INVOLVEMENT OF AUXIN IN THE LOSS OF APICAL DOMINANCE AND PLANT-GROWTHPOTENTIAL 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 durin g long-term storage of potato (Solanum tuberosum L.) seed tubers. Trea tment of single-eye seed cores from 18-month-old seed tubers with 1-na phthaleneacetic acid (NAA) restored apical dominance and increased dry matter partitioning ing to roots, stems, and leaves, thus partially m itigating the deleterious effects of advanced seed-tuber age on growth potential. Conversely, NAA treatment of seed cores from 6-month-old t ubers substantially inhibited plant growth. In contrast to NAA, IAA wa s totally ineffective at counteracting the deleterious effects of adva nced tuber age on plant growth, whereas the effect of IAA on overall g rowth of plants from 6-month-old seed cores remained slightly inhibito ry. The difference in efficacy of these two auxins appears to be relat ed to age-induced differences in ability of tissues to transport and c atabolize IAA. The specific activity of IAA oxidase (IAAox) was 4 time s higher in tissue from 20-month-old seed tubers at planting and incre ased at a faster rate during sprouting compared with that from 8-month -old tubers. Hence, the higher potential for oxidation of IAA in tissu e from older seed cores is well correlated with the inability of this auxin to alter growth. In translocation studies, etiolated sprouts fro m 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 ra diolabeled IAA at a much faster rate on a dry weight basis than those from younger seed cores. The specific activities of IAAox and peroxida se in the sprout apex, sprout base, and tuber tissue from 18-month-old seed cores were substantially higher than in similar tissues from 6-m onth-old seed cores, and tissue concentration of the radiolabel was ne gatively correlated with IAAox activity. Hence, aging of potato seed t ubers not only reduces the ability of sprouts to transport auxin basip etally, but it also increases the capacity for auxin catabolism during sprouting. The physiological consequence of this may be the release o f lateral meristems from correlative inhibition, and in effect, reduce d apical dominance and shoot growth potential during plant establishme nt from aged seed tubers.