Effect of environmental conditions and genotype on nectar secretion in sunflower (Helianthus annuusL.)

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.

New challenges for sunflower ideotyping in changing environments and more ecological cropping systems

“…Similar results were obtained by Stejskalová et al (2018). This could be potentially related to melliferous characteristics (Chabert et al, 2020) but also to other floral traits (Prasifka et al, 2018). Nicolson and Human (2013) indicated that the protein content of sunflower pollen is relatively low and the cultivars of such mass-flowering crops may also vary in value for pollinators.…”

“…According to Terzic et al (2017), temperatures above 27°C reduce nectar production, while those above 33°C completely stop it. Chabert et al (2020) observed that nectar sugar mass per floret increased with air temperature over the range of 16-32°C and appeared to decrease beyond 32°C on the overall range of cultivars sampled.…”

“…For seriously addressing these questions, a lot of additional phenotyping would be necessary: pollen and nectar composition and secretion, tubular floret length (Vear, 2016;Chabert et al, 2020). Pollinators make foraging decisions based on numerous floral traits, including nectar and pollen rewards, and associated visual and olfactory cues (Mallinger and Prasifka, 2017).…”

Nectar Dynamics and Pollinators Preference in Sunflower

“…Sunflower honey is also highly demanded for its aromatic and nutritional properties, which includes the abundance of nutraceutical and antioxidant substances as polyphenolic compounds [ 7 ]. Despite the large interest for this species in agriculture, beekeeping and food markets, studies suggest that in recent decades, the honey production from sunflower has decreased dramatically worldwide [ 8 , 9 , 10 ].…”

“…However, despite global warming and climate change, other causes generally attributed to the decline of sunflower honey production include the availability of the pest of the insect, the pesticide exposure, the beekeeping mismanagement, the long-distance transport, and the decreased genetic diversity [ 13 ]. In particular, the introduction of new self-fertile hybrids respect to old population seems another verified option [ 8 , 9 ]. In a study performed in India, six hybrids and two old sunflower cultivars were tested with respect to nectar secretion and honey production capacity, utilizing a quantification method based on dry nectar sugar (DNS).…”