Temperature and Soil Moisture Drive Coumestrol Concentration in Annual Medicago spp. in the Presence but Not Absence of Phoma Black Stem and Leaf Spot (Phoma medicaginis)

Abstract

Studies were undertaken to determine the impact of environmental variables temperature (12.5/9.5, 20/17, 27/24 °C day/night) and soil moisture (100, 50% WHC), and their interaction with Phoma medicaginis infection, on production of the phytoestrogen coumestrol in annual Medicago rugosa cv. Paraponto and M. scutellata cv. Sava. Disease factors measured included leaf disease incidence/severity, petiole/stem disease incidence/severity, and leaf yellowing severity. Coumestrol levels were determined using gas chromatography–mass spectrometry (GC–MS). Increasing temperature from 12.5/9.5 °C to 27/24 °C in inoculated plants significantly (p < 0.05) increased coumestrol from 193 mg kg−1 to 390 mg kg−1, but there were no differences in coumestrol production across all three temperatures in uninoculated plants. Reducing soil moisture from 100% to 50% WHC at the highest temperature (27/24 °C) caused the greatest increase in coumestrol production from 156 to 269 mg kg−1 in inoculated plants. The greatest coumestrol production (600 mg kg−1) was under 27/24 °C/50% WHC for Sava infected with P. medicaginis and least coumestrol (1.6 mg kg−1) was Sava under 20/17 °C/50% WHC in the absence of P. medicaginis. Clearly, situations of higher temperatures in conjunction with lower soil moisture levels cause greatest elevation in coumestrol in the presence of P. medicaginis, levels far exceeding the animal risk threshold of 25 mg kg−1.