Sunflower (Helianthus annuus L.) Floral Nectar Characterization and Gene Expression Analysis of Sucrose Hydrolyzing Gene HaCWINV2

Abstract

Pollination enhances the seed setting of cross-pollinated crops and maintains the floral diversity in the ecosystem. Honeybees are benefited as they pollinate and obtain nectar as their main source of food. However, the honeybee population is declining due to excessive use of pesticides, climate change, and lack of nectar source plants, which could lead to a decline in honey production in the beekeeping industry. Sunflower has the potential to become a source of nectar and the honey derived is one of the most in-demand due to its nutraceuticals and flavor. This study was conducted to evaluate floral parts and nectar of sunflower germplasm collected by the Central Luzon State University (CLSU) and identify accessions with potential as a nectar source for bees. The focus was on nectar volume, sugar concentration and composition, and relative gene expression levels of the cell wall invertase 2 gene (HaCWINV2) responsible for hydrolyzing sucrose to glucose and fructose, as pollinators have different sugar preferences. Among the 23 accessions, six were single heads, and 17 were multiflorous, which contributed to the difference in the amount of nectar. CL-SF18 accession had the highest average nectar volume. In terms of sugar concentration based on total soluble solids, five accessions had values higher than the viscous dippers pollinators such as the bee’s optimum sugar requirement. They were the sweetest accessions – namely, CL-SF1 with 71.83, CL-SF9 with 63.50, CL-SF27 with 63.33, CL-SF14 with 61.67, and CL-SF4 with 61.50 oBrix, respectively. The morphological parameters gathered in this study were not significantly correlated to the nectar volume except for the number of heads. Moreover, there was a weak positive linear association between sugar concentration and head diameter, and the disc floret. High expression of the gene has a negative, moderate linear association with low sucrose content. Meanwhile, glucose and fructose had a strong linear association with the gene expression of HaCWINV2. Gene expression analysis of the HaCWINV2 gene showed that CL-SF27 had the highest gene expression level among 10 selected accessions. The same accession had the highest glucose (2.01 g/L) and fructose (1.14 g/L) content. The selected sunflower accessions, CLSF1 and CL-SF 27, are recommended as a potential source of nectar for the beekeeping industry