Abstract
The sunflower crop provides an important honey flow for beekeepers. In France, beekeepers observed a decrease in honey yield from this crop these past years compared to the 1980s–1990s. They suspect the new cultivars to be less productive in nectar compared to the older ones, but no data is available to support this, and it is known that climate conditions have a strong impact on nectar secretion. This study aimed to explore the effect of abiotic environmental conditions on nectar secretion in sunflower, as well the range of variation of this secretion in a sample of current cultivars. Thirty-four current sunflower hybrid cultivars were sampled in test plots for their nectar secretion under varying conditions of temperature, air humidity and soil moisture. Air humidity controlled the sugar concentration of nectar, and thus its volume. To study nectar secretion independently from this effect, analyses subsequently focused on nectar sugar mass per floret. The nectar sugar mass increased with temperature up to an optimum of 32 °C, while the variation range of soil water tension was not sufficient to detect an effect on nectar sugar mass. This varied by up to 100% among the 34 cultivars (from 101 to 216 μg sugar per staminate floret in average), with a similar range to those reported in the literature for older cultivars. Likewise, oleic cultivars, a new type introduced since the early 2000s, were found to secrete the same amounts of nectar as linoleic cultivars, an older conventional type. The more self-fertile cultivars also showed no reduction in nectar secretion. Finally, we tested the method that measures the nectar gross secretion rate in one hybrid, and we observed that this hybrid secreted in average 28 μg sugar per hour per staminate floret. The potential benefits of this method were discussed.
Highlights
Studying floral nectar secretion of entomophilous crops can be of importance for crop production, as the more a crop secretes floral nectar, the more it is visited by pollinators (Prasifka et al, 2018)
This study aimed to explore the effect of abiotic environmental conditions on nectar secretion in sunflower, as well the range of variation of this secretion in a sample of current cultivars
Sugar concentration increased with vapour pressure deficit (VPD), from a predicted mean concentration of 23.2% when the air was saturated in humidity, a concentration probably close to that at which the nectar is secreted, to a mean maximum concentration of 55.6% at a VPD of 0.75 kPa (Tab. 1; Fig. 4a)
Summary
Studying floral nectar secretion of entomophilous crops can be of importance for crop production, as the more a crop secretes floral nectar, the more it is visited by pollinators (Prasifka et al, 2018). It is especially so in the case for sunflower (Helianthus annuus; Asteraceae) (Tepedino and Parker, 1982; Mallinger and Prasifka, 2017), for which an average of 0.2 bees per head is sufficient to maximise seed yield and oil content (Chabert et al, 2019, in prep). No official data is available to support these contentions
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