| Potassium, an element required in slightly lower quantities than nitrogen, is one of the essential major nutrients in forest ecosystems. In the present experiments, eight typical ectomycorrhizal fungi (Pisolithus tinctorius 2144, 441, 715 and XC1, Laccaria bicolor 270 and S238N, Paxillus involutus COU and Cenococcum geophilum SIV), some aspects of the biology of which have been intensively studied, were selected to investigate the absorption, utilization and mobilization of potassium from various media by ectomycorrhizal fungi and mycorrhized eucalyptus seedlings. The objectives were to determine the role of these fungi in potassium nutrition and to provide some information that may help in the selection of fungal species for improvement of potassium nutrition in forestry.Ectomycorrhizal fungal species and strains used in the experiments varied significantly in kinetic parameters of potassium absorption. The Km value ranged from 14.02 to 24.75 molL-1, Imia from 28.95 to 65.74 molg-1 DWh-1 and Cmin from 1.28 to 5.51 molL-1. The results indicated that the' mycorrhizal fungi had an effective system of potassium absorption from soil solutions. Moreover, the absorption of potassium by ectomycorrhizal fungi was greatly inhibited by ammonium in solution, especially at high concentrations. For example, a depression of potassium absorption of 19.52-74.02% compared to the control (without ammonium) was observed at 100 and 200 rnol L-1 of ammonium, even though there were different depression rates among the fungal species at the same ammonium concentrations. IAA and hypaphorine, the betaine of tryptophane, stimulated the efflux of oxalate and protons by ectomycorrhizal fungi, and the degree of stimulation varied among the species.Typical growth of ectomycorrhizal fungi was found at low potassium supply in contrast to those in solutions with high potassium and those with no potassium, in which growth was retarded. One explanation for this could be the low Cmin values. Potassium concentrations in culture solutions significantly influenced the efflux of oxalate and protons from the fungi in the sequence: no potassium > low potassium > middle potassium > high potassium. The secretion rate varied from 1.83 to 7.40 (oxalate) and 3.37 to 17.36 times (protons) compared to control (without potassium). It seems reasonable to suggest that greater efflux of oxalate and protons under conditions of potassium deficiency could lead to biological mobilization of lattice potassium and release of interlayer potassium from clay minerals. Furthermore, fungal species and strains varied noticeably in the efflux of oxalate and protons.The variable efflux of oxalate and protons from ectomycorrhizal fungi (which was related to potassium supply) influenced the ability of the fungi to utilize potassium from clay minerals. In general, the greater the efflux of protons by the fungi, the larger was the proportion of potassium in the hyphae that was derived from the exchangeable and interlayer potassium in clay minerals. In addition, the greater the secretion of oxalate by the fungi, the larger was the proportion of the potassium in hyphaederived from the lattice of clay minerals. The accumulation of potassium in the leaves of Pt XC1- and Pt 2144-mycorrhized eucalyptus seedlings was increased by 26-112% and the growth by 37-64.5 % (height) and 86-160% (biomass) compared to nonmycorrhized seedlings. Moreover, the infected seedlings (especially those infected by Pt XC1) were able to utilize the potassium in minerals digested by HF-HC1O4 in contrast to nonmycorrhized seedlings.More than 6,000 ectomycorrhizal fungi have been found, representing a wide variation in biological characteristics. Only a few of these fungi (fewer than 20 species or strains), however, have been studied in respect of their potassium absorption from culture solutions and mobilization from clay minerals. From our present experiments, we can conclude that the eight ectomycorrhizal fungi studied varied significantly in potassium absorption, efflux of oxalate and proto...
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