Intraspecific Competition Mediated by Mycorrhizal Fungi Alters Photosynthetic Response to Herbivory in Soybean

Abstract

Application of mycorrhizal fungi in agriculture is becoming increasingly common; the fungi can aid plant nutrient uptake and reduce the negative effects of many abiotic soil stressors. However, little is known about how networks formed by mycorrhizal colonization affect the defense response and resource allocation of plants simultaneously stressed by competition and herbivory. Here, we study the immediate photosynthetic response of herbivore-stressed plants connected to both stressed and unstressed plants via mycorrhizal network. Specifically, we use A/Ci curves to measure maximum photosynthetic capacity (Amax), maximum Rubisco carboxylation rate (Vcmax), maximum rate of RuBP regeneration (Jmax), and maximum rate of triose phosphate utilization (TPU). Soybean, Glycine max (L.), grown in pairs with mycorrhizal fungi were either separated by an impermeable barrier or by a mesh that only allowed mycorrhizal connection (no root contact). Initially, Spodoptera exigua larvae were deposited on one plant in the pair, while the other plants received larvae two days later. Plants which initially received larvae had significantly greater Amax after four days compared to initially unstressed plants, indicative of a compensatory response in stressed plants. In addition, of the plants which initially received larvae, those in pairs separated by mesh had significantly reduced Amax, Jmax and TPU compared to those in pairs separated by a barrier. These findings indicate that intraspecific competition mediated by mycorrhizal networks drastically affects the immediate, plastic responses of soybean to herbivory and, more specifically, negatively affects its compensatory photosynthetic response.