Intra-annual dynamics of phloem formation and ultrastructural changes in sieve tubes inFagus sylvatica

Abstract

Despite increased interest in the timing and dynamics of phloem formation, seasonal changes in the structure of phloem sieve elements remain largely unexplored. To understand better the dynamics of phloem formation and the functioning of sieve tubes in the youngest phloem in Fagus sylvatica L., we investigated repeatedly taken phloem samples during the growing season of 2017 by means of light microscopy, and transmission and scanning electron microscopy. Phloem formation started with the expansion of the overwintered early phloem sieve tubes adjacent to the cambium and concurrent cambial cell production. The highest phloem growth rate was observed in general 1 week after the onset of cambial cell production, whereas the transition from early to late phloem occurred at the end of May. Cambial cell production ceased at the end of July. The final width of the phloem increment was 184 ± 10 μm, with an early phloem proportion of 59%. Collapse of older phloem tissue is a progressive process, which continuously occurred during the sampling period. Collapse of early phloem sieve tubes started shortly after the cessation of cambial cell production. Prior to the onset of radial growth, late phloem from the previous year represented 80% of the total non-collapsed part; during the growth period, this percentage decreased to 20%. Differences were observed in both sieve tube ultrastructure and sieve plate geometry between the youngest and older phloem. However, sieve plates were never completely occluded by callose, suggesting that processes affecting the functionality of sieve tubes may differ in the case of regular collapse or injury. The youngest parts of the phloem increment from the previous year (i.e., previous late phloem) continue functioning for some time in the current growing season, but the two-step development of overwintered phloem cells also ensures a sufficient translocation pathway for photosynthates to the actively growing tissues.

Seasonal Vascular Tissue Formation in Four Boreal Tree Species With a Focus on Callose Deposition in the Phloem

“…Nonetheless, callose formation was only found in some sieve plates, indicating that freezing temperatures per se did not trigger the consistent formation of callose in the current year's phloem. Our findings agree with a recent study on Fagus sylvatica (Prislan et al, 2018). These authors found open sieve pores in phloem formed in the previous year, prior to the beginning of cambial activity in March.…”

“…Expedited phloem development in spring serves the tree during the important period of bud burst when there may only be a small population of functional phloem cells near the cambium to support the unfolding leaves and other meristematic zones with food (Zamski and Zimmermann, 1979;Lavrič et al, 2017;Prislan et al, 2018). Radial translocation of sugars in rays will also be important at that time (Zamski and Zimmermann, 1979).…”

“…This is surprising, given the importance of the phloem in delivering photosynthates to the cambium and the role of the phloem in distributing signaling molecules across plant organs (Lucas et al, 2013). Much of what we know about seasonal phloem development comes from the pioneering studies of Evert and co-workers (Evert, 1990, and literature cited therein); only recently have researchers focused their attention on the phloem again (e.g., Prislan et al, 2018).…”

“…Callose deposition on sieve plates has a major influence on phloem transport rates (Stanfield et al, 2018), so it would follow that this may be an indication of transport cessation. To date, few studies have demonstrated changes in callose deposition on sieve plates on a seasonal basis (but see Davis and Evert, 1970;Prislan et al, 2018). Given that seasonal callose deposition remains poorly documented, many questions remain regarding the functionality of callose deposition on sieve plates.…”

Značilnosti lesnih in floemskih prirastkov pri trepetliki (Populus tremula L.) / Characteristics of wood and phloem increments in Eurasian aspen (Populus tremula L.)

“…Za razliko od lesnega prirastka floemski prirastek nastane vsako leto, četudi je zelo ozek in zajema le nekaj celic. Njegov vsakoletni nastanek je ključen za preživetje dreves, saj sitasti elementi prevajajo produkte fotosinteze 1-2 rastni sezoni, potem so celice podvržene sekundarnim spremembam, katerih rezultat je propad celic in s tem prenehanje njihovega delovanja (Prislan et al 2018a).…”