Root proliferation in response to P stress and space: implications for the study of root acclimation to low P supply and P acquisition efficiency

The experiment was conducted to understand how root morphological traits contributed to the contrasting P-acquisition efficiencies of two Trifolium subterraneum cultivars. Phosphorus acquisition, root length proliferation and root acclimation to P deficiency were investigated by varying the P supply (20–300 mg P kg−1) and planting density (1–12 plants pot−1) of clover micro-swards. The shoot yield of both cultivars increased in response to increased P supply and increased planting density. Higher planting densities increased shoot yield by reducing the time to leaf canopy closure. In P-deficient soil, the ‘less P-efficient’ cultivar acquired less P and yielded relatively poorly compared with the ‘more P-efficient’ cultivar when planting density was low (<6 plants pot−1). However, the P acquisition and yields of the cultivars were equivalent when planting density was high (~6–12 plants pot−1). Both cultivars proliferated nutrient-foraging roots in response to P limitation and space (i.e. lower planting densities). However, the proliferation responses of the cultivars differed, particularly in response to space. Differential proliferation responses, combined with differences in specific root length, explained how the cultivars differed in P-acquisition efficiency. The results indicate that inappropriate cultural conditions (e.g. planting densities) may confound attempts to define P-efficient root traits and to identify genotypes with improved P-acquisition efficiency.