the complex has been determined by x-ray crystallography of the 99Tc analog. In comparison with previously reported 99mTc complexes of alkyl-phosphines, the tetrofosmin species shows substantially increased clearance from nontarget tissue, especially blood and liver. A freeze-dried kit formulation has been developed. The kit provides a product of high radiochemical purity up to 8 hr after reconstitution at room temperature.