Spatial patterns of radial oxygen loss and anatomical traits associated to root elongation in Lotus tenuis under anoxia

MANZUR MILENA ELISA; GRIMOLDI AGUSTÍN ALBERTO; INSAUSTI PEDRO; STRIKER GUSTAVO GABRIEL

Chascomús, Buenos Aires, Argentina

Taller; V Taller interdisciplinario sobre aspectos genéticos, moleculares y eco-fisiológicos de Lotus spp. y sus simbiontes.; 2010

IIB-Intech, Universidad de San Martín - CONICET

Root aerenchyma provides a pathway of lower resistance for longitudinal oxygen transport from the root base towards the root apex, a crucial process for root aeration under anaerobic conditions. However, oxygen can be lost from the root towards the rhizosphere (i.e. radial oxygen loss: ROL) before reaching the apex. For that reason, the magnitude of ROL along a root may influence its elongation by altering root apex oxygenation. In this respect, some species are able to develop a physical barrier limiting ROL by increasing suberin deposition in the outer layers of root cortex (e.g. wetland graminoids). This work investigates the anatomical traits defining ROL along roots, and their relationship with root elongation under anoxic conditions in the forage legume Lotus tenuis. Consequently, adult plants of L. tenuis were grown under hydroponics culture and subjected to aerated (7.9 mg O2 L-1) and anoxic (0.5 mg O2 L-1) conditions for 10-d. Aerenchyma root proportion, ROL pattern along roots (0.5–5 cm from apex), suberin deposition in the outer layer of the cortex and elongation rate in adventitious roots were assessed. Results showed that aerenchyma was higher under anoxic than under aerated conditions (26.2% vs. 12.9%). Under anoxic conditions L. tenuis showed higher ROL along the root than control (85 vs. 33.7 nmol m-2 s-1). Suberin deposition in the outer layer of the cortex at two root positions (0.5 and 4 cm from the apex) was low (<2% in both positions) and similar between treatments (p>0.05). Although the aerenchyma tissue was two-fold higher under anoxic conditions, root elongation rate was strongly reduced (from 3.11 to 0.33 cm d-1). This result appears to be related with the excessive loss of oxygen from the roots towards rhizosphere.  Therefore, L. tenuis does not develop a barrier against ROL when it  is subject to anoxic conditions, as it was reported in some wetland graminoids.  Although release of oxygen towards the rhizosphere can be seen as negative, because it constrains the root elongation rate, the benefits of maintaining an oxidized microenvironment surrounding the root can be part of the strategy of this flood-tolerant species to avoid the entrance of toxic forms of iron and manganese into the root apoplast. This idea and its implications deserve further experimental investigation.