The structure and development of polyphenolic parenchyma cells in Norway spruce (Picea abies) bark

Abstract

A developmental and structural characterization of polyphenolic parenchyma cells (PP cells) in Norway spruce bark was undertaken as part of our studies on their role in defense against bark beetles and pathogenic fungi. PP cells form multiple circumferentiallayers of cells within the secondary phloem. A layer of PP cells begins differentiation at the start of each growth season, delineating annual growth increments in the secondary phloem. The PP cells grow in size over a number of years, and remain viable even in the oldest phloem layers of trees 100 years old. While most spruce clones examined had PP cell layers that are one cell thick, in one clone the PP cell layer is 2 cells thick with additional PP cells scattered throughout the intervening blocks of sieve cells. The additional cells develop from undifferentiated axial parenchyma cells during the first 5–8 years after formation of the PP cell layer. Division of PP cells in phloem layers older than 8 years give rise to additional PP cells. This accommodates the expansion of the stem circumference while maintaining the intactness of this defense barrier. The importance of phenolic accumulation is also indicated by examination of early stem development. PP cells are produced during the earliest stages of interfascicular cambium formation, and well organized layers are produced by the second year of growth. PP cells in all layers of 25 year old tree bark contained starch, lipids and polyphenolics, which changed in amount or character in a seasonal pattern. Plasmodesmata are abundant between adjacent PP cells and PP cells and ray parenchyma, where they are probably important to nutrient and defense signal transport in the radial and axial directions. The formation of a new PP cell layer each season, the maintenance of the cells for many years, the early organization of this layer in the primary stem, and the dynamic physiological activity even older cells exhibit, supports previous work suggesting that PP cells are an important protective tissue in the secondary phloem.