ON THE RELATIONSHIP BETWEEN A PLANTS MYCORRHIZAL DEPENDENCY AND RATE OF VESICULAR-ARBUSCULAR MYCORRHIZAL COLONIZATION

This study tests the hypothesis that plant species of low mycorrhizal dependency (MD) tend to limit vesicular-arbuscular mycorrhizal (VAM) colonization more than species of high MD. Mycorrhizal dependency (MD = VAM plant dry weight/non-mycorrhizal plant dry weight) was determined for six citrus rootstocks in three greenhouse trials in a sterilized, P-deficient soil. In order to isolate the process of VAM colonization from shoot growth responses, colonization of the rootstocks with the same scion was examined in high-P soil in a rootstock field trial. Colonization of roots of known age was evaluated in soil cores extracted from beneath the canopy of mature trees. Initially, the soil was replaced in the resultant holes after dry sieving to remove roots and mix VAM fungal propagules (disturbed soil). Extraradical (ER) colonization of young roots in disturbed soil increased rapidly up to 5 weeks and approached that of mixed-age roots in undisturbed soil by 19 weeks. Intraradical (IR) colonization of young roots increased up to 19 weeks and was less than that of mixed-age roots. Differences in IR colonization of rootstock species that developed with time were correlated with their ER colonization at 10 and 19 weeks. Variation among citrus rootstocks in colonization of new roots was not related to root diameter or root extension rate. Colonization of rootstocks at 19 weeks was positively correlated (r = 0.88-0.99, P < 0.05) with rootstock MD in P-deficient soil. Therefore, our data support the hypothesis that species that have evolved root systems that are less dependent on mycorrhizas have also evolved presently unknown mechanisms to regulate mycorrhizal colonization and presumably, carbon expenditure on the fungus.