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)