Greater resistance to flooding of seedlings of Ulmus laevis than Ulmus minor is related to the maintenance of a more positive carbon balance

Key message Flooding reduces leaf photosynthesis but initially enhances plant respiration. Differences in flood-induced restrictions to plant net carbon gain could underlie distinct susceptibility to flooding between species. Flooding affects plant physiology and development, ultimately determining species' ecology and distribution. Ulmus laevis Pallas and Ulmus minor L. are two European riparian trees facing habitat degradation and Dutch elm disease. Here, we have investigated the sensitivity to flooding of 2-year-old seedlings of these species to ascertain their level of tolerance in relation to future reforestations. Gas exchange of leaves, stems and roots, hydraulic conductivity and growth were measured in a controlled experiment. Seedlings of U. minor died by the 60th day of flooding, but not those of U. laevis, which partly recovered physiological functions after 30 days of adequate watering. Light-saturated net photosynthesis rate (Pn) and stomatal conductance progressively declined after flooding started. Forty-six days later, Pn was 2 and 3 times lower in flooded compared to control U. laevis and U. minor plants, respectively; at this time, the percentage loss of root hydraulic conductivity increased by fourfold relative to control plants. Rates of respiration initially increased with flooding in leaves, stems and roots, and then were similar in flooded and control plants. Aerenchyma was not formed on either species, but lenticels at the water line became increasingly hypertrophied and could help in providing oxygen and sustaining respiration. Whole-plant net carbon gain was 3 and 9 times lower in flooded than control plants in U. laevis and U. minor, respectively. Our data suggest that the inability to maintain a positive carbon balance somehow compromises seedling survival under flooding, earlier in U. minor than U. laevis, partly explaining their differential habitats.