Root-Zone CO(2) Concentration Affects Partitioning and Assimilation of Carbon in Oriental Melon Seedlings

Root-zone CO(2) is essential for plant growth and metabolism. However, the partitioning and assimilation processes of CO(2) absorbed by roots remain unclear in various parts of the oriental melon. We investigated the time at which root-zone CO(2) enters the oriental melon root system, and its distribution in different parts of the plant, using (13)C stable isotopic tracer experiments, as well as the effects of high root-zone CO(2) on leaf carbon assimilation-related enzyme activities and gene expressions under 0.2%, 0.5% and 1% root-zone CO(2) concentrations. The results showed that oriental melon roots could absorb CO(2) and transport it quickly to the stems and leaves. The distribution of (13)C in roots, stems and leaves increased with an increase in the labeled root-zone CO(2) concentration, and the δ(13)C values in roots, stems and leaves increased initially, and then decreased with an increase in feeding time, reaching a peak at 24 h after (13)C isotope labeling. The total accumulation of (13)C in plants under the 0.5% and 1% (13)CO(2) concentrations was lower than that in the 0.2% (13)CO(2) treatment. However, the distributional proportion of (13)C in leaves under 0.5% and 1% (13)CO(2) was significantly higher than that under the 0.2% CO(2) concentration. Photosynthetic carbon assimilation-related enzyme activities and gene expressions in the leaves of oriental melon seedlings were inhibited after 9 days of high root-zone CO(2) treatment. According to these results, oriental melon plants’ carbon distribution was affected by long-term high root-zone CO(2), and reduced the carbon assimilation ability of the leaves. These findings provide a basis for the further quantification of the contribution of root-zone CO(2) to plant communities in natural field conditions.