Exploring Composition and Within-Population Variation in the Phloem Exudate “Manna” in Eucalyptus viminalis

TL;DR

This study explores the composition and variation of manna, a sugary exudate from Eucalyptus viminalis, revealing how factors like tree size and climate influence its production.

Contribution

The study introduces a repeatable method to extract and analyze manna, revealing intra-specific variation in its sugar composition and quantity.

Findings

Manna contains six sugars, with sucrose and raffinose as the dominant components.

Manna quality and quantity vary with tree size, branch circumference, and environmental conditions like aridity.

Seasonal and annual variations in manna production were observed, with quality increasing under drier conditions.

Abstract

Sugary phloem exudates are produced by many plant species and play key roles in carbon storage, defense, and ecological interactions. Among eucalypts, one such exudate, manna, is an important carbohydrate source for birds, mammals, and insects. Despite its ecological relevance, little is known about the composition and intra-specific variability of manna. Here, we investigated patterns of manna production in Eucalyptus viminalis, a widespread foundation tree species in southeastern Australia. We developed a repeatable ex situ method to extract and analyze manna, allowing us to characterize its sugar composition and examine variation within and between trees. Across years, manna contained six sugars, with sucrose and raffinose dominant. We found substantial variation in both the quality (sucrose/raffinose ratio) and quantity (mg) of manna produced. Both declined with increasing tree size (DBH), while quality increased with branch circumference. Seasonal and annual variation in manna was also evident, with quality increasing under drier conditions (positive correlation with aridity). Our findings demonstrate substantial intra-specific variation in phloem exudates (manna), shaped by temporal and tree-level factors. These patterns offer a foundation for future research into the ecological and physiological drivers of exudate variation and resource availability in foundation species like E. viminalis.

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