Root strength and trampling tolerance in the grass Paspalum dilatatum and the dicot Lotus glaber in flooded soil

1. We studied the differences in root strength of species with contrasting root structural types (the grass Paspalum dilatatum and the dicot Lotus glaber ), and their relationship with tolerance to simulated cattle trampling under flooding conditions. 2. Root strength was analysed through measurement of the pressure required to cause root collapse. The responses of aerenchyma and plant mass to flooding and trampling were studied. 3. Root aerenchyma increased from 28·0 to 40·2% in P. dilatatum and from 12·9 to 19·7% in L. glaber under flooding conditions. The increase in aerenchyma did not affect root strength in the relatively trampling-resistant roots of P. dilatatum : roots cracked at >380 kPa in all treatments. In contrast, roots of L. glaber were weaker, cracking at 260 kPa; flooded roots with air spaces irregularly dispersed in the cortex cracked at 115 kPa. 4. Trampling, flooding or their combination did not affect the biomass of P. dilatatum . Conversely, the isolated effects of either trampling or flooding both decreased biomass accumulation in L. glaber . The combination of both treatments killed all Lotus plants . 5. In conclusion, root strength was positively associated with soil trampling tolerance. The effect of aerenchyma tissue generation on root strength varies among root structural types. Aerenchyma tissue increases root weakness in the less stable structural type of the dicot species, but had no effect on the strength of the grass.