WATER RELATIONS OF XERIC GRASSES IN THE FIELD: INTERACTIONS OF MYCORRHIZAS AND COMPETITION.

The effects of vesicular-arbuscular mycorrhizal inoculum and competition from non-mycorrhizal annual plants were tested on Agropyron smithii Rydb. and Agropyron dasystachyum (Hook.) Scribn. growing in an inoculum-poor soil. Mycorrhizal A. smithii decreased stomatal resistance (r ) and increased leaf water potential compared to the non-inoculated plants, but only during the driest portion of the growing season. The presence of annuals caused increased r of A. smithii, but only of non-inoculated plants. Mycorrhizal plants had the same r with or without annuals, indicating that mycorrhizas may alleviate the detrimental effects of competition on r of A. smithii. By contrast, neither inoculation nor annuals significantly affected water relations of A. dasystachyum. Mycorrhizas also did not increase N and P concentrations or percentage cover of either grass, but both had delayed phenology when mycorrhizal. This research was done during a wet year when the effects of mycorrhizas may be subtle. Long-term observations are necessary to determine the importance of mycorrhizas under field conditions.