Effects of elevated CO2 on the phytoalexin production of two soybean cultivars differing in the resistance to stem canker disease

The excess of carbon leads to changes in the production of plant defensive secondary chemicals, which in turn may modify the plant responses to herbivores and pathogens. Age and/or developmental stage as well as environmental conditions influence race-specific accumulation of phytoalexins in soybean. Thus, we hypothesized that elevated CO2 atmospheres could lead to distinct phytoalexin production in soybean cultivars differing in their resistance to fungal attack. Two cultivars of soybean (Glycine max, IAC-14 and IAC-18) differing in the resistance to the causal agent of the stem canker disease were subjected to 320 and 720 μmol mol−1 of CO2 in open top chambers. Plants were kept under both CO2 atmospheres from germination up to 9 days (VC stage), when CO2 assimilation rates, chlorophyll a fluorescence, leaf and cotyledon area, dry mass and root: shoot ratio were measured. Phytoalexin content were evaluated in detached cotyledon diffusates of both cultivars elicited with β-glucan elicitor from mycelial walls of Phythophytora sojae. Exposure to elevated CO2 stimulated photosynthesis, leaf and cotyledon areas and the enhancement of glyceollin production by the elicitor in the resistant cultivar (IAC-18). No changes were observed in growth parameters (biomass and area of photosynthetic tissues) and phytoalexin response in susceptible cultivar (IAC-14), despite the stimulation of photosynthetic assimilation in cotyledons under high CO2. These findings corroborate the assumption that exposure to enriched CO2 atmospheres can change inducible defensive responses in soybean against pathogens. These changes occurred in individual metabolites and were dependent on the cultivar resistance patterns.