THE EFFECT OF FUNGICIDES ON VESICULAR-ARBUSCULAR MYCORRHIZAL SYMBIOSIS III - THE INFLUENCE OF VA MYCORRHIZA ON PHYTOTOXIC EFFECTS FOLLOWING-APPLICATION OF FOSETYL-AL AND PHOSPHONATE

This study compares the effects of the fungicide Aliette(R) (fosetyl-A l) on non-mycorrhizal and mycorrhizal onion (Allium cepa L.) with effe cts of the degradation products, aluminium (applied as aluminium chlor ide) and phosphonate (applied as dimethyl phosphonate). We sought to d etermine the extent to which the plants absorb and accumulate phosphon ate and ascertain why, as shown previously, toxic effects of fosetyl-A l on mycorrhizal plants are less severe than effects on equivalent non -mycorrhizal plants. Fosetyl-Al markedly reduced growth, especially of roots, and also inhibited mycorrhizal colonization. Dimethyl phosphon ate caused smaller effects on growth and did not decrease colonization . Aluminium chloride did not affect growth of non-mycorrhizal or mycor rhizal plants, or mycorrhizal colonization. In all cases, mycorrhizal plants grew better than equivalent non-mycorrhizal plants (with no add ed soil phosphate), and non-mycorrhizal plants given supplementary soi l phosphate grew best. Concentrations and contents of total P in shoot s were increased by dimethyl phosphonate and more so by fosetyl-Al. Co ncentrations of P were also increased in roots. P-31 nuclear magnetic resonance (NMR) spectroscopy was used to determine relative concentrat ions of phosphonate and phosphate. Application of fosetyl-Al led to ac cumulation of more phosphonate than did application of dimethyl phosph onate, and non-mycorrhizal plants treated with fosetyl-Al accumulated much more phosphonate than did equivalent mycorrhizal plants (both wit h no added soil phosphate). Internal phosphate concentrations increase d, especially in mycorrhizal plants, following application of both dim ethyl phosphonate and fosetyl-Al. Non-mycorrhizal plants given supplem entary soil phosphate also showed restricted growth when treated with fosetyl-Al even though they accumulated phosphate in relatively high a mounts and had lower ratios of phosphonate to phosphate, particularly in shoots. In general, high internal concentrations of phosphonate wer e correlated with large reductions in plant growth among treatments. P ossible causes of these effects include conversion of phosphonate to p hosphate in the tissues (especially in mycorrhizal plants), enhanced u ptake of phosphate via mycorrhizal fungi and additional accumulation o f P in plants with restricted growth. Competition for uptake between p hosphate and phosphonate is also discussed.