SUCROSE SYNTHASE AND SUCROSE-PHOSPHATE SYNTHASE IN POTATO - DISTRIBUTION OF ACTIVITIES AND VARIATIONS IN LEAVES

Activities of sucrose synthase (SS, EC 2.4.1.13) and sucrose phosphate synthase (SPS, EC 2.4.1.14) in leaf, stem, stolon, tuber and root of potato plant (Solanum tuberosum L. cv Kufri Sindhuri) were determined in vitro. SS was determined in both sucrose synthesis (SSsyn) and sucrose cleavage (SScle) directions. Both SS and SPS were present in fairly detectable quantities in immature and mature leaves, stem, stolon, developing tubers and root. The ratio of cleavage to synthesis was 2.5-3.0. Both SSsyn and SPS activities and sucrose contents varied diurnally when measured in mature leaves. Maximum SPS was at 16 h and the minimum at 8 h while the maximum SSsyn was at 20 h and the minimum at 12 h. The variation rhythm of leaf sucrose content synchronized with that of SSsyn. Both SSsyn and SPS started to decline 80 days after planting (DAP) and remained approximately equal to each other between 40 and 120 DAP. On organ basis, in developing tubers SPS was 2.2 and 2.5 times and SS was 79 and 62 times that in the leaves at 60 and 80 DAP, respectively. The relative levels of SS and SPS in the photosynthetic tissue and a higher total SPS in developing sink than in photosynthetic tissue of potato do not conform to the current belief that SPS is higher than SS in tissues concerned with export of sucrose and also that SS is engaged solely for cleavage of sucrose. The results are discussed in terms of currently assigned roles to SPS and SS in the metabolism of sucrose in plants.