Diurnal changes in radial oxygen loss and ethanol metabolism in roots of submerged and non-submerged rice seedlings

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Summary

The effects of shoot submergence on root aeration were examined using floodwater Hushed with 1 or 2 kPa CO2 plus 10 or 21 KPa O2 or with air; the rooting media were stagnant. Oxygen regimes within the roots were evaluated from measurements of radial O2 loss to polarographic electrodes sleeving the extension zone of nodal roots, or, from dissolved O2 and ethanol in the rooting medium.

With shoots submerged, root O2 and extension, as well as O2 and ethanol concentrations in the rooting medium underwent marked diurnal changes. In the dark, the radial O2 loss fell rapidly until root surface O2 became (0.001 mol m−3 (gas-phase equivalent < 0.1 kPa) and root extension slowed down or ceased. Oxygen, previously accumulated in the rooting medium, also declined markedly, whilst ethanol accumulated rapidly. When the lights come on, radial O2 loss resumed within 3 min and surged to a peak at c. 30 min before decreasing gradually to a lower level. Root extension also resumed, and a steady rise in dissolved O2 was accompanied by a decline in ethanol concentration. The expression of a diurnal cycling of O2, was attributed to high boundary-layer resistance to gas exchange between leaf and water, to C O2 supply and to a buffering effect by oxygen in the Hoodwater. Thus, during the day the escape of photosynthetic O2 was hindered, while at night, O2 flow from Hoodwater to leaf was restricted. Fluctuations in ethanol were attributed to the generation and subsequent consumption of ethanol by the roots in response to lower and higher internal O2 concentration. The O, surge at dawn was partly attributed to enhanced photosynthesis from accumulated internal CO2 and partly to a decline in O2 demand during the night as a result of substrate depletion in the plant. Stagnant floodwaters around the shoots led to much higher daytime rates of radial O2 loss from the roots, and to unexpected oscillations in radial O2 loss attributed to pressurization and de-pressurization during the expansion and subsequent release of O2-enriched bubbles from the leaves.

With non-submerged plants, the diurnal cycles in O2 and ethanol concentrations were smaller and in the case of O2, different in pattern: although, initially at night, radial O2 loss declined, it often rose again later. This trend would be consistent with a reduction in respiration during the night following rapid substrate depletion at the high temperatures. Short roots grew day and night, but as roots became longer, apical O2 concentration declined and, because of this, growth ceased at c SO mm. The considerable influence of high temperatures on root aeration was confirmed by rapid increases in radial O2 loss when temperatures were lowered from 32 to 23 °C.

The ecological significance of the findings is discussed and it is concluded that (a) a diurnal periodicity of root growth and of localized internal anoxia may be a normal feature of roots in submerged and even non-submerged rice, and (b) ethanol production may play an important role in root survival during the night.

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