The pots were then watered with deionized water and one Romaine seed was placed ca.
3.2 mm under the SSVT. Each tray was placed in a growth chamber
with light:dark cycles of 16:8 h at 25:18°C to allow germination and
seedling growth up to six weeks to allow for colonization of roots by
G. intraradices (see Fig. 1). The seedlings were fertilized weekly with
10 ml of Hoagland's nutrient solution without phosphorus (Hoagland
and Arnon, 1938) until used for the study. Just prior to initiating experiments
with the pathogens, a subsample (n= 3) of seedlings was harvested
for measurement of shoot dry weight (80 °C) and phosphorus
content. Roots were retained for staining to determine extent of AM
fungus colonization. Shoot tissue was ground to pass a 20 mesh sieve,
digested in H2SO4/H2O2, and analyzed for P via the method of Murphy
and Riley (1962).