CARDOSO, ELKE J.B.N, MARCO A. NOGUEIRA & RAYMOND S. PACOVSKY
ESALQ - Univ. São Paulo, Dept. Soil Science, Av. Pádua Dias, 11, C.P. 9, 13418 - 900 Piracicaba, SP, BRAZIL.
Citrus limonia L. Osbeck (cv.
Limão cravo) plants were grown in a sterilized P-fixing soil and were
either inoculated with the arbuscular mycorrhizal (AM) fungus Glomus intraradices
or were left non-inoculated. Plants were grown with either 25, 200 or 1000 mg/kg
P as supertriple phosphate and were harvested over five months. Plant dry weight
and nutrient contents were measured. Internal fungal colonization, along with
total external mycelium (TEM) and active external mycelium (AEM) were quantified
using microscopic methods. Internal colonization and mycelial lengths were converted
to biomass to determine fungal C content, and the growth respiration (based
on TEM) and maintenance respiration (based on AEM) were estimated to determine
fungal C consumption.
Mycorrhizal citrus plants grown at 25 or at 200 mg/kg P had significantly (P <0.05)
more dry weight than non-AM plants grown at the same P input following month
2. AM citrus grown at 1000 mg/kg P went through a transient growth depression
at the third and fourth month compared to non-inoculated plants. C loss in these
AM plants was between 200 and 400 mg C. Carbon in fungal biomass (18 or 30 mg),
attributable to fungal growth respiration (36 or 60 mg) or AM maintenance respiration
(3.3 or 5.7 mg/month) was estimated between 60 and 100 mg total C -- only 25
% of host C loss. Apparently C use by the AM fungus is not the sole reason but
other mechanisms, inaddition to the commonly acepted "C drain", seem to play
a role in transient or absolute growth depression in AM plants at high external
P.