Stem hypertrophic lenticels and secondary aerenchyma enable oxygen transport to roots of soybean in flooded soil

Background and Aims Aerenchyma provides a low-resistance O-2 transport pathway that enhances plant survival during soil flooding. When in flooded soil, soybean produces aerenchyma and hypertrophic stem lenticels. The aims of this study were to investigate O-2 dynamics in stem aerenchyma and evaluate O-2 supply via stem lenticels to the roots of soybean during soil flooding. Methods Oxygen dynamics in aerenchymatous stems were investigated using Clark-type O-2 microelectrodes, and O-2 transport to roots was evaluated using stable-isotope O-18(2) as a tracer, for plants with shoots in air and roots in flooded sand or soil. Short-term experiments also assessed venting of CO2 via the stem lenticels. Key Results The radial distribution of the O-2 partial pressure (pO(2)) was stable at 17 kPa in the stem aerenchyma 15 mm below the water level, but rapidly declined to 8 kPa at 200-300 mm inside the stele. Complete submergence of the hypertrophic lenticels at the stem base, with the remainder of the shoot still in air, resulted in gradual declines in pO(2) in stem aerenchyma from 17.5 to 7.6 kPa at 13 mm below the water level, and from 14.7 to 6. 1 kPa at 51 mm below the water level. Subsequently, re-exposure of the lenticels to air caused pO(2) to increase again to 14-17 kPa at both positions within 10 min. After introducing O-18(2) gas via the stem lenticels, significant O-18(2) enrichment in water extracted from roots after 3 h was confirmed, suggesting that transported O-2 sustained root respiration. In contrast, slight O-18(2) enrichment was detected 3 h after treatment of stems that lacked aerenchyma and lenticels. Moreover, aerenchyma accelerated venting of CO2 from submerged tissues to the atmosphere. Conclusions Hypertrophic lenticels on the stem of soybean, just above the water surface, are entry points for O-2, and these connect to aerenchyma and enable O-2 transport into roots in flooded soil. Stems that develop aerenchyma thus serve as a 'snorkel' that enables O-2 movement from air to the submerged roots.