Ultrastructural Changes in Chloroplasts of Cucumis sativus L. and Secale cereale L. during Low-Temperature Hardening.

Chloroplast ultrastructural changes were investigated in chilling-sensitive (cucumber, Cucumis sativus L.) and cold-tolerant (winter rye, Secale cereale L.) plants during low-temperature hardening. Cold hardening was found to shorten the total length of thylakoid membranes in chloroplasts of both species by 20–30%. The reduction in total membrane length in cucumber was due to the decline in membrane length of both grana and stroma thylakoids, whereas the membrane length of grana thylakoids in winter rye was reduced without changes in length of stroma thylakoid membranes. In cucumber plants, the chloroplast section area decreased during hardening in parallel with more than threefold increase in starch grain area. In winter rye, the chloroplast area increased after hardening, while starch grains were completely absent. Both S. cereale and C. sativus retained photosynthetic activity at hardening temperatures, although photosynthetic rates decreased by 2–3 times compared to those in untreated plants. The content of soluble sugars in leaves increased by 20% in cucumber and increased more than threefold in winter rye. The accumulation of starch in cucumber chloroplasts provides evidence that a significant part of photosynthetic products was removed from the osmotic pool, which limited the efficiency of hardening in C. sativus plants. In winter rye plants, the thylakoid system rearrangement and the lack of starch synthesis in chloroplasts caused a severalfold increase in the content of intracellular soluble sugars, which ensured highly efficient low-temperature hardening of these plants. [ABSTRACT FROM AUTHOR]