K. Herbinger

Complex interactive effects of drought and ozone stress on the antioxidant defence systems of two wheat cultivars

The response of antioxidative defence systems in flag leaf tissues of wheat to combinations of ozone and drought stress was investigated. Sensitive (Triticum aestivum Desf. cv. Nandu) and resistant (Triticum durum L. cv. Extradur) cultivars of wheat were grown in open-top chambers under two ozone (ambient; ambient plus 50 ppb) and two water regimes (well-watered and 40% of soil water capacity). Concentrations of antioxidants ascorbate, glutathione, and tocopherol as well as the contents of chloroplast pigments were determined to evaluate the capacity of radical scavenging systems. Additionally leaf water potential, stomatal conductance and chlorophyll fluorescence were measured. Leaf water potentials of the resistant cultivar were significantly lower than that of the sensitive cultivar grown under the same imposed drought exposure. Drought decreased total ascorbate and total chlorophyll concentrations, increased the protective carotenoids and the de-epoxidation state of the xanthophyll cycle as well as total glutathione and tocopherol concentrations. Drought stress combined with ozone exposure did not further affect the antioxidative system.

Extraordinary drought of 2003 overrules ozone impact on adult beech trees (Fagus sylvatica)

The extraordinary drought during the summer of 2003 in Central Europe allowed to examine responses of adult beech trees (Fagus sylvatica) to co-occurring stress by soil moisture deficit and elevated O3 levels under forest conditions in southern Germany. The study comprised tree exposure to the ambient O3 regime at the site and to a twice-ambient O3 regime as released into the canopy through a free-air O3 fumigation system. Annual courses of photosynthesis (Amax), stomatal conductance (gs), electron transport rate (ETR) and chlorophyll levels were compared between 2003 and 2004, the latter year representing the humid long-term climate at the site. ETR, Amax and gs were lowered during 2003 by drought rather than ozone, whereas chlorophyll levels did not differ between the years. Radial stem increment was reduced in 2003 by drought but fully recovered during the subsequent, humid year. Comparison of AOT40, an O3 exposure-based risk index of O3 stress, and cumulative ozone uptake (COU) yielded a linear relationship throughout humid growth conditions, but a changing slope during 2003. Our findings support the hypothesis that drought protects plants from O3 injury by stomatal closure, which restricts O3 influx into leaves and decouples COU from high external ozone levels. High AOT40 erroneously suggested high O3 risk under drought. Enhanced ozone levels did not aggravate drought effects in leaves and stem.

Enhanced ozone strongly reduces carbon sink strength of adult beech (Fagus sylvatica) – Resume from the free-air fumigation study at Kranzberg Forest

Ground-level ozone (O(3)) has gained awareness as an agent of climate change. In this respect, key results are comprehended from a unique 8-year free-air O(3)-fumigation experiment, conducted on adult beech (Fagus sylvatica) at Kranzberg Forest (Germany). A novel canopy O(3) exposure methodology was employed that allowed whole-tree assessment in situ under twice-ambient O(3) levels. Elevated O(3) significantly weakened the C sink strength of the tree-soil system as evidenced by lowered photosynthesis and 44% reduction in whole-stem growth, but increased soil respiration. Associated effects in leaves and roots at the gene, cell and organ level varied from year to year, with drought being a crucial determinant of O(3) responsiveness. Regarding adult individuals of a late-successional tree species, empirical proof is provided first time in relation to recent modelling predictions that enhanced ground-level O(3) can substantially mitigate the C sequestration of forests in view of climate change.

How sensitive are forest trees to ozone? - New research on an old issue.

Studies on juvenile individuals under artificial environments dominate knowledge about the sensitivity of trees to O3. Field approaches based on free-air O3 fumigations of adult forest trees are a novel choice. Such a case study on beech and spruce (at the Kranzberg Forest near Munich, Germany) is used to address four long-standing issues in O3 research: (1) Can a “unifying theory” of O3 sensitivity be verified? (2) Are responses to O3 consistent at different scaling levels in trees? (3) Are branch-bag experiments relevant for O3 risk assessment of crowns? (4) Are saplings surrogates of adult trees when both are assessed under the same field conditions? Preliminary evidence from the ongoing long-term study confirms (1) and (3) but negates (2) and (4). In the absence of acute risks for adult trees, responsiveness of leaves cannot rule out long-term constraints by chronic O3 stress.

Photosynthesis, chloroplast pigments, and antioxidants in Pinus canariensis under free-air ozone fumigation.

High O3 levels, driving uptake and challenging defense, prevail on the Canary Islands, being associated with the hot and dry summers of the Mediterranean-type climate. Pinus canariensis is an endemic conifer species that forms forests across these islands. We investigated the effects of ozone on photosynthesis and biochemical parameters of P. canariensis seedlings exposed to free-air O3 fumigation at Kranzberg Forest, Germany, where ambient O3 levels were similar to those at forest sites in the Canary Islands. The twice-ambient O3 regime (2 x O3) neither caused visible injury-like chlorotic or necrotic spots in the needles nor significantly affected violaxanthin, antheraxanthin and zeaxanthin levels and the de-epoxidation state of the xanthophyll cycle. In parallel, stomatal conductance for water vapour, net photosynthesis, intercellular CO2 concentration, chlorophyll fluorescence parameters, as well as antioxidant levels were hardly affected. It is concluded that presently prevailing O3 levels do not impose severe stress on P. canariensis seedlings.

Multivariate analysis of physiological parameters reveals a consistent O3 response pattern in leaves of adult European beech (Fagus sylvatica)

• Increasing atmospheric concentrations of phytotoxic ozone (O(3) ) can constrain growth and carbon sink strength of forest trees, potentially exacerbating global radiative forcing. Despite progress in the conceptual understanding of the impact of O(3) on plants, it is still difficult to detect response patterns at the leaf level. • Here, we employed principal component analysis (PCA) to analyse a database containing physiological leaf-level parameters of 60-yr-old Fagus sylvatica (European beech) trees. Data were collected over two climatically contrasting years under ambient and twice-ambient O(3) regimes in a free-air forest environment. • The first principal component (PC1) of the PCA was consistently responsive to O(3) and crown position within the trees over both years. Only a few of the original parameters showed an O(3) effect. PC1 was related to parameters indicative of oxidative stress signalling and changes in carbohydrate metabolism. PC1 correlated with cumulative O(3) uptake over preceding days. • PC1 represents an O(3) -responsive multivariate pattern detectable in the absence of consistently measurable O(3) effects on individual leaf-level parameters. An underlying effect of O(3) on physiological processes is indicated, providing experimental confirmation of theoretical O(3) response patterns suggested previously.