Tissue magnesium and calcium affect arbuscular mycorrhiza development and fungal reproduction

Applications of high levels of MgSO₄ resulted in reduced root colonization and sporulation by Glomus sp. (INVAM isolate FL329) with sweet potato and onion in aeroponic and sand culture, respectively. Onion shoot-Mg concentrations were elevated when a nutrient solution containing 2.6 or 11.7 mM MgSO₄ was applied. Magnesium application depressed tissue-Ca levels. With lower Ca in the tissue, colonization was reduced from > 30% of root length to < 10%, and sporulation from > 1200 to ca. 200 spores per plant, 10 weeks after transplantation and the start of nutrient application. These effects on colonization and sporulation were independent of changes in tissue-P concentration. High Mg/low Ca tissue concentrations induced premature root senescence, which may have disrupted the mycorrhizal association. Our results confirm the importance of Ca for the maintenance of a functioning mycorrhiza.