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 inocu lants 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-fertilizat ion) and that likely to occur one or two years later when soil nutrient con centrations 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 n ursery phase, seedlings of Eucalyptus globulus, pre-inoculated with 10 diff erent mycorrhizal fungi, were given optimal nutrients to encourage the deve lopment of mycorrhizas. At the end of this period, six of the 18 replicate plants were harvested and assessed for shoot growth and mycorrhizal infecti on while the remainder received for a further 12 weeks two different nutrie nt solutions (10 and 3 mg l(-1) phosphorus (P)). When these plants were har vested at 24 weeks, shoot growth and mycorrhizal development were again ass essed. At the end of the first phase (0-12 weeks), all seedlings, irrespective of fungal inoculant, had become mycorrhizal. However, the extent of mycorrhiza l infection achieved by the 10 different fungi ranged from 1.4% to 40.7%. S ignificant differences were also found between the fungi in their effect on stem diameter, stem dry weight, and shoot dry weight but not stem height o r 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, irrespectiv e 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 h igher 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 thos e which received 3 mg l(-1) P. However. inoculation with the remaining fung us, 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 re gime. The results indicate that isolates can be selected with the potential to ac hieve significant gains over a broad spectrum of nutrient concentrations. T he implications for improving the establishment of E. globulus plantations at a wide spectrum of outplanting sites is discussed. (C) 2000 Elsevier Sci ence B.V. All rights reserved.