Sugar and dihydroxyacetone ratios in floral nectar suggest continuous exudation and reabsorption in Leptospermum polygalifolium Salisb

Leptospermum polygalifolium Salisb. can accumulate high concentrations of dihydroxyacetone (DHA), precursor of the antimicrobial compound methylglyoxal found in honey obtained from floral nectar of Leptospermum spp. Floral nectar dynamics over flower lifespan depends on internal and external factors that invariably impact nectar quality. Current models to estimate nectar quality in Leptospermum spp. overlook time of day, daily (24 h), and long-term dynamics of nectar exudation and accumulation over flower lifespan.To explain the dynamics of nectar quality over flower lifespan, accumulated nectar from flowers of different ages was collected from two L. polygalifolium clones, and then re-collected 24 h later from the same flowers. HighPerformance Liquid Chromatography was used to quantify DHA amount and total equivalents of glucose + fructose (Tsugar) per flower in the nectar. DHA and Tsugar amount per flower differed with flower age and between clones. In accumulated nectar, the amount of DHA and Tsugar per flower rose to a broad peak postanthesis before decreasing. Immediately after peaking DHA declined more quickly than Tsugar in accumulated nectar due to a greater decrease in the exudation of DHA than for Tsugar. The DHA : Tsugar ratios in accumulated nectar and in nectar exuded over the next 24 h were similar and decreased with flower age, indicating that exudation and reabsorption occurred concomitantly across flower development. Hence there is a balance between exudation and reabsorption. A quantitative model suggested that flowers have the potential to exude more DHA and Tsugar than actually accumulated.

Automated summary

The nectar quality of Leptospermum polygalifolium flowers undergoes dynamic changes throughout the flower lifespan, with the age of the flowers and the genetic differences between clones affecting the amount of DHA and Tsugar in the nectar. After reaching a peak, the amount of DHA in the accumulated nectar decreases faster than Tsugar, indicating a greater decrease in the exudation of DHA. The ratios of DHA to Tsugar in the accumulated nectar and in the nectar exuded over the next 24 hours decrease with flower age, suggesting simultaneous exudation and reabsorption during flower development.