Root traits of cup plant, maize and lucerne grass grown under different soil and soil moisture conditions

The cup plant (Silphium perfoliatum L.) is presently discussed as a promising alternative to silage maize for biomethane production in Germany. It is assumed that the cup plant develops a profound root system, contributing decisively to the drought tolerance of this crop. This study is aimed at providing the first experimental data on root growth and water uptake of this novel biogas crop. Root morphological characteristics of the cup plant were studied at six sites differing in soil type. Root samplings were made at the time of maximum root expansion (flowering). In a 2- year field experiment at an additional location, continuous measurements of root development and soil water acquisition during the growth cycle were taken under contrasting water supply, together with maize (Zea mays L.) and lucerne grass (mixture of Medicago sativa L. with Festuca pratensis Huds. and Phleum pratense L.) as reference crops. The cup plant attained maximum rooting depths of 80–240 cm. The root length density was comparable to that of maize, but markedly lower than that of lucerne grass. Despite the cup plant’s higher potential evapotranspiration and similar water-use efficiency, its soil water extraction ability was significantly lower than that of lucerne grass. Compared with maize and lucerne grass, the cup plant showed no outstanding ability to cope with drought stress by means of its root system. Because of its high potential evapotranspiration, the cup plant can attain biomass yields comparable to those of maize only at sites with high water supply.