EFFECTS OF A VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS AND OTHER SOIL-MICROORGANISMS ON GROWTH, MINERAL NUTRIENT ACQUISITION AND ROOT EXUDATION OF SOIL-GROWN MAIZE PLANTS

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil i n pots divided by 30-mu m nylon nets into three compartments, the cent ral one for root growth and the outer ones for hyphal growth. Sterile soil was inoculated with either (1) rhizosphere microorganisms other t han vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere micr oorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd .) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and har vested after 3 or 6 weeks. VAM plants had higher shoot:root ratios tha n non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, w hereas Mn concentrations had significantly decreased. Root exudates we re collected on agar sheets placed on the interface between root and h yphal compartments. Six-week-old VAM and non-VAM plants had similar ro ot exudate compositions of 72-73% reducing sugars, 17-18% phenolics, 7 % organic acids and 3% amino acids. In another experiment in which roo t exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, three- to sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibioti cs were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered f rom roots.