Radial Oxygen Losses from Intact Rice Roots as Affected by Distance from the Apex, Respiration and Waterlogging

Abstract

Radial oxygen losses (ROL) from the roots of intact rice plants were assayed by the cylindrical Pt electrode technique. At 23°C losses from roots grown in waterlogged soil proved to be about double those from non‐waterlogged plants. Cooling which lowers respiratory activity led to increased ROL and it was estimated that at 23°C respiratory activity had been reducing oxygen loss by 8 to 10 10–8 g O 2 cm–2 root surface min–1 (c. 50 %) in the non waterlogged, and by 4.5 to 5.5 10–8 g O 2 cm–2 min–1 (2C–30 %) in the waterlogged roots. Lacunae formation occurred nearer to the apex and was eventually more extensive in the waterlogged roots while the presence of more intact and presumably functional tissue in the non‐waterlogged roots coincides with the greater respiratory effect noted. Estimated flux rates at 23°C (respiration inactive) were respectively 15–17 × 10–8 g O 2 cm–2 min–1 (non‐waterlogged) and 20–23 × 10–8 g O 2 cm–2 min–1 (waterlogged). A major part of this difference can probably be accounted for directly by the differences in root porosity, and Meakiness' superimposed upon lower porosity in the non‐waterlogged plants may account for the remainder. ROL was also examined in relation to distance from the apex. With respiratory activity lowered by cooling, two patterns of oxygen loss were detected. Pattern I was a property of younger roots of length between 5–9 cm, while pattern 2 was found in longer roots 11–16 cm bearing numerous emergent laterals. In both, ROL fell rapidly towards the base and at 4–5 cm approached zero in pattern 1 and near zero to about 16% of the maximum in pattern 2. The rapid drop in oxygen loss in both patterns which indicates a concomitant decrease in root wall permeability was associated with the appearance of cortical lacunae at 2–3 cm from the apex. In pattern 2 a rise in ROL began at 5–6 cm from the apex. The presence of lateral root initials in both the pericycle and unbroken segments of cortex was associated with maintained permeability in this pattern as well as with the basal increase in ROL. With a 3‐electrode system placed around the apical 3 cm regions of waterlogged roots, it was found that ROL was substantially affected by respiratory activity at 0.5 cm, a little less so at 1.7 cm, but much less or not at all at 3 cm from the apex. The drop in respiratory effect parallelled the formation of cortical lacunae.