Fig. 2.

The experimental system used to supply ¹⁸O₂ gas to the base of the stems of soybean plants. Control: ¹⁸O₂ was supplied to the basal region of the stem of irrigated control plants that lacked both aerenchyma and hypertrophic lenticels. Flood Ae-Open (Ae, aerenchyma): ¹⁸O₂ was supplied to the basal region of the stem aerenchyma and hypertrophic lenticels of flooded plants. Water level was maintained at 3·5 cm that was the same level as in pre-flooded conditions, so aerenchyma and lenticels just above the water surface could be exposed to ¹⁸O₂ gas layer. Flood Ae-Submerged: ¹⁸O₂ was supplied to the basal region of the stem, but above the region that contained aerenchyma and lenticels in flooded plants. Aerenchyma and hypertrophic lenticels were well-developed in the basal stem, but the water level was raised to cover them. Water level was maintained at 6 cm that was 2·5 cm higher than the level of pre-flooded conditions, so aerenchyma and lenticels of the stem base were completely submerged.