Seasonal changes in mesophyll ultrastructure of needles of Norway spruce (Picea abies)

Seasonal changes in the ultrastructure of the mesophyll of needles of Norway spruce are described. During the growing season, the tannin is in the form of a ribbon along the margin of the vacuole or in the form of small, more or less scattered particles. The cytoplasm is dark and its organelles are along the cell wall; only the chloroplasts with large starch grains and the nucleus can be clearly resolved. During and after September, the cytoplasm is very netlike, all of the cytoplasmic organelles clump together, the amount of lipid droplets increases, and ribosomes are clearly visible. The tannin is in the form of rather large granules and it increases over its growing-season amount. The shape of chloroplasts changes. Chloroplasts lose their starch and the stroma becomes sparsely granular and rich in ribosomes. This structure is maintained throughout the winter. During spring activation, the cell structure is reorganized and reassumes the condition typical of the growing season. At this time, the needles have some collapsed cells and cells in which some chloroplasts are not quite intact. The injuries were probably caused by weather conditions (a warm period in March followed by a frost).

Developmental and seasonal patterns of mesophyll ultrastructure in Abies balsamea

Changes in mesophyll ultrastructure with development and season are described for Abies balsamea. Cells mature sequentially during expansion of the needles. Most cells appear to be fully mature and actively photosynthesizing at the time of budbreak. Tannins appear early and accumulate throughout the growing season. Winter dormancy is marked by an accumulation of lipids throughout the cell, an aggregation of organelles around the nucleus, some loss of chloroplast structure, and a failure of chloroplasts to form starch grains. Reorganization of cell structure and resumption of synthetic activity in the spring occurs about 2 months before budbreak.