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

A developmental and structural characterization of polyphenolic parenchyma cells (PP cells) in Norway spruce bark was undertaken as part of our studie s on their role in defense against bark beetles and pathogenic fungi. PP ce lls form multiple circumferential layers of cells within the secondary phlo em. A layer of PP cells begins differentiation at the start of each growth season, delineating annual growth increments in the secondary phloem. The P P 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 exami ned had PP cell layers that are one cell thick, in one clone the PP cell la yer is 2 cells thick with additional PP cells scattered throughout the inte rvening blocks of sieve cells. The additional cells develop from undifferen tiated 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 year s 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 produc ed 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 o r character in a seasonal pattern. Plasmodesmata are abundant between adjac ent PP cells and PP cells and ray parenchyma, where they are probably impor tant to nutrient and defense signal transport in the radial and axial direc tions. 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 prim ary 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.