Oilseed rape (Brassica napus) as a resource for farmland insect pollinators: quantifying floral traits in conventional varieties and breeding systems

Abstract

Oilseed rape (OSR; Brassica napus L.) is a major crop in temperate regions and provides an important source of nutrition to many of the yield-enhancing insect flower visitors that consume floral nectar. The manipulation of mechanisms that control various crop plant traits for the benefit of pollinators has been suggested in the bid to increase food security, but little is known about inherent floral trait expression in contemporary OSR varieties or the breeding systems used in OSR breeding programmes. We studied a range of floral traits in glasshouse-grown, certified conventional varieties of winter OSR to test for variation among and within breeding systems. We measured 24-h nectar secretion rate, amount, concentration and ratio of nectar sugars per flower, and sizes and number of flowers produced per plant from 24 varieties of OSR representing open-pollinated (OP), genic male sterility (GMS) hybrid and cytoplasmic male sterility (CMS) hybrid breeding systems. Sugar concentration was consistent among and within the breeding systems; however, GMS hybrids produced more nectar and more sugar per flower than CMS hybrid or OP varieties. With the exception of ratio of fructose/glucose in OP varieties, we found that nectar traits were consistent within all the breeding systems. When scaled, GMS hybrids produced 1.73 times more nectar resource per plant than OP varieties. Nectar production and amount of nectar sugar in OSR plants were independent of number and size of flowers. Our data show that floral traits of glasshouse-grown OSR differed among breeding systems, suggesting that manipulation and enhancement of nectar rewards for insect flower visitors, including pollinators, could be included in future OSR breeding programmes.

Keywords: canola; cytoplasmic male sterility; floral traits; genic male sterility; hybrids; nectar; open‐pollinated; pollinator declines; rapeseed; sustainable intensification.

Figure 1

Mean metrics of oilseed rape floral traits, by breeding system (average n = 54). GMS, genic male sterility hybrids; CMS, cytoplasmic male sterility hybrids; OP, open‐pollinated. Panels a–d are metrics per flower, while panels e and f are per plant. Error bars show 95% confidence intervals; data in all panels, except c and d, are back‐transformed to the natural scale.

Figure 2

Mean metrics of oilseed rape floral traits by variety (n = 7). Panels a–d are metrics per flower, while panels e and f are per plant. Closed circles: genic male sterility (GMS) hybrids; open circles: cytoplasmic male sterility (CMS) hybrids; closed triangles: open‐pollinated (OP). Error bars show 95% confidence intervals; data in all panels, except c and d, are back‐transformed to the natural scale.