Variation in floret size explains differences in wild bee visitation to cultivated sunflowers

Abstract

Wild and managed bees are needed to move sunflower (Helianthus annuus L.) pollen, both to create hybrid seed and to encourage high, consistent yields when those hybrids are subsequently grown. Among floral traits that influence bee preference, floret size may be critical, as the depth of the corolla affects the accessibility of nectar. Sampling and observation of inbred maintainer (HA) lines were used to assess variation in floret size, and to measure any effects of floret size on pollinator visitation. Among 100 inbreds sampled, there was significant variation among the lines, with floret lengths of 6.8–9.9 mm. Floret length, measured before anthesis, was closely related to corolla depth during anthesis and was consistent between 2 years (environments). Pollinator observations on 30 inbred lines showed floret size explained a majority (52%) of the variation in wild bee preference, with a reduction in floret length of 2 mm more than doubling pollinator activity. Though honey bee, Apis mellifera L., colonies were located ≈ 60 m from the plots, near-zero honey bee activity in the sunflowers precluded an assessment of how strongly this managed pollinator is affected by floret length. Production of inbreds and hybrids with smaller florets could enhance sunflower pollination, but genetic markers for floret size are needed to facilitate selection, and an understanding of potential trade-offs also is required. Information on variation and heritability of other traits, such as pollen and nectar rewards, could help explain residual variation in wild bee visitation to sunflowers.

Flowers as viral hot spots: Honey bees (Apis mellifera) unevenly deposit viruses across plant species

“…A plants’ propensity to harbor viruses could also be related to pollinator visitation rates with highly attractive plants more likely to act as fomites. Floral traits such as floral size, corolla depth [31], color [32], as well as nectar production rate and composition [33,34] may all influence pollinator visitation rates. Flowering plant diversity may also be an important factor as resource availability may impact floral preference of foraging bees [35,36].…”

Using Nectar-Related Traits to Enhance Crop-Pollinator Interactions

“…Additionally, inbred lines with shorter corollas (=easier access to nectar) were found to receive more pollinator visits. Subsequent work in 2016–2017 ( Portlas et al, 2018 ) focused on the effect of floret size because deeper corollas prevent nectar sampling by short-tongued bees, and because phenotyping floret size should be more rapid and precise than assessing nectar volume. Evaluation of 100 female lines showed total floret length ranged from 6.8 to 9.9 mm.…”

“…When a subset of these lines was grown again and bee visits counted daily, most of the variation in wild bee counts was explained by floret size. Data from Portlas et al (2018) suggest that for lines with the longest florets, a reduction in floret size of only 1.0 mm should double bee visitation; further reductions in floret size beyond 1.0 mm provide even greater benefits, likely because proboscis (“tongue”) lengths vary both within ( Waddington and Herbst, 1987 ) and between bee species ( Cariveau et al, 2016 ).…”

“…To validate the gene-trait association in sunflowers, inbred lines with unknown nectar types but SNP haplotypes matching high or low sucrose lines ( n = 10, each group) were grown and nectar sugars determined by high-performance anion exchange chromatography, which clearly supported the role of CWINV2 in determining sugar composition in sunflower nectar ( Figure 1B ). While sucrose may influence bee foraging in sunflowers ( Waller, 1972 ; Pham-Delègue et al, 1994 ; Mommaerts et al, 2013 ), finding markers for another nectar-related trait, floret size, is a priority because small changes in floret size have dramatic effects on nectar access (and sunflower visitation) by wild bees ( Figure 1C ; from data of Portlas et al, 2018 ). Previous identification of genes that govern flower size in other plants ( Krizek and Anderson, 2013 ) suggests that this is achievable, and analysis of a broader panel of sunflower lines has identified several quantitative trait loci (QTL) for this trait (Hulke, unpublished data).…”

“… Relationships between nectar-related traits in sunflower and genetic markers or pollinator behavior. Panels indicate (A) expression of cell wall invertase ( CWINV2 ) from control tissue (leaf) and nectaries of Helianthus annuus maintainer (HA) lines previously phenotyped for sucrose content, (B) sucrose content (% by mass of sucrose + fructose + glucose) of nectars associated with two SNP haplotypes at CWINV2 ( n = 10 inbred lines per group), and (C) illustration of the range of floret sizes in cultivated sunflowers and the effect of decreasing floret size (from start to end of arrows) on visitation by wild bees as observed by Portlas et al (2018) . Significant differences between pairs in (A,B) indicated by differing lowercase letters.…”