Seed set of male-sterile and male-fertile oilseed rape (Brassica napus) in relation to pollinator density

Apidologie, Jul 2018

Ingolf Steffan-dewenter

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Seed set of male-sterile and male-fertile oilseed rape (Brassica napus) in relation to pollinator density

Apidologie Seed set of male-sterile and male-fertile oilseed rape (Brassica napus) in relation to pollinator density Ingol 0 STEFFAN-DEWENTER 0 0 Agroecology, University of Göttingen , Waldweg 26, 37073 Göttingen , Germany - The effects of pollinator density on the seed set of a male-sterile (MS) and a male-fertile (MF) line of winter oilseed rape (Brassica napus) were studied in 24 isolation cages (7.5 m2). Pollinator treatments comprised of high pollinator density (a small honeybee colony and 10 mason bees, Osmia rufa), and a gradient of increasing O. rufa densities from zero (control) up to 100 bees per cage. High pollinator densities increased the seed weight per plant from 6.5 g to 56.9 g for the MS line. However seed weight did not differ between high pollinator densities and controls for the MF line. Increasing densities of O. rufa had a significant effect on almost all yield components of the MS line, but only marginal effects on the MF line. The number of seeds per pod, the seed weight per plant and the harvest index (seed weight/plant dry weight) of the MS line showed a significantly steeper increase with increasing bee density than that of the MF line. The results suggest that solitary bees could be used successfully to replace honeybees as pollinators of MS oilseed rape in isolation cages. 1. INTRODUCTION Winter oilseed rape (Brassica napus L. ssp. oleifera (Metzg.), Brassicaceae) is an important crop for the production of oilseed in temperate agricultural regions. In Germany in 1999 approximately 1.1 million hectares (12% of the total agricultural area) were sown with oilseed rape (Statistisches Bundesamt, 2002) . Oilseed rape is a predominantly self-pollinated crop with about one-third outcrossing (Becker et al., 1992) . Pollen transport by insects, wind, or gravity as vectors is necessary (Williams, 1978; Eisikowitch, 1981; Free, 1993; Westcott and Nelson, 2001) . Earlier studies have shown that the insect pollination of oilseed rape can lead to higher seed set and yield (Williams and Simpkins, 1989; Westcott and Nelson, 2001) . However, these effects are dependent on cultivar, environmental growing conditions, and the compensatory capacity of the crop (Williams et al., 1987; Mesquida et al., 1988; Free, 1993; Westcott and Nelson, 2001) . Further replicated experiments comparing yields with and without bees are still needed (Williams et al., 1987; Mesquida et al., 1988; Kevan and Phillips, 2001) . The significant heterosis for seed yield in oilseed rape has created interest in the development of hybrid cultivars (Riaz et al., 2001) . Oilseed rape hybrids are based on male sterility mainly using the MSL system (Pinochet and Bertrand, 2000) . Several oilseed rape hybrids have been recommended to farmers since 1994 (Pinochet and Bertrand, 2000) . Hybrid composites consisting of a male-sterile component and a male-fertile component have been widely used in France and the United Kingdom. However, at several locations in Europe low seed set occurred, presumably due to pollen limitation (Pinochet and Bertrand, 2000) . This could have been caused by low temperatures and rain reducing pollen transfer by wind and flight activity of insects. Additionally, the destruction of semi-natural habitats results in lower pollinator abundance in intensively managed agricultural landscapes thereby possibly causing pollinator limitation and reduced seed set (Steffan-Dewenter and Tscharntke, 1999; Steffan-Dewenter et al., 2001, 2002) . More recently, restored hybrid cultivars producing normal amounts of pollen are grown, which successfully replace composite hybrids (Pinochet and Bertrand, 2000) . In 2001, restored hybrids covered an area of about 670 000 ha in Europe and 345 000 ha in Germany (NPZ Hohenlieth, personal communication). However, for the breeding of restored hybrid cultivars and seed production for commercial growing, the combination of male-sterile (MS) and male-fertile (MF) lines is still necessary. In closed environments in particular, pollinators are required in order to achieve sufficient seed set. In addition to the honeybee, Apis mellifera L., which is often advocated as the only practical pollinator, the use of solitary bees and flies has been proposed (Torchio, 1990; Richards, 1993; Delaplane and Mayer, 2000) . Yields of MS oilseed rape in isolation cages were increased by honeybee pollination (Mesquida and Renard, 1981) and varied with leafcutting bee stocking rates (Soroka et al., 2001) . Pollination of other plant species in closed environments was improved successfully by using the solitary mason bee Osmia rufa Losinski and the syrphid fly Eristalis tenax (L.) (Jarlan et al., 1997; Schittenhelm et al., 1997) . Because most studies compare control versus pollinator treatments, pollinator densities required for optimal seed set are as yet still unknown. In this research, yield components of malesterile (MS) and conventional male-fertile (MF) oilseed rap (...truncated)


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Ingolf Steffan-dewenter. Seed set of male-sterile and male-fertile oilseed rape (Brassica napus) in relation to pollinator density, Apidologie, pp. 227-235, Volume 34, Issue 3, DOI: doi:10.1051/apido:2003015