The effect of elevated ozone on leaf growth was investigated at the same experimental field site in 2004 (Christ et al. 2006). Ozone concentration was set 20% above ambient concentration; this increase is expected to be seen by 2050. Ozone concentration of the lower tropospheric layer has increased from pre-industrial values of around 10 ppb to a current summer average of about 40 ppb (Pritchard and Amthor 2005; Morgan et al. 2006). Ozone is a powerful oxidizing agent that is already expected to be leading to crop yield losses in the range of some billion $ in the US (Murphy et al. 1999; Lorenzini and Saitanis 2003). The sensitivity for ozone damage differs between plant species; Glycine max is known as a generally very sensitive species (Lesser et al. 1990).
In contrast to expectations based on laboratory experiments and "open-top-chamber-studies" (Reid and Fiscus 1998; Rogers et al. 2004; Fiscus et al. 2005) no decrease in crop yield of the investigated cultivar Spencer was observed. This might have been caused by optimal growing conditions for Glycine max in Illinois leading to extremely high crop yield in 2004 (Leakey et al. 2006). Yet, the results of Christ et al. (2006) showed that growth, photosynthesis and carbohydrate content of leaves from the upper canopy were reduced markedly in the treated plants. Leaves of the upper canopy are developing during the life phase of pod filling; a developmental stage of the plant when it is especially sensitive to ozone damage (Morgan et al. 2004). Those leaves are the prime source for assimilates going into the pods and developing seeds (Thrower 1962; Gifford and Evans 1981). The fact that those leaves remained smaller than leaves of control plants shows that the plants were able to divert their reserves towards generative organs at the cost of the vegetative growth of the leaves of the upper canopy. This implicates that at least the investigated cultivar still has high potential for optimizing crop yield. In future studies, it will be important to compare the vegetative growth performance of upper canopy leaves in different environmental situations to optimize the selection of lines or cultivars for certain regions and climatic scenarios.
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