Phosphorus Acquisition in the Soil-Root System of Eucalyptus pilularis Smith Seedlings. II the Effect of Ectomycorrhizas on Seedling Phosphorus and Dry Weight Acquisition

Abstract

Ectomycorrhizas were visible on Eucalyptus pilularis Smith seedling root systems 30 days after sowing when seedlings were grown under glasshouse conditions in a severely phosphorus-deficient A1 horizon soil collected from a study area in the Myall Lakes National Park, N.S.W. The types of ectomycorrhizas were similar to those observed on seedlings collected from the study area. Black ectomycorrhizas were the initial and dominant symbiont on seedlings until at least 6 months after sowing.

The ectomycorrhizal component of the A₁ horizon soil could not be eliminated by partial sterilisation of the soil without a significant change in the available phosphorus supply. However, a comparison of seedlings grown in autoclaved soil with or without black ectomycorrhizas showed that this symbiont had the capacity to increase the phosphorus acquisition of seedlings.

A correlative approach, quantifying the relationship between black ectomycorrhizal infection intensity and seedling growth, was utilised to indicate whether the black ectomycorrhizas improved phosphorus and dry weight acquisition of seedlings grown in A₁ horizon soil. The pattern of phosphorus acquisition and the dry weight increment of seedlings grown in the A₁ horizon soil under glasshouse conditions were characterised by two phases, the second indicated by a three-fold increase in both seedling parameters 116 days after sowing.

At a harvest in the first phase of growth (96 days after sowing) seedlings with larger numbers of black ectomycorrhizas per unit root length had significantly higher phosphorus contents (P< 0.05) but not dry weights. In comparison, a harvest selected in the second phase of growth (176 days after sowing) showed that both seedling phosphorus content and dry weight were highly correlated with the numbers of black ectomycorrhizas per unit root length (r² = 0.81 and r² = 0.88 respectively). These results, when considered in combination with the inoculation study, indicate that black ectomycorrhizas do improve the phosphorus acquisition of seedlings grown in A₁ horizon soil. The black ectomycorrhizas increased the inflow of phosphorus by 2.2 x 10^-6μg P cm infected root length^-1 s^-1. We propose that dry weight was promoted by this increase only when substantial amounts of phosphorus were transferred to the shoot. This has been shown to occur primarily in the second phase of seedling growth.