Avocado fruit stalks add sugar-carrying tubes as fruits develop

Developing avocados depend on resources delivered through their stalks. A study of Hass avocado trees in southern Spain examines how the microscopic plumbing supporting that delivery changes between flowering and fruit maturity. Its central finding is a two-stage pattern: individual sugar-carrying conduits enlarge early, while later development is associated mainly with an increase in their number.

The study, by María Agulló-Samper and colleagues, was posted on bioRxiv on November 2, 2026. It is a preprint and has not been certified by peer review. Its evidence combines anatomical observations, carbohydrate measurements and hydraulic calculations—not direct measurements of sugar transport through living stalks.

Following the transport tissue

The researchers studied mature, irrigated trees at the La Mayora research orchard in Málaga province. Different measurements used subsets of an orchard population of 41 trees. They sampled leaves, flowers and developing fruits across the seasonal cycle, with key anatomical comparisons involving flowers, young fruits in July and mature fruits in December.

Their focus was the phloem, the tissue that carries carbohydrates through the plant. Its conducting cells, called sieve tube elements, connect through porous end walls known as sieve plates. Tube dimensions, the size and number of these pores, and the number of parallel tubes all contribute to the hydraulic resistance represented in the researchers’ model.

Fluorescence microscopy revealed tube dimensions, while scanning electron microscopy allowed measurements of sieve plate pores. The team also used an antibody marker to help confirm the identity of the cells being counted. Tube-dimension measurements included hundreds of elements from 10 flower stalks and 10 fruit stalks at each of the two main fruit stages.

Larger passages first, more tubes later

Flower stalks had the smallest sieve tube elements and the narrowest sieve plate pores. Average pore radius increased from about 0.2 micrometres in flowers to about 0.4 micrometres in young fruits. Tube length and width also increased between those stages.

After that initial change, the measured tube dimensions and sieve plate characteristics showed no statistically significant differences through maturity. Pore number per plate did not differ significantly among the flower and fruit stages either.

Tube abundance, however, increased substantially. The reported number per stalk cross-section rose from roughly 6,000 in flowers to nearly 20,000 in mature fruits. Tube density remained approximately constant, indicating that the increase accompanied expansion of the phloem area rather than tighter packing. For the largest samples, total tube numbers were estimated from counts in selected regions rather than obtained by counting every tube.

This internal expansion occurred even though overall stalk diameter remained broadly unchanged after its early increase. The authors interpret the pattern as continued formation of phloem, or phloemogenesis: adding conducting tissue rather than continually widening each conduit.

What the models establish—and what they do not

Using the anatomical measurements, the researchers calculated conductivity, a property of individual conduits, and conductance, which also accounts for the number of tubes and transport distance. Their models indicated low conductivity in flower stalks and increasing total conductance during fruit development, with additional tubes making a major contribution.

These calculations support a physical explanation for how the observed anatomy could accommodate transport. They do not demonstrate that more tubes caused faster fruit growth, or that actual sugar flow increased by the modeled amount. The analysis assumed uniform phloem geometry along the transport route, explored source-to-fruit distances of 0.1 and 1 metre, and used an assumed sap sucrose concentration of 18% by weight when estimating flow from growth.

Carbohydrate assays provided another association. Flowers contained about 20% fructose and 35% glucose on a dry-weight basis, much higher than leaves or fruits. Those are tissue concentrations, not measurements of phloem sap. Their coincidence with narrow conduits does not, by itself, establish the authors’ proposed transport bottleneck.

The work therefore offers an anatomical basis for refining models of resource delivery to avocado fruits. Its scope remains one cultivar in one orchard over a seasonal cycle. Whether the same pattern occurs elsewhere, what regulates new tube formation, and how closely modeled capacity tracks living transport remain open questions.