CHANGES IN LIPID-PEROXIDATION AND LIPOLYTIC AND FREE-RADICAL SCAVENGING ENZYME-ACTIVITIES DURING AGING AND SPROUTING OF POTATO (SOLANUM-TUBEROSUM) SEED-TUBERS

Previous research has shown that cell membranes of potato (Solanum tub erosum L. cv Russet Burbank) seed-tubers lose integrity between 7 and 26 months of storage (4-degrees-C, 95% relative humidity), and this lo ss coincides with a significant decrease in growth potential. The age- induced decline in membrane integrity is apparently due to increased p eroxidative damage of membrane lipids. Malondialdehyde (MDA) and ethan e concentrations (sensitive markers of lipid peroxidation and membrane damage) increased in seed-tuber tissues with advancing age. Moreover, in vivo ethane production from discs of cortex tissue from 13- and 25 -month-old seed-tubers was 87% greater (on average) than that from dis cs from 1-month-old tubers. Calcium suppressed ethane production from all ages of tissue discs, and the effect was concentration dependent. Linoleic acid enhanced ethane production from 5- and 17-month-old tube rs by 61 and 228%, respectively, suggesting that older tissue may cont ain a higher free-radical (FR) titer and/or lower free polyunsaturated fatty acid content. In addition, throughout plant establishment, the internal ethane concentration of older seed-tubers was 54% higher than that of younger seed-tubers. MDA concentration of tuber tissue declin ed by about 65% during the initial 7 months of storage and then increa sed 267% as tuber age advanced to 30 months. The age-induced trend in tuber reducing sugar concentration was similar to that of MDA, and the two were linearly correlated. The age-dependent increase in reducing sugars may thus reflect peroxidative degeneration of the amyloplast me mbrane, leading to increased starch hydrolysis. Compared with 5-month- old seed tubers, 17- and 29-month-old seed-tubers had significantly hi gher levels of lipofuscin-like fluorescent compounds (FCs), which are produced when MDA reacts with free amino acids. Age-dependent increase s in MDA, ethane, and FCs were not associated with higher activities o f phospholipase and lipoxygenase in tissue from older tubers. In fact, 8-month-old seed-tubers had significantly higher activities of these enzymes than 20-month-old seed-tubers. However, the activities of supe roxide dismutase, peroxidase, and catalase in 20-month-old tubers were substantially higher out of storage, and increased at a faster rate d uring plant establishment, than in 8-month-old seed-tubers. Collective ly, these results suggest that a gradual build-up of FRs leads to pero xidative damage of membrane lipids during aging of potato seed-tubers.