# Bee on Boron—Sufficient Boron Supply of Brassica napus Is Crucial for Attracting Pollinating Insects to Ensure Seed Yield

## TL;DR

This study shows that enough boron in rapeseed plants is essential for healthy flowers and attracting pollinators, which helps ensure good seed yield.

## Contribution

The study quantifies how boron deficiency affects both plant fertility and pollinator attraction in rapeseed.

## Key findings

- Boron-deficient rapeseed plants had more deformed flowers and fewer pollinator visits.
- Insect pollination increased seed yield in boron-inefficient cultivars but not in boron-efficient ones.
- Sufficient boron supply is crucial for both flower development and pollinator visitation in rapeseed.

## Abstract

A sufficient supply of the micronutrient boron (B) is crucial for plant development and fertility. Boron limitations in crops, such as rapeseed (
Brassica napus
), cause deformed or infertile flowers and may consequently affect yield both directly and indirectly via reduced flower fertility or insect pollination, respectively. This latter point is particularly relevant for rapeseed, as its yield increases via insect pollination, which relies on healthy‐looking flowers to attract pollinators. This study aimed to quantify the direct and indirect effects induced by B limitation on yield. Thus, we determined the extent to which B‐deficiency in rapeseed affects plant–pollinator interactions by cultivating B‐inefficient (Daktari and CR3153) and B‐efficient (CR2267) cultivars at B‐sufficient and B‐deficient substrate levels in pot trials and exposing them to a natural pollinator community in a cafeteria experiment. All cultivars displayed phenotypic B‐deficiency symptoms at B‐deficient levels with 9% of the total flowers appearing deformed in B‐efficient CR2267 compared to 34% and 18% in Daktari and CR3153, respectively. Both B‐inefficient cultivars were visited by a higher number of pollinating insects at B‐sufficient levels (Daktari by 59%, p < 0.001; and CR3153 by 37%, p = 0.002) than under B‐deficiency. Compared to B‐inefficient cultivars, pollinator visits to CR2267 were less abundant and not affected by B‐deficiency (p = 0.252). Across cultivars, pollinator visitation decreased with an increase in the number of deformed flowers (p < 0.001). Insect pollination increased seed yield compared exclusively to self‐pollination only for the B‐inefficient rapeseed cultivars by around 20% on either B‐deficient or B‐sufficient supply levels (p < 0.05), but not for CR2267, suggesting a cultivar‐dependent effect of insect pollination on yield. Sufficient B supply is crucial for successful rapeseed flower growth and fertility as well as for attracting pollinating insects, thus securing yield in a pollinator‐dependent crop.

The study demonstrates that sufficient B supply is crucial for successful rapeseed flower growth and fertility as well as for attracting pollinating insects, thus securing yield in a pollinator‐dependent crop. It is illustrated that climate change can pose a threat to crop yield via both direct effects, that is, an impaired nutrient uptake, and indirect effects, that is, reduced pollinator visitation.