Jp. Syvertsen et Jh. Graham, Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2], PLANT SOIL, 208(2), 1999, pp. 209-219
We hypothesized that greater photosynthate supply at elevated [CO2] could c
ompensate for increased below-ground C demands of arbuscular mycorrhizas. T
herefore, we investigated plant growth, mineral nutrition, starch, and net
gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and m
ycorrhizas at elevated atmospheric [CO2]. Half of the seedlings of sour ora
nge (C. aurantium L.) and `Ridge Pineapple' sweet orange (C. sinensis L. Os
beck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus i
ntraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants we
re grown at ambient or 2X ambient [CO2] in unshaded greenhouses for 11 week
s and fertilized daily with nutrient solution either without added P or wit
h 2 mM P in a low-P soil. High P supply reduced AM colonization whereas ele
vated [CO2] counteracted the depressive effect of P on intraradical coloniz
ation and vesicle development. Seedlings grown at either elevated [CO2], hi
gh P or with G. intraradices had greater growth, net assimilation of CO2 (A
(CO2)) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch
and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decrease
d by elevated [CO2], P, and AM colonization. Mycorrhizal seedlings had high
er leaf-P status but lower leaf N and K concentrations than nonmycorrhizal
seedlings which was due to growth dilution effects. Starch in fibrous roots
was increased by elevated [CO2] but reduced by G. intraradices, especially
at low-P supply. In fibrous roots, elevated [CO2] had no effect on C/N, bu
t AM colonization decreased C/N in both Citrus spp. grown at low-P supply.
Overall, there were no species differences in growth or A(CO2). Mycorrhizas
did not increase plant growth at ambient [CO2]. At elevated [CO2], however
, mycorrhizas stimulated growth at both P levels in sour orange, the more m
ycorrhiza-dependent species, but only at low-P in sweet orange, the less de
pendent species. At low-P and elevated [CO2], colonization by the AM fungus
increased A(CO2) in both species but more so in sour orange than in sweet
orange. Leaf P and root N concentrations were increased more and root starc
h level was decreased less by AM in sour orange than in sweet orange. Thus,
the additional [CO2] availability to mycorrhizal plants increased CO2 assi
milation, growth and nutrient uptake over that of NM plants especially in s
our orange under P limitation.