The objective of the proposed research is to determine the mechanisms by which mycorrhizal fungi influence the major components of plant fitness. Mycorrhizal fungi are present in nearly all terrestrial ecosystems and have been shown to influence nutrient uptake, growth and reproductive host plants under controlled conditions. Unfortunately, very little is known about the effects of mycorrhizal fungi on the population biology of their host plants, particularly under field conditions. Experiments in the laboratory have shown that mycorrhizal infection can increase both fecundity and offspring quality. For example, it was shown that infection of the maternal generation increased reproductive output in the offspring generation. Therefore, the effects of infection on maternal fecundity and offspring quality will be assessed separately in the proposed study. A combination of field and greenhouse experiments incorporating demographic and physiological measurements is proposed to establish the mechanistic basis for observed phenomena. In one study, maternal generation host plants (Abutilon theophrasti) will be grown in the field in a 3 x 2 factorial experiment (3 levels of P amendment; mycorrhizal, non-mycorrhizal). Growth, nutrient uptake, reproductive output, leaf photosynthesis and root phosphatase activities will be monitored for two seasons. In the second year of this study, a third factor (natural reestablishment vs. thinning to a standard density) will be incorporated in order to determine whether, in addition to influencing fecundity, maternal treatments have independent effects on offspring performance within the maternal environment. In another 2 year study, seeds collected from mother plants (from the various maternal treatments) will be transplanted to another location in the field to determine whether interactions between offspring environment (M/NM x high P/low P) and maternal treatment (see above) exists for offspring vigor. Seeds collected from mother plants will also be used in controlled greenhouse studies in which the effects of maternal treatment on offspring vigor mediated by seed weight will be distinguished from those mediated by seed P content. In addition, greenhouse experiments in which offspring from contrasting maternal treatments will be allowed to interact will allow a determination as to whether maternal treatment can influence competitive behavior. The results from these experiments should greatly increase the understanding of the effect of mycorrhizal fungi on plant fitness.
