Specific temperature, storage times, and medium composition enable initiation of regular arrays of intramembranous particles on the exoplasmic fracture face during prolonged storage of isolated chloroplasts at 4 °C, producing about 2 – 10 regular arrays with 2 – 30 particles in each array, with a period of about 36 nm, oriented in 1 – 4 directions. The particle sizes do not change throughout the time of storage (1 – 4 weeks). The second type of particle regularity arises during prolonged storage of chloroplasts in greater than 1 M sucrose at −18 °C. Rounded areas of small particles tightly packed into paracrystalline arrays are found among less densely packed particles. The density of small particles is 4700 particles/μm2, and the mean size is 11 nm, whereas the particle density of the background is 1600 particles/μm2 with a mean particle size of 13 nm compared with 1200 particles/μm2 and mean size 16 nm in fresh chloroplasts. Based on the reduction of particle sizes and manner of packing on the fracture face, it is proposed that the small particles are a light-harvesting complex, separate from photosystem II and aggregated into paracrystalline arrays. The thylakoid lipids may participate in formation of particle regularity. Key words: thylakoid membrane, freeze fracture, particle regularity, low temperatures.
