Quantification of carbon-water dynamics in soil-perennial grass (Bothriochloa ischaemum) feedbacks under drought stress following a double isotope-labelled pulse experiment

Abstract

Current studies have described photosynthetic and hydraulic processes in plants using parameters from the whole plant or particular organs. Quantification of carbon and water resource consumption and coupling between plant organs is poorly understood but important for predicting carbon and water coupled processes in grassland ecosystem drought responses. We determined the correlations between water transport parameters and carbon allocation in leaves, stems, branches, and roots of the grass Bothriochloa ischaemum, sampled 0, 6, 24, 48, 216, and 360 h after 13C-18O pulse-labeling under three drought treatments: control (CK), mild stress (MS) and serious stress (SS). We found that: (1) The sum of 13C in roots, soil, and rhizosphere soil was higher than the amount of 13C respired by roots under all treatments. The ratio of stored to respired 13C in the SS had a significantly lower than CK, indicating that the underground grass-soil system was a carbon sink rather than a carbon source. (2) 216 h after the application of 18O-labeled water, the proportion of 18O-labeled water in the aboveground portion of plants was up to 60% under MS, significantly greater than under SS (36.5%) and under CK (45.9%). Root density was mainly distributed in the 0–15 cm soil layer under mild drought stress. Both results suggest that grasses respond to mild drought by increasing topsoil root density to absorb soil water and improve the water transfer ratio to the aboveground portion of the plants. (3) During the double-labeling experiment, the ratio of 13C accumulated to 18O consumed in roots under mild drought was significantly higher than in other treatments. In contrast, these ratios in old leaves and stems under drought were significantly lower than in the control treatment, indicating that B.ischaemum experiences a trade-off between increasing root carbon and water utilization and reducing old leaf and stem carbon and water utilization under mild drought stress.