Among closely related citrus genotypes growing in high phosphorus
(P)
orchard soils, there is a tendency for less
mycorrhizal-dependent (M-dependent) species to have lower rates of root
colonization than more M-dependent
species. We hypothesized that the less M-dependent the citrus species the
more
limited is their carbohydrate
(CHO) allocation to the M fungus. In a glasshouse study at low and high
P supply,
lower total incidence of Glomus
intraradices FL 208, intensity of vesicles formation, and accumulation
of
16[ratio ]1W5cis fungal fatty acid as measures
of root colonization were related to lower mycorrhizal dependency (MD;
M plant
d. wt/non-mycorrhizal (NM)
plant d. wt expressed at low P supply) in five citrus genotypes. At high
P supply,
when host carbon (C) production
was not affected by P nutrition, less M-dependent genotypes had consistently
lower starch concentrations in their
root and shoot tissues than did more M-dependent genotypes, irrespective
of M
inoculation. At low P supply, M
plants were more heavily colonized by G. intraradices and had
lower
starch levels than the M plants supplied with
additional P. At high P, M plants of the more dependent genotypes allocated
more
C to starch pools relative to
the NM plant than the least dependent genotype. Concentration of sucrose
in
tissues did not vary consistently with
the dependency of citrus genotypes or with M inoculation except in high
P NM
plants, when less M-dependent
genotypes had lower levels of sucrose in their roots than the more dependent
species. At high P supply, sucrose
concentrations were lower in colonized roots than in NM roots. Across citrus
genotypes, sucrose in M fibrous roots
decreased relative to that in NM roots with increase in root colonization
suggesting that more sucrose was allocated
for growth and maintenance of G. intraradices in roots of M-dependent
species. The concentration of reducing
sugars in root tissues varied in relation to MD of citrus genotypes in
the same
way as that of starch and sucrose,
but was less responsive to M colonization. Responses of total non-structural
CHO in tissues indicated that more
heavily colonized plants at low P expended more C to acquire P than did
M plants
of similar biomass and P status
grown at high P supply. Total CHO pools increased with MD of citrus genotypes,
providing evidence than C
allocation patterns in the host affect M colonization.