Medicarpin Delays Alfalfa Seed Germination and Seedling Growth

The objective of this study was to identify a compound(s) that may be responsible, at least in part, for alfalfa autotoxicity. We found that medicarpin (3‐hydroxy‐9‐methoxypterocarpan) is a compound produced by alfalfa ( Medicago sativa L.) that contributes to this. clay loam soil from a declining alfalfa stand with two plants per 930 cm 2 and its 95% ethanol extract inhibited alfalfa emergence and seedlingrowth whereas extracted soil did not. The soil extracts contained at least 10 mg kg −1 medicarpin. Medicarpin, 4‐methoxymedicarpin, sativan, and 5′‐methoxysativan were isolated from alfalfa foliage with 95% ethanol, purified, identified by gas chromatography‐ mass spectroscopy, and then exogenously applied to alfalfa and velvetleaf ( Abutilon theophrasti L.) seeds using an agar bioassay. Medicarpin at 1 ✕ 10 −7 tool/seed reduced both alfalfa and velvetleaf seedling length by 39% after 72 h. Sativan and both methoxy derivatives had no affect on germination or seedingrowth. In a timecourse study, 2 ✕ 10 −7 mole/seed medicarpin delayed germination 12 h and seedling growth 44 h, during this time the alfalfa seedlings metabolized 85% of the medicarpin. Seedling growth then resumed at the control rate. Medicarpin is produced by mature alfalfa, is present in soil from a declining alfalfa stand, and is absorbed by alfalfa seedlings where it is metabolized and exerts a transient phytotoxic effect. Medicarpin may permit established alfalfa plants to control the ecology in their immediate proximity by inhibiting seedling establishment of nearby plants, thereby gaining a competitive advantage.