Effects of phosphate supply and inoculation with a vesicular‐arbuscular mycorrhizal fungus on the death of the root cortex of wheat, rape and subterranean clover

The effect of phosphorus supply and mycorrhizal infection on the death of the cortex of roots (RCD) was examined in two glasshouse experiments. RCD was observed in both Australian and English cultivars of wheat (Triticum aestivum L.), and in rape (Brassica napus L.), but not in subterranean clover (Trifolium subterraneum L.). It occurred most rapidly in wheat plants moderately deficient in phosphorus (i.e. in plants producing approx. 60% of shoot weight with phosphorus not limiting growth) but was delayed both in plants severely deficient in phosphorus and in plants with adequate and luxury supplies of phosphorus compared to those which were moderately deficient. Although inoculation with a vesicular‐arbuscular mycorrhizal fungus did not markedly affect plant growth, it delayed RCD in phosphorus‐deficient plants and accelerated RCD in phosphorus‐adequate plants. The effects of phosphorus supply and mycorrhizal infection on RCD cannot be readily explained by effects of these treatments on plant growth or phosphorus concentrations within plants. Moreover, it is unlikely that effects of these treatments on RCD are related to effects on the availability of assimilates required to maintain the cortex.