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  • Author or Editor: Philip G. Gibson x
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Peach [Prunus persica (L.) Batsch (Peach Group)] trees bloom in response to chilling and postrest heat accumulation. The peach cultivar Coronet exposed to a graft-transmissible, infectious agent known as peach latent mosaic viroid (PLMVd) blooms at a different time than noninoculated trees of the same cultivar. To determine if chilling requirements differed between trees inoculated with PLMVd and noninoculated controls, fruiting shoots collected from the orchard and artificially chilled containerized trees were forced in a greenhouse. Additional artificially chilled containerized trees were forced under constant temperatures in growth chambers to determine if postrest heat accumulation requirements differed. There was no difference in the chilling requirement of the fruiting shoots collected from the field although the shoots exposed to PLMVd had a delayed response and fewer responded to greenhouse forcing conditions. The containerized trees also showed no differences in chilling requirements during winter 1999 or 2000. Trees inoculated with PLMVd had a significant delay in bloom. Growth chamber data revealed a significantly higher base temperature for heat accumulation in the PLMVd inoculated trees.

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Delaying bloom to reduce spring frost risk and reducing labor costs by increasing orchard efficiency are important goals of peach producers. At the Musser Fruit Research Center near Clemson, S.C., `Coronet' peach trees were inoculated with Peach Latent Mosaic Viroid (PLMVd) to induce bloom delay and reduce shoot vigor. Trees were grown in a high-density, Y-trained orchard system to determine the potential benefit of reduced shoot vigor on labor efficiency in summer pruning operations. In Aug. 1997, `Ta Tao 5' buds were grafted onto the scaffolds of 2-year-old `Coronet' peach trees to transmit PLMVd. Transmission was confirmed by dot-blot on N+ nylon membranes using cRNA probes. Bloom was not delayed in the following year, Spring 1998, but bloom was delayed 7 days in Spring 1999. Yields were unaffected in 1998, but the total fruit weight produced on PLMVd-treated trees was significantly less in the 1999 harvest. Individual fruit weight, firmness, and color were improved on the treated trees. Shoot vigor was reduced on the PLMVd treated trees in the summers of 1998 and 1999, resulting in a significant reduction in pruning time and pruning weights. Light penetration was significantly increased in the treated trees when compared to the untreated controls in the summers of 1998 and 1999. Fruiting shoot length and the number of fruit per shoot was unaffected by the PLMVd presence. The trunk cross-sectional area was significantly less on the treated trees when compared to the untreated controls after each year of growth. Autumn defoliation occurred earlier on the treated trees in Fall 1998 and Fall 1999. The manipulation of peach tree growth and development was accomplished using graft transmissible agents as PLMVd induced several beneficial growth and developmental modifications in established peach trees.

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Graft-transmissible agents found in `Ta Tao 5' peach have been associated with phenological changes, including delay in bloom, reduced shoot vigor, and early autumn defoliation. Peach Latent Mosaic Viroid (PLMVd) is present as a graft-transmissible agent in `Ta Tao 5'. In order to further characterize the changes occurring in trees exposed to PLMVd from `Ta Tao 5' grafts, total fatty acid content was measured for peach buds during chilling accumulation and release from dormancy in `Coronet' peach trees and `Coronet' trees treated with `Ta Tao 5' bud grafts. Palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids were the major fatty acids in dormant and releasing peach buds of both the controls and treated trees. The degree of unsaturation increased immediately following completion of chilling requirement in both the untreated controls and in the treated trees. However, the desaturation of linoleic acid to linolenic acid was significantly inhibited in the trees treated with `Ta Tao 5' bud grafts, which was accompanied by a concomitant delay in the resumption of growth. The disparity between the control and treated trees in the trend toward increased fatty acid unsaturation continued through the resumption of growth. The changes in degree of fatty acid saturation correlated with a response to forcing conditions and the release from dormancy. The presence of PLMVd in `Coronet' peach trees affects membrane fatty acid saturation during chilling accumulation and dormancy release. These findings suggest that metabolic pathways involving fatty acid desaturation are linked to the phenotypic variation in trees exposed to PLMVd.

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Peach [Prunus persica (L.) Batsch (Peach Group)] trees infected with peach latent mosaic viroid (PLMVd) have been associated with phenological changes including delay in bloom, reduced shoot vigor, and early autumn defoliation. In order to further characterize the changes occurring in trees inoculated with PLMVd, total fatty acid content was measured for floral buds during release from dormancy in `Coronet' peach trees. Palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids were the major fatty acids in dormant and releasing peach buds of both control and PLMVd-inoculated (VI) trees. The degree of unsaturation increased immediately following release from dormancy in both the control and VI trees. However, desaturation of linoleic acid to linolenic acid was significantly inhibited in VI trees, which was accompanied by a concomitant delay in the resumption of growth. The disparity between the control and VI trees in the progression of increased fatty acid unsaturation continued through petal fall. The presence of PLMVd in `Coronet' peach trees slowed membrane fatty acid desaturation during release from dormancy and suggested that metabolic pathways involving fatty acid desaturation were linked to the delayed phenology of the VI trees.

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