Flooding tolerance of Carex species. II. Root gas-exchange capacity

Abstract.

Root CO₂ and O₂ gas exchange were measured in young Carex extensa Good. (flooding sensitive), C. remota L. and C. pseudocyperus L. (both flooding tolerant) plants, precultured either aerobically or anaerobically. Temperature changes form 21 to 11 °C had small effects on root CO₂ release from respiration. In C. extensa, root respiration rates decreased when plants were precultured anaerobically, while in C. pseudocyperus preculture conditions had no effect on root CO₂ release. In contrast to CO₂, temperature decrease significantly enhanced radial oxygen loss from the roots during the light phase, indicating that at 20 °C the O₂ transported form the shoot to the root met the demand for root respiration quite well, while at 11 °C excess O₂ entered the root and was released into the anaerobic nutrient solution. In C. remota and C. pseudocyperus, the maximal O₂ concentration of a previously anaerobic nutrient solution was attained after several days of equilibration with the atmosphere through the plant body and was approximately one-third of that found in C. extensa, indicating that the diffusion resistance of the root/rhizosphere interface to O₂ is much lower in the C. extensa root than in the flooding-tolerant Carex species. A calculation of the maximal attainable root length that can be sustained by pure O₂ diffusion from CO₂ exchange, and anatomical data obtained earlier, revealed that longitudinal diffusion of O₂ through the root is sufficient for the oxygen supply of the root. It is concluded that the postulate of a gas mass-flow into the root is not necessary for the understanding of flooding tolerance of Carex species.