The cup plant (Silphium perfoliatum L.) is discussed as an alternative energy crop for biogas production in Germany due to its ecological benefits over continuously grown maize. Moreover, a certain drought tolerance is assumed because of its intensive root growth and the dew water collection by the leaf cups, formed by fused leaf pairs. Therefore, the aim of this study was to estimate evapotranspiration (ET), water-use efficiency (WUE) and the relevance of the leaf cups for the cup plant's water balance in a 2-year field experiment. Parallel investigations were conducted for the two reference crops maize (high WUE) and lucerne-grass (deep and intensive rooting) under rainfed and irrigated conditions. Root system performance was assessed by measuring water depletion at various soil depths. Transpiration-use efficiency (TUE) was estimated using a model approach. Averaged over the 2 years, drought-related above-ground dry matter reduction was higher for the cup plant (33 %) than for the maize (18 %) and lucerne-grass (14 %). The WUE of the cup plant (33 kg ha À1 mm À1 ) was significantly lower than for maize (50 kg ha À1 mm À1 ). The cup plant had a lower water uptake capacity than lucerne-grass. Cup plant dry matter yields as high as those of maize will only be attainable at sites that are well supplied with water, be it through a large soil water reserve, groundwater connection, high rainfall or supplemental irrigation.

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.

Following winter oilseed rape cultivation, considerable numbers of volunteer oilseed rape plants may occur in subsequent years in following crops. The appearance of volunteer oilseed rape plants is based on the capability of the seeds to become secondary dormant and to survive in this stage for many years in the soil. Genetic reduction of secondary seed dormancy in oilseed rape could provide a means to reduce the frequency of volunteer plants and especially the dispersal of transgenic oilseed rape. The objective of the present study was to analyse the inheritance of primary and secondary seed dormancy in a winter oilseed rape doubled haploid population derived from the cross Express 617 × R53 and to study correlations to other seed traits. Field experiments were performed in Germany for 2 years at two locations with two replicates. Seeds harvested from open pollinated plants were used for all analyses, including a laboratory test for seed dormancy. A previously developed molecular marker map of the doubled haploid population was used to map QTL of the relevant traits. For primary, secondary and total seed dormancy, the results showed significant effects of the genotypes and their interactions, with years and locations. Two, four and five QTL were detected for primary, secondary and total seed dormancy which explained 19, 35 and 42 % of the phenotypic variance, respectively. Results show that secondary seed dormancy is a heritable trait and that selection for low secondary seed dormancy is possible.

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