Influence of genotype, floral stage, and water stress on floral nectar yield and composition of mānuka (Leptospermum scoparium).

Michael J Clearwater,Merilyn Manley-Harris,Maria Revell,Stevie Noe

doi:10.1093/aob/mcx183

Abstract

Background and AimsFloral nectar can be variable in composition, influencing pollinator behaviour and the composition of honey derived from it. The non-peroxide antibacterial activity of mānuka (Leptospermum scoparium, Myrtaceae) honey results from the chemical conversion of the triose sugar dihydroxyacetone (DHA), after DHA accumulates for an unknown reason in the nectar. This study examined variation in nectar DHA, glucose, fructose and sucrose content with floral stage of development, between mānuka genotypes with differing flower morphology, and in response to water stress.MethodsSix mānuka genotypes were grown without nectar-feeding insects. Stages of flower development were defined, nectar was harvested and its composition was compared between stages and genotypes, and with floral morphology. Water stress was imposed and its effect on nectar composition was examined.Key ResultsNectar was present from soon after flower opening until the end of petal abscission, with the quantity of accumulated nectar sugars rising, then stabilizing or falling, indicating nectar secretion followed by reabsorption in some genotypes. The quantity of DHA, the ratio of DHA to other nectar sugars and the fructose to glucose ratio also varied with stage of development, indicating differences in rates of production and reabsorption between nectar components. Nectar composition and yield per flower also differed between genotypes, although neither was positively related to nectary area or stomatal density. Drying soil had no effect on nectar composition or yield, but variation in nectar yield was correlated with temperature prior to nectar sampling.ConclusionsMānuka nectar yield and composition are strongly influenced by plant genotype, flower age and the environment. There were clear stoichiometric relationships between glucose, fructose and sucrose per flower, but DHA per flower was only weakly correlated with the amount of other sugars, suggesting that accumulation of the triose sugar is indirectly coupled to secretion of the larger sugars by the nectary parenchyma.

Highlights

Floral nectar composition and volume are often variable between flowers and plants of a given species (Pacini and Nepi, 2007)
The amount of accumulated nectar sugar was highest at Stage 3 (P < 0.001), before declining in at least two of the six genotypes (NT and Red Ensign (RE)) during Stage 4 (Fig. 4A), suggesting that nectar reabsorption can occur if pollinators are not present
This temporal pattern was matched by the accumulation of nectar DHA (P < 0.001), except that DHA levels fell more sharply than sugars in all genotypes at Stage 4, suggesting faster reabsorption or degradation of DHA compared to glucose and fructose

Summary

Introduction

Floral nectar composition and volume are often variable between flowers and plants of a given species (Pacini and Nepi, 2007). Composition for a given species is usually dominated by the major sugars glucose, fructose and sucrose in relatively consistent proportions, but a wide variety of other compounds may be present in variable amounts (Carter and Thornburg, 2004; Nicolson and Thornburg, 2007). The floral nectars of mānuka (Leptospermum scoparium) and some other Leptospermum species are dominated by fructose and glucose, and contain small but variable amounts (usually

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Conclusion