The distribution of current assimilates between competing zones of potential growth in the peach tree (Prunus persica (L.) Batsch) was studied using limb girdling, which altered the balance between reproductive growth and vegetative growth in a similar manner to the aging process. Fruit matured earlier, and leaf senescence and abscission were advanced in girdled limbs. which supported normal fruit loads but had only half the leaf area. Lateral growth and secondary thickening were reduced by 50% but vegetative growth approached normal rates at times when fruit growth was minimal, indicating that girdling reduced the ability of vegetative growth to compete with reproductive growth for assimilates. Starch and soluble sugars did not accumulate above the girdles. The data are consistent with the hypothesis that girdling alters the balance between endogenous growth regulators which favour either vegetative or reproductive development. We suggest that the initial effects on the girdled limb are attributable to accumulation of growth regulators produced above the girdle. The reduced flow of growth regulators to the roots eventually results in lowered levels of root-produced hormones which subsequently causes effects throughout the tree.
For 2-3 days after all except one of the secondary limbs of a peach tree were girdled, all limbs (including the non-girdled limb) shrank. Shrinkage and subsequent growth was greatest in the zone immediately above the girdle whereas growth was least immediately below. After initial shrinkage the growth rate of the cambium, at specific positions in relation to the girdle, was more-or-less constant and consistent with the long-term effect of the girdle at each position.
The concentration of indole-3-ylacetic acid (IAA) extracted from the bark immediately above the girdle first increased sharply but then decreased to the level in the non-girdled limb. On the other hand, beneath the girdle the IAA concentration decreased by 75% and remained low.
Thus, immediately above the girdle IAA concentration in the bark increased while the limb shrank, and declined before the cambial growth rate increased. Beneath the girdle, however, the concentration of IAA was severely depleted while growth of the limb was inhibited. The possible implications of these observations in relation to control mechanisms of cambial growth are discussed.
During the first stage of dry weight increase (DW I) of peach fruit, ethephon inhibited growth but stimulated the short-term uptake of 14C-labelled assimilate as well as the natural abscission of the fruitlets. Abscission appeared to be the primary effect, while inhibited growth was a secondary effect of ethephon. This stage of growth @W I) includes the first half of the lag phase of fresh weight increase (FW 11). During the remainder of FW I1 (after the start of DW 11), ethephon did not affect growth except at the highest concentration, which inhibited growth. After commencement of FW 111, which is the final stage of rapid fresh weight increase, ethephon stimulated fruit growth.The data indicated that growth could be stimulated with ethephon during periods when the rate of fresh weight growth was rising but not when it was falling. Thus, endogenous ethylene appears to limit growth of peach fruit during the stages of rapid increase in fresh weight.
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