Morphological and Anatomical Changes of Roots Relate to Waterlogging Tolerance in Brachiaria Forage Grasses

Oxygen is a limiting factor for plant growth under waterlogged soil conditions. To avoid asphyxiation, roots of plants adapted to waterlogging show traits that enhance ventilation and sustain growth under oxygen-limited conditions. A combination of traits such as large aerenchyma formation, thicker nodal roots and cells of the outer part of the root, narrower steles, and reduction in number of lateral roots developed from nodal roots under waterlogged conditions might contribute to improve internal aeration and reduce oxygen demand. The objective of this study was to determine differences in such traits in seven Brachiaria genotypes with different levels of tolerance to waterlogging stress conditions. Tolerant genotypes showed well-developed aerenchyma in nodal roots under drained conditions that was further increased under waterlogging. Aerenchyma developed in all genotypes under waterlogged conditions but the extent of development was greater in tolerant genotypes. Greater level of aerenchyma development in tolerant genotypes in combination with narrower steles, thicker nodal roots and cells of the outer part of the root, and a reduced number of lateral roots could contribute to superior tolerance to waterlogging and could allow deeper root penetration into waterlogged soil. A reduction in number of lateral roots might explain the greater tolerance of moderately tolerant genotypes than sensitive genotypes. The long delay in the development of new nodal roots under waterlogging may explain why the most sensitive genotype was the more affected by waterlogging.