The effect of reduced phosphorus concentration on mycorrhizal development and growth of Eucalyptus globulus Labill. seedlings inoculated with 10 different fungi

In order to improve the procedure for selecting effective mycorrhizal inoculants for fast-growing eucalypts, a wide range of isolates associated with Eucalyptus globulus Labill. was tested at two contrasting nutrient regimes, representative of that present following outplanting (i.e, post-fertilization) and that likely to occur one or two years later when soil nutrient concentrations will have returned to a much lower level. It was felt that this would provide a more accurate prediction of a selected isolate's potential performance during early plantation establishment. As a consequence, during an initial 12-week period, representative of the nursery phase, seedlings of Eucalyptus globulus, pre-inoculated with 10 different mycorrhizal fungi, were given optimal nutrients to encourage the development of mycorrhizas. At the end of this period, six of the 18 replicate plants were harvested and assessed for shoot growth and mycorrhizal infection while the remainder received for a further 12 weeks two different nutrient solutions (10 and 3 mg l(-1) phosphorus (P)). When these plants were harvested at 24 weeks, shoot growth and mycorrhizal development were again assessed. At the end of the first phase (0-12 weeks), all seedlings, irrespective of fungal inoculant, had become mycorrhizal. However, the extent of mycorrhizal infection achieved by the 10 different fungi ranged from 1.4% to 40.7%. Significant differences were also found between the fungi in their effect on stem diameter, stem dry weight, and shoot dry weight but not stem height or leaf dry weight. When assessed at 24 weeks, mycorrhizal development ranged from 22.3% to 77.7% on seedlings given 10 mg l(-1) P and 23.1-77.3% for those given 3 mg l(-1) P. Three of the 10 fungi formed >50% infection on seedlings, irrespective of the nutrient regime applied. Most of the fungi formed higher levels of infection on seedlings given 10 mg l(-1) P; however, two isolates formed higher levels of infection on seedlings given 3 mg l(-1) P. Infection by nine of the 10 isolates led to a significant increase in leaf and shoot dry weight by seedlings given 10 mg l(-1) P in comparison to those which received 3 mg l(-1) P. However. inoculation with the remaining fungus, an isolate of Pisolithus tinctorius, led to seedlings given 3 mg l(-1) P forming shoots as large as those attained by plants given the higher P regime. The results indicate that isolates can be selected with the potential to achieve significant gains over a broad spectrum of nutrient concentrations. The implications for improving the establishment of E. globulus plantations at a wide spectrum of outplanting sites is discussed.