The Relationship Between Growth and Oxygen Uptake in Hypoxic Rice Seedlings

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Abstract
Atwell, B. J. and Green way, H. 1987. The relationship between growth and oxygen uptake in hypoxic rice seedlings.—J. exp. Bot. 38: 454–465. Rice seedlings (Oryza saliva L.) were grown in the dark for up to 4 d in solutions containing various concentrations of O2. Compared with seedlings grown at 0·250 mol O2 m–3, the dry weight of the growing seedling was 14% lower at 0·110 mol O2 m–3 and 60% lower at 0 mol O2 m–3. Decreases in fresh weight were similar but not identical to decreases in dry weight, possibly because leaf growth was suppressed even above 0·110 mol O2 m–3. Oxygen deficiency inhibited root growth more severely than coleoptile growth. Coleoptiles from seedlings grown in aerated solution were exposed to an atmosphere of pure N2 for 30 min. Anoxia caused a decline in ATP content and energy charge, suggestive of decreased oxidative phosphorylation. It is not clear whether the decline in oxidative phosphorylation was solely responsible for impaired growth in hypoxia. In seedlings growing at O2 concentrations less than 0·110 mol O2 m–3, significant amounts of ethanol were synthesized. The rate of O2 uptake decreased markedly below 0·06 mol O2 m–3; this was presumably near the external O2 concentration at which oxidative phosphorylation became limited by the supply of O2. The stage of development of the seedlings appeared to influence O2 uptake, possibly through changes in conductance of the tissue to O2. Uncouplers were used to confirm that the critical O2 concentration was dependent on O2 diffusion rather than enzyme kinetics. Impaired growth above 0·110 mol O2 m–3 may have been due to a decreased activity of oxygenases of relatively low affinity for O2, which in turn altered cell metabolism.