Growth responses of diploid and tetraploid perennial ryegrass (Lolium perenne) to soil-moisture deficit, defoliation and a root-feeding invertebrate

Growth responses of perennial ryegrass (Lolium perenne L.) diploid cv. Alto and a tetraploid breeding line of Alto infected with wild-type fungal endophyte to three types of stress were studied in a semi-controlled environment. The stresses imposed were: soil-moisture deficit (wilting point vs field capacity), severe defoliation (2.5 vs 6 cm cutting height), and root-feeding invertebrate infestation (nil vs 600 grass grubs (Costelytra zealandica) m(-2)). Stress treatments were applied simultaneously in a fully factorial design for 8 weeks followed by a 4-week recovery period. Total plant biomass was reduced to a similar extent for both ploidies by soil-moisture deficit (43% reduction), severe defoliation (31% reduction) and root-feeding invertebrates (13% reduction) at the end of the treatment period. At the end of the recovery period, feeding by grass grubs reduced root biomass by 34% and total plant biomass by 25% in the tetraploid cultivar, but there was no effect in the diploid cultivar. Although compensatory growth occurred during the recovery period, one or more aspects of plant growth (e.g. tillering, biomass) remained lower in previously stressed plants at the end of the recovery period. The lower tiller density and total 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.