Vascular Cavity Formation Enhances Oxygen Availability during Flooding in Root Tips of Phaseolus coccineus L. Primary Roots

Abstract

Premise of research. Vascular cavities (VCs) form in the primary roots of many species of cool-season herbaceous legumes when the growing medium is saturated and relatively warm (>15°–20°C). Circumstantial evidence has suggested that VCs that rapidly form following flooding of these roots are a form of inducible aerenchyma that confers tolerance to waterlogging. This study was undertaken to directly confirm that, once formed, VCs improve internal and rhizosphere oxygen levels in and around tips of roots in which they occur.Methodology. Seedlings of Phaseolus coccineus were flooded after various periods of growth for 24 and 48 h and compared to unflooded plants. Some roots were freehand sectioned to confirm and locate VCs. Some root segments were embedded, sectioned, and photographed for image analysis of cortical intercellular porosity. An oxygen photomicroprobe was used to measure oxygen concentration in the rhizosphere at root surfaces and internally by penetration through roots.Pivotal results. Flooding rapidly depleted oxygen in the rhizosphere and within root-tip tissue. VCs formed within 24 h near root tips. Surface measurements showed that, after formation, VCs elevated rhizosphere oxygen levels at root tips compared to those measured 2 h after flooding. Internal measurements showed very low oxygen concentration in vascular tissue 2 h after flooding that significantly improved after VC formation 22 h later. In addition to VC formation, flooding induced an increase in cortical porosity along the root axis.Conclusions. VCs enhance internal and rhizosphere oxygen concentrations in P. coccineus primary roots within 24 h after flooding. VCs are therefore verified as a form of inducible aerenchyma.