EFFECTS OF DROUGHT AND FLOODING ON ROOT ANATOMY IN 4 TROPICAL FORAGE GRASSES

Seasonal drought and flooding severely limit pasture growth in savannas, although tropical grasses show broad variation in their tolerance to these water stresses. In this study we analyzed and compared the responses in morphology and anatomy of the roots of four important perennial C-4 forage grass species to short-term flooding and moderate drought under controled conditions. The plants studied were the dryland bunchgrasses Andropogon gayanus and Hyparrhenia rufa and the wetland stoloniferous Echinochloa polystachya and Brachiaria mutica. The responses to flooding were more conspicuous than those to drought, and all species were able to alter their root form and structure under flooding. The most evident response was the development of aerenchymatous tissue in the root cortex, which was propertionally highest in H. rufa. The wetland grasses already had constitutively high proportions of aerenchyma tissue in the cortex, which changed little with waterlogging, up to 40%-49% of total transverse root area. Andropogon gayanus developed the least aerenchyma space in the cortex and was the most affected by flooding. Other effects of flooding included root dimorphism and suberization of both exodermis and endodermis cells in all grasses.