INVOLVEMENT OF AUXIN IN THE LOSS OF APICAL DOMINANCE AND PLANT-GROWTH POTENTIAL ACCOMPANYING AGING OF POTATO SEED TUBERS

Studies were conducted to further characterize a role for auxin in the loss of apical dominance and plant growth potential that occurs during long-term storage of potato (Solanum tuberosum L.) seed tubers. Treatment of single-eye seed cores from 18-month-old seed tubers with 1-naphthaleneacetic acid (NAA) restored apical dominance and increased dry matter partitioning ing to roots, stems, and leaves, thus partially mitigating the deleterious effects of advanced seed-tuber age on growth potential. Conversely, NAA treatment of seed cores from 6-month-old tubers substantially inhibited plant growth. In contrast to NAA, IAA was totally ineffective at counteracting the deleterious effects of advanced tuber age on plant growth, whereas the effect of IAA on overall growth of plants from 6-month-old seed cores remained slightly inhibitory. The difference in efficacy of these two auxins appears to be related to age-induced differences in ability of tissues to transport and catabolize IAA. The specific activity of IAA oxidase (IAAox) was 4 times higher in tissue from 20-month-old seed tubers at planting and increased at a faster rate during sprouting compared with that from 8-month-old tubers. Hence, the higher potential for oxidation of IAA in tissue from older seed cores is well correlated with the inability of this auxin to alter growth. In translocation studies, etiolated sprouts from aged seed tubers showed a reduced ability to translocate [1-C-14]IAA basipetally compared with those from younger tubers. Moreover, intact etiolated sprouts growing from older seed cores decarboxylated the radiolabeled IAA at a much faster rate on a dry weight basis than those from younger seed cores. The specific activities of IAAox and peroxidase in the sprout apex, sprout base, and tuber tissue from 18-month-old seed cores were substantially higher than in similar tissues from 6-month-old seed cores, and tissue concentration of the radiolabel was negatively correlated with IAAox activity. Hence, aging of potato seed tubers not only reduces the ability of sprouts to transport auxin basipetally, but it also increases the capacity for auxin catabolism during sprouting. The physiological consequence of this may be the release of lateral meristems from correlative inhibition, and in effect, reduced apical dominance and shoot growth potential during plant establishment from aged seed tubers.