Plant enemy-derived elicitors increase the foliar tannin concentration of Onobrychis viciifolia without a trade-off to growth.

BACKGROUND AND AIMS Molecular experiments suggest that the regulation of the biosynthesis of condensed tannin (CT) is sensitive to the presence of plant enemies. The enemy-specific response of CT concentrations to simulated attacks by pathogenic fungi, bacteria or herbivores was studied in Onobrychis viciifolia grown at four levels of nutrient availability. It was hypothesized that CT concentrations increase in response to an attack, and that constitutive and induced levels of CT are higher at low than at high nutrient availability. Investment in CT was also predicted to be negatively related to plant growth. METHODS Recently discovered substances by which plants recognize their opponents (i.e. elicitors) were used to simulate attacks to Onobrychis viciifolia grown at 0.0027, 0.075, 0.67 or 2 mm phosphorus in the nutrient solution. KEY RESULTS Relative growth rate and final biomass (P < 0.001) were highest at 0.67 mm of phosphorus. CT concentrations decreased with increasing phosphorus availability, from 94.9 to 69.0 mg g(-1) leaf dry weight (P < 0.001). Compared with unscathed plants, sterile mere mechanical wounding reduced tannin concentrations from 83.8 to 69.3 mg g(-1) leaf dry weight (P < 0.01). Local CT concentrations were higher when wounded leaves were additionally treated with fungal (+15.9 %), bacterial (+19.6 %) or insect (+31.0 %) elicitors (each elicitor; P < 0.05); however, only the insect elicitor (saliva of the lepidopteron Spodoptera littoralis) induced CT concentrations higher than those of unscathed leaves. CONCLUSIONS CT concentrations were inducible in the vicinity of the wound but the level of induction was unrelated to the nutrient status of the plant. There was no evidence of a growth-defence trade-off. The inverse relationship between CT concentrations and nutrient availability appears to reflect passive growth dilution at high nutrient availability, rather than surplus CT production at low nutrient availability.