Phloem conductance from leaf to fruits in avocado during development

Reproductive function in seed plants depends on efficiently allocating nutrients from sources to developing fruits. Hydraulic transport of carbohydrates largely depends on phloem architecture, but how phloem structure in fruit pedicels changes with development remains unexplored for most seed plants. Using the one-year phenology of avocado trees, we quantified water content, dry matter, and non-structural carbohydrates (NSCs) in mature leaves, flowers, young fruits, and fully developed fruits. Concomitantly, we measured phloem conduit geometry in different vein orders of leaves and in flower-fruit pedicels over time, including sieve tube numbers in cross sections. NSCs were below 5% in leaves and fruits but reached the highest concentrations in flowers, especially fructose (20%) and glucose (35%), and the pedicels of these flowers showed the smallest conduits. While sieve tube elements in leaves scaled their geometry from the minor veins to the petioles, in fruit pedicels, their geometry changed only from flower to initial fruit set, increasing their number linearly up to fruit maturation. Our models show that phloem transport toward the flowers is impaired, but total conductance increases gradually due to phloemogenesis.