Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics

Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochromemediated respiration contributed 70% and cyanideinsensitive (alternative) respiration 30% of the total respiration rate, and apparent Km values of cytochrome oxidase were lower than those obtained for the alterna­ tive oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent Km for oxygen, were strongly correlated with differences in root porosity and root diameter. Km(0 2) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be esti­ mated. Root respiration of Rumex maritimus and R. crispus, both with high root porosity, but differing in root diameter, had a low Km for oxygen (3-7 mmol m-3). In contrast with this were the responses of R. thyrsiflorus, which has thin roots but low root porosity: a high Km (10-20 mmol m-3) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding. Key-words: Rumex; critical oxygen pressure; Km( 0 2); root respiration; root porosity; cytochrome-coupled respiration; Arrhenius plot. Abbreviations: SHAM , salicyl hydroxamic acid; MES, 2-N-morphoIino ethane sulphonic acid; COPR , critical oxygen pressure for respiration.