Ultrastructure of the mesophyll in scots pine and Norway spruce: Seasonal variation and molarity of the fixative buffer

Abstract

Resolution of the ultrastructure of the needles of both Scots pine (Pinus silvestris L.) and Norway spruce [Picea abies (L.) Karst.] is strongly influenced by the molarity of the buffer used in fixation. When 0.2 M or 0.1 M buffer is used in fixation during the summer, the constituents of the cytoplasm are precipitated, resulting in poor resolution of the membranes and lamellae and often in negative staining. The tannin in the central vacuole appears as a thick ribbon. By using correct molarities of buffer during each season (0.1 M for autumn and winter and ca. 0.05 M for the growing season), the best possible resolution will be achieved. With good resolution the tannin in the central vacuole appears in granular form throughout the year, and the cytoplasm and its organelles are clearly distinguishable during every season. During the growing season, the chloroplasts in the needles of Scots pine are spread to the cell walls and have large starch grains; the stroma and grana lamellae are well developed; the stroma and cytoplasm are rich in polysomes. Mitochondria and microbodies can be clearly resolved. During hardening and afterwords throughout the winter, the chloroplasts, which at this time contain no starch, and other cytoplasmic organelles aggregate in the corners of the cells. The chloroplast envelopes and the stroma and grana lamellae stay intact. The cytoplasm is netlike and rich in ribosomes, mitochondria and microbodies, all of which are intact and clearly distinguishable. During spring activation the structure returns to that described for the growing season.