Stem photosynthesis not pressurized ventilation is responsible for light-enhanced oxygen supply to submerged roots of alder (Alnus glutinosa)

Background and Aims Claims that submerged roots of alder and other wetland trees are aerated by pressurized gas flow generated in the stem by a light-induced thermo-osmosis have seemed inconsistent with root anatomy. Our aim was to seek a verification using physical root-stem models, stem segments with or without artificial roots, and rooted saplings. Methods Radial O-2 loss (ROL) from roots was monitored polarographically as the gas space system of the models, and stems were pressurized artificially. ROL and internal pressurization were also measured when stems were irradiated and the xylem stream was either CO2 enriched or not. Stem photosynthesis and respiration were measured polarographically. Stem and root anatomy were examined by light and fluorescence microscopy. Key Results Pressurizing the models and stems to <= 10 kPa, values much higher than those reportedly generated by thermo-osmosis, created only a negligible density-induced increase in ROL, but ROL increased rapidly when ambient O-2 concentrations were raised. Internal pressures rose by several kPa when shoots were exposed to high light flux and ROL increased substantially, but both were due to O-2 accumulation from stem photosynthesis using internally sourced CO2. Increased stem pressures had little effect on O-2 transport, which remained largely diffusive. Oxygen flux from stems in high light periods indicated a net C gain by stem photosynthesis. Chloroplasts were abundant in the secondary cortex and secondary phloem, and occurred throughout the secondary xylem rays and medulla of 3-year-old stems. Diurnal patterns of ROL, most marked when light reached submerged portions of the stem, were modified by minor variations in light flux and water level. Low root temperatures also helped improve root aeration. Conclusions Pressurized gas flow to submerged roots does not occur to any significant degree in alder, but stem photosynthesis, using internally sourced CO2 from respiration and the transpiration stream, may play an important role in root aeration in young trees and measurably affect the overall carbon balance of this and other species.