Seasonal movement of chloroplasts in mesophyll cells of two Picea species.

Cold acclimation in evergreen conifers of temperate zone is associated with seasonal structural changes of mesophyll cells. Photoprotective reactions include the movement of chloroplasts from summer position when they are located along the cell walls to winter arrangement with their aggregation in one part of the cell. Special spatial arrangement of mesophyll in Picea species with chloroplasts located along the two opposite cell walls causes the very specific pattern of chloroplast movement. To reveal the intracellular apparatus involved in the seasonal organelle position changes, 3D reconstruction of mesophyll cell structure was applied. Two Picea species, P. obovata and P. pungens, from two geographic regions were studied in a 3-year course. The involvement of small transparent vacuoles in the development of cytoplasmic strands penetrating through the central vacuole and connecting two opposite lateral sides of the cell was shown. The nucleus and cytoplasmic organelles including chloroplasts move inwards the strand forming the cytoplasmic conglomerate enclosed by the vacuole at the cell center. Two Picea species have distinct differences in spatial organization of winter mesophyll cells and in structural events leading to its formation. Analysis of Picea species from two geographic regions over 3 years of monitoring reveals dependence of seasonal organelle movement on the dynamics of temperature decline in autumn.