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

Abstract

S ummary The effects of shoot submergence on root aeration were examined using floodwater Hushed with 1 or 2 kPa CO 2 plus 10 or 21 KPa O 2 or with air; the rooting media were stagnant. Oxygen regimes within the roots were evaluated from measurements of radial O 2 loss to polarographic electrodes sleeving the extension zone of nodal roots, or, from dissolved O 2 and ethanol in the rooting medium. With shoots submerged, root O 2 and extension, as well as O 2 and ethanol concentrations in the rooting medium underwent marked diurnal changes. In the dark, the radial O 2 loss fell rapidly until root surface O 2 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 O 2 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 O 2 was accompanied by a decline in ethanol concentration. The expression of a diurnal cycling of O 2 , was attributed to high boundary‐layer resistance to gas exchange between leaf and water, to C O 2 supply and to a buffering effect by oxygen in the Hoodwater. Thus, during the day the escape of photosynthetic O 2 was hindered, while at night, O 2 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 O 2 concentration. The O, surge at dawn was partly attributed to enhanced photosynthesis from accumulated internal CO 2 and partly to a decline in O 2 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 O 2 loss from the roots, and to unexpected oscillations in radial O 2 loss attributed to pressurization and de‐pressurization during the expansion and subsequent release of O 2 ‐enriched bubbles from the leaves. With non‐submerged plants, the diurnal cycles in O 2 and ethanol concentrations were smaller and in the case of O 2 , different in pattern: although, initially at night, radial O 2 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 O 2 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 O 2 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.