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.