Radial oxygen loss and physical barriers in relation to root tissue age in species with different types of aerenchyma

Plant root aeration relies on aerenchyma and barrier formation in outer cortex influencing the radial oxygen loss (ROL) from roots towards the rhizosphere. Plant species display large variation in strategies for both responses. We investigated the impacts of root-zone hypoxia on aerenchyma formation and development of ROL apoplastic barriers in the outer cortex as a function of root tissue age using three lowland grassland species, each with alternative aerenchyma structure. All species increased root aerenchyma and continued with root elongation after imposing hypoxia. However, ROL barrier development differed: (i) Rumex crispus L. displayed only 'partial' barrier to ROL evidenced at older tissue ages, (ii) Cyperus eragrostis Lam. initiated a 'tighter' barrier to ROL following exposure to hypoxia in tissues older than 3 days, and (iii) Paspalidium geminatum (Forssk.) Stapf demonstrated highly effective inhibition of ROL under aerated and hypoxic conditions at all tissue ages related to constitutive 'tight' apoplastic barriers in outer cortex. Thus, hypoxic conditions affected root elongation and 'tightness' of apoplastic barriers depending on species. The physiological implications of the different ROL responses among species in relation to the differential formation of barriers are discussed.