Seasonal patterns of callose deposition and xylem embolism in five boreal deciduous tree species.

Premise: Phloem tissue allows for sugar transport along the entirety of a plant and, thus, is one of the most important anatomical structures related to growth. It is thought that the sugar-conducting sieve tube may overwinter and that its cells persist multiple seasons in deciduous trees. One possible overwintering strategy is to build up callose on phloem sieve plates to temporarily cease their function. We tested the hypothesis that five deciduous tree species produce callose on their sieve plates on a seasonal basis.

Methods: Young shoots of five deciduous tree species were sampled periodically between April 2019 and February 2020 in Edmonton, Alberta, Canada. After enzymatic digestion of cytoplasmic constituents, cross sections were imaged using scanning electron microscopy to observe and quantify the level of callose deposition at monthly intervals, and sieve plate pore size was measured. Using a conductivity apparatus, we measured xylem native embolism during these sampling periods.

Results: Contrary to past work on some of the same species, we found little evidence that sieve tubes overwinter by becoming occluded with callose. Instead, we found that most sieve plates remain open. Xylem embolism was minimal during the peak growing season, but increased over winter.

Conclusions: Many species had been assumed to deposit callose on sieve plates over winter, though anatomical and phenological phloem data were sparse. Our data do not support this notion.