Effects of arbuscular mycorrhizal fungi on constitutive and induced plant resistances to spider mites

Plant constitutive and induced resistances have effects of altering interactions between plant and herbivorous arthropods. Although the functions of these resistances have been focused in aboveground studies, we know little about the roles of soil microbes to support the plant resistances. Arbuscular mycorrhizal fungi (AMF) can improve nutrient absorption of plants and thus alter plant susceptibility to herbivores. We conducted a factorial experiment to test the hypothesis that AMF improves both of the constitutive and induced resistance of Lotus corniculatus to the spider mite (Tetranchus urticae). We set four treatments of plants with AMF (Gigaspora margarita) or AMF-free filtrate and with high-P or low-P fertilization. Seven weeks after sowing, the plants were exposed to ten female of adult mites for 3 days. To evaluate effects of AMF on the plant resistance to the spider mites, we took leaves one day before mite introduction, 1, 3, 5, 8 days following mite introduction,. We provided each leaf to one female mite for 3 days and counted number of eggs laid. AMF increased plant growth and phosphorus contents in low-P fertilization, but did not affect in high-P fertilization. In the absence of herbivory, AMF increased the number of eggs in low-P fertilization but did not affect in high-P fertilization. However, AMF induced earlier reduction of the number of eggs after herbivory on both nutrient conditions. Therefore, AMF enhanced the induced resistance to the spider mites, although the constitutive resistance was not affected.