EFFECTS OF SOIL-PH ON THE ECTOMYCORRHIZAL RESPONSE OF EUCALYPTUS-UROPHYLLA SEEDLINGS

To examine the effects of soil pH on ectomycorrhizal formation and fun ction on Eucalyptus urophylla S. T. Blake, seedlings inoculated with n ine ectomycorrhizal fungi (seven isolates of Pisolithus spp., Sclerode rma cepa and Laccaria laccata collected under eucalypt stands in Austr alia and the Philippines) were transplanted into pots containing a non -sterile acid (pH 4.6) sandy loam amended with four levels of CaCO3 th at raised the soil pH from 4.6 to 6.6 (5 mM CaCl2). Pots were placed i n temperature-controlled water baths (28 +/- 2 degrees C) inside an ev aporatively cooled glasshouse for 9 wk. Increase in soil pH from 4.6 t o 6.6 significantly decreased plant d. wt and shoot nutrient content o f uninoculated and inoculated seedlings. Inoculation with four Pisolit hus spp. (H445, H2144, M56 and H4003) significantly increased the grow th of E. urophylla seedlings at pH 4.6. At pH 6.6, eight ectomycorrhiz al isolates significantly improved total d. wt compared with those of the uninoculated seedlings. Pisolithus isolates stimulated seedling gr owth more than L. laccata whereas S. cepa was ineffective at all pH le vels. Total d. wt of H445 inoculated plants grown in P-deficient (8 mg P kg(-1) soil) soil was 147 % more than that of uninoculated plants g iven the same P rate and was 70 % that of plants fertilized with 64 mg P kg(-1) soil (P64) at pH 4.6. At soil pH 5.8 and 6.6, M56 was the be st growth-promoter for E. urophylla. These results indicate that soil pH can significantly alter the development and function of ectomycorrh izal fungi. Soil pH did not significantly affect mycorrhizal formation by the different ectomycorrhizal fungi. However, the percentages of m ycorrhizal root tips formed by the different ectomycorrhizal fungi dif fered significantly. Pisolithus isolate H445 formed the highest percen tage of colonized roots and highest total d. wt at pH 4.6 and 5.2, imp lying its potential for commercial use in acidic conditions.