139 SCIENTIFIC WORKS OF THE LITHUANIAN INSTITUTE OF HORTICULTURE AND LITHUANIAN UNIVERSITY OF AGRICULTURE. SODININKYSTĖ IR DARŽININKYSTĖ. 2008. 27(2). Alteration of source-sink relations in the leaves of in vitro plants of two Solanum tuberosum L. genotypes under hypothermia Nina Astakhova, Alexander Deryabin, Maxim Sinkevich, Stanislav Klimov, Tamara Trunova Timirayzev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya st. 35, Moscow, 127276 Russia, e-mail: trunova@ippras.ru Growth, ultrastructural organization of photosynthetic apparatus and CO 2 -exchange were investigated in the leaves of potato (Solanum tuberosum L.) plants cv. ‘Desiree’ of wild type (control) and transformed with vector carrying yeast invertase gene under the control of tuber-specific patatin promoter B33 class I, fused with proteinase II inhibitor leader peptide to provide enzyme location in apoplast. Plants grown in vitro on the Murashige and Skoog medium supplemented with 2 % sucrose. At optimal growth temperature (22 °C), the transformed plants differed from the plants of wild type by retarded growth and a lower rate of photosynthesis as calculated per plant. Photosynthesis and leaf dry weight ratio in transformed plants was higher than in control plants. Under hypothermia (5 °C), dark respiration and especially photosynthesis of transformed plants turned out to be more intense than in control plants. After a prolonged exposure to low temperature (6 days at 5 °C), in the plants of both genotypes, the ultrastructure of chloroplasts changed. Absolute areas of sections of chloroplasts increased in transformed plants. By some ultrastructural characteristics: the number of granal thylakoids (per a chloroplast section area), transformed plants turned out to be more cold resistant than control plants. The obtained results are discussed in connection with changes in source-sink relations in transformed potato plants. These changes modify the balance between photosynthesis and retarded efflux of assimilates, causing an increase in the intracellular level of sugars and a rise in the resistance to chilling. Key words: Solanum tuberosum, chilling stress, chloroplast ultrastructure, photosynthesis, respiration, yeast invertase gene. Introduction. The potato plant transformed with vector carrying yeast invertase gene under the control of tuber-specific patatin promoter B33 class I, fused with proteinase II inhibitor leader peptide to provide enzyme location in apoplast is an organism with modified source-sink relations (Stitt et al., 1990). Sink ability therein is reduced because of an elevated activity of acid insoluble invertase cleaving the sucrose to monosaccharides (glucose and fructose); as a result, the efflux of assimilates from photosynthesizing tissues is suppressed, and they accumulate in the leaves (Deryabin et al., 2003; 2007). In transformed plants, the threshold concentration of sucrose necessary to initiate the formation of tubers was low (1–2 %). The process of tuberization therein was intensified when the content of sucrose in the growth medium increased up to 10 %. At the same time, in the plants