Mineral and lipid composition of <i>Glycine ‐ Glomus ‐ Bradyrhizobium</i> symbioses

Abstract

Soybean [ Glyeine max (L.) Merr. cv. Amsoy 71] plants were inoculated with either the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum . with a strain of Bradyrizobobium Japonicum . or with both endophytes in combintion. Noninoculated controlplantes were fertilized with levels of N and P previously found to compensate for nutrient input following infection by Bradyhizobium or Glomus Temporal differences in N and P assimilation in nodulated or mycorrhizal plants indicated that Glomus was most effective during early vegetative growth and Bradyrhizobion was active until the mid‐pod‐fill stage in soybean. In general. soybeans colonized by Glomus contained more Cu but less Mn and P than corresponding P‐fertilized plants. Soyubean roots infected with G. fasciculaum contained five unusual fatty acids: [16: 1 (11c): 8:3 (6c, 9c, 12c): 20:3 (8c, 11c, 14c): 20:4 (5c, 8c, 11c, 14c): 20:5 (5c, 8c, 11c, 14c, 17c)] that were absent in non‐infected roots. Fatty acid 16:1 (11c) comprised 43% of total fatty acids in Glomus‐infected roots at week 9 and 29% of total root fatty acids at week 15. This isomer of hexadecenoic acid was positively correlated with vesicle number (r = 0.92**). and 16:1 (11c) was probably the principalstorage fatty acid in fungal vesicles. These five unusual fatty acids were not found in the leaves. pods or seeds of either VAM or non‐VAM plants. Specific leaf area increased with time in nodulated soybeans. but these plants contained lessCu than corresponding N‐fertilized plants. Soybeans nodulated with Bradyrthizobium contained more total lipid and proportionately more fatty acid 16:0 than N‐fertilized plants. Infection by Glomus or Bradyrhizobium also altered the fatty acid composition of above‐ground plant parts, although these changes were subtle compared to the markedly different fatty acid found in Glomus ‐infected roots. These findings suggest that seed quality may be altered due to the physiological changes resulting from infection by N 2 ‐fixing bacteria and/or endomycorrhizal fungi. Observed differences in the plant nutrition of inoculated soybeans could not be replicated by fertilizer addition alone.