Growth responses of a diploid and a tetraploid perennial ryegrass cultivar

The growth response of a diploid and tetraploid perennial ryegrass (cv. Alto infected with wildtype endophyte) to water stress (wilting point vs field capacity), defoliation (2.5 vs 6 cm) and root feeding invertebrate stress (with and without 600 grass grub (Costelytra zealandica) m-2) was determined in a semi-controlled environment. Stresses were applied for 8 weeks followed by a 4 week recovery period. Plant biomass was reduced by all three stresses. Tiller production was reduced by water stress and to a lesser extent by severe cutting. Although compensatory growth occurred during the recovery period, one or more aspects of plant growth remained lower in previously stressed plants at the end of the recovery period. Root and total biomass was reduced by grass grub to a greater extent in the tetraploid than diploid. There was a consistent trend of the tetraploid allocating more resources than the diploid to shoot growth. The lower tiller density and biomass of the tetraploid, in combination with greater allocation of resources to shoot growth and greater susceptibility to root feeding invertebrates may compromise its persistence in the field.