Chronic ozone exposure and photosynthate partitioning into starch in soybean leaves

Photosynthesis, photosynthate partitioning, and translocation are reportedly affected by ozone (O-3), but the sensitivity of these processes in relation to other potential targets is not known. In this study, rates of net photosynthetic CO2 assimilation (Pn) and carbohydrate accumulation in fully expanded leaves in soybean canopies in the field were examined following season-long exposure either to low-O-3 conditions (20-30 nL L-1 7-h daily average, charcoal-filtered air) or to chronic midlevel concentrations (50-60 nL L-1, ambient air plus supplemental O-3). O-3-sensitive (cv. Forrest) and less sensitive (cv. Essex) lines of soybean were compared at two growth stages corresponding to altered sink demand (i.e., vegetative and early pod fill). Starch was a significant sink for photosynthate, accounting for ca. 29% of carbon assimilation at the vegetative growth stage and 35% at the later reproductive stage. Effects of O-3 on Pn or the proportion of photosynthate partitioned into starch were not detected; differences on the order of 15% would have been significant. The lack of O-3 effects on partitioning indicates that translocation of photosynthate from the leaf was probably not altered by O-3 during the light in this study. Transient prior effects of O-3 on partitioning are not excluded since average starch levels were reduced-whereas average sucrose levels were increased-in leaves of Forrest exposed to elevated O-3. In contrast, leaf growth was affected by O-3 under these conditions for both cultivars, but growth was affected in different ways. Leaf area was reduced significantly at both growth stages for cv. Forrest, whereas specific leaf mass was increased significantly in cv. Essex. These results indicate that low levels of O-3 can affect leaf growth and development without producing long-term effects on photosynthesis or partitioning of photosynthate.