SEASONAL STRUCTURAL CHANGES OF PHLOEM CELLS IN GINKGO BILOBA L.

Ginkgo biloba is a non-coniferous gymnosperm tree species growing in China and widely cultivated as ornamental tree. Exploring seasonal structural changes in phloem cells of Ginkgo biloba could provide better understanding of the relationship between phloem structure and function. Stems of Ginkgo biloba saplings were sampled at regular intervals during different seasons and then prepared for observation with light microscopy and transmission electron microscopy. Light microscopy was used for identification of non-conducting and conducting phloem and for histometric comparison between sampling dates Ultrastructure (cell organelle presence and distribution) of phloem sieve cells and parenchyma was observed to better understand seasonal changes in phloem cells. Our results showed that most early phloem sieve cells collapsed at the end of the growing season, while most late phloem sieve cells did not collapse till the next spring. Sieve cells in the youngest phloem were characterized by square shape with larger radial diameter and slightly thicker cell walls compared to cambial cells. Their lumina were mostly empty, although some organelles (endoplasmic reticulum, plastids) could be found close to the cell walls. In the older (nonconducting) phloem the shape of sieve cells became partly collapsed with completely empty lumina. Furthermore, pores are mostly open and callose was not present on sieve plates neither in the youngest nor in the older phloem. Most phloem parenchyma cells contain large vacuoles, lipid droplets and amyloplasts, with those in the youngest phloem showing evident seasonal changes, i.e., the cytoplasm was denser in spring and winter compared to summer and autumn, and lipid droplets appeared to be at the highest density/frequency in winter compared with other seasons.In conclusion, phloem sieve cells of G. biloba undergo obvious seasonal structural changes depending on their ages, which is also in accordance with their seasonal conducting functions in these deciduous trees. Youngest phloem parenchyma cells also showed seasonal structural variation with regards to cytoplasm density and frequency of lipid droplets.