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A.R. Biggs and R. Scorza

Suberin accumulation in mechanically wounded bark tissue was determined fluorimetrically in greenhouse-grown peach [Prunus persica (L.) Batsch] and F2 progeny from peach × almond [P. amygdalus (Mill.) DA. Webb] hybrids. In general, suberin accumulation following wounding was significantly greater for progeny from almond-type than for peach-type hybrids. Hybrids from parents with almond tree type combined with peach fruit type accumulated the highest suberin levels. These data may partially explain the differences observed among peach and peach × almond hybrids in relative susceptibility to Leucostoma canker [Leucostoma persoonii Hohn. and L. cincta (Fr.) Hohn.] and injury caused by lesser peachtree borer. The association of higher suberin accumulation with specific phenotypic characteristics could simplify the selection of desirable seedlings in a breeding program that includes canker resistance as an objective.

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M.R. Pooler and R. Scorza

Shoots were regenerated from cotyledons of mature stored seed of three peach rootstock cultivars (`Flordaguard', `Nemared', and `Medaguard'). Shoot regeneration rates were highest when cotyledons were cultured for 3 weeks in darkness on Murashige and Skoog (MS) medium with 2.5% sucrose and a combination of IBA (1.25 or 2.5 μm) and TDZ (6.25 or 12.5 μm). Regeneration rates for `Flordaguard', `Nemared', and `Nemaguard' were as high as 60%, 33%, and 6%, respectively. Length of seed storage (1 to 3 years) did not affect regeneration rates. Seventy percent of regenerated shoots produced rooted plants. This regeneration method is rapid and simple, and stored seed can be used year-round. It may be a useful regeneration system for gene transfer in seed-propagated peach rootstocks. Chemical names used: 5 indole-3-butyric acid (IBA); thidiazuron (TDZ).

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W.R. Okie and R. Scorza

Willow-leaf peaches, reported by Lesley (1957) as a product of inbreeding but also mentioned as far back as 1887 (Hedrick, 1917), are characterized by a narrow leaf shape. We received willow-leaf germplasm from Wayne Sherman (Univ. of Florida, Gainesville), who selected a peach seedling with unusually narrow leaves from a group of seedling rootstocks. His original willow-leaf tree bore very small, poor-quality fruit. In 1983, it was used in breeding at the USDA-ARS breeding program at Byron to develop willow-leaf peaches with improved fruit types. After four generations, current selections are approaching commercial fruit standards in size, color, firmness, and attractiveness. Inheritance studies indicate the character is at least partially dominant and is expressed in some F1 seedlings of crosses with wild-type parents. However, the precise mode of inheritance remains unclear as the ratios do not fit common patterns. Progeny show a range of leaf narrowness, complicating characterization of genotype. The character may be useful in standard-type trees to enhance spray penetration, speed drying of the foliage to reduce disease, improve light penetration and photosynthetic efficiency, and make the fruit more visible to speed picking.

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M.R. Pooler and R. Scorza

Pollen from the doubled haploid peach [Prunus persica (L.) Batsch] `Hall-D' was irradiated with 0, 290, 530, 820, 1000, 5000, or 9000 Gray (Gy) of gamma radiation, 113 μW·cm-2 of ultraviolet (UV) radiation, or exposed to 100 °C for 2 h. In vitro pollen germination percentages were recorded and pollen was used to pollinate more than 10,000 emasculated or male-sterile peach flowers. Although pollen germination in vitro was stimulated by <1000 Gy of gamma irradiation, seed set following pollination was greatly reduced in all treatments. These results suggest that low levels of irradiation are sufficient to render pollen infertile while still maintaining germination capacity. Such results may be useful for pollination-induced parthenogenetic egg division for the production of maternally derived haploids and for the production of interspecific hybrids.

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T.J. Tworkoski and R. Scorza

Peach trees (Prunus persica L.) with diverse shoot growth habits have been developed, but little is known about their root systems. Characterizing shoot and root systems can improve basic understanding of peach tree growth and be important in the development of rootstocks and own-rooted trees. This research determined shoot and root characteristics of four peach tree growth habits (compact, dwarf, pillar, and standard). Seed from four peach growth habits were planted in 128-L containers, grown outside during the 1998 growing season, and then harvested. Compact tree leaf number (1350/tree) was twice, but leaf area (6 cm2/leaf) was half, that of pillar and standard trees. The number of lateral branches in compact trees (34) was nearly three-times more than in pillar and standard trees. The leaf area index (LAI) of pillar trees was greater than compact and standard trees (13 compared with 4 and 3, respectively) due to a narrower crown diameter. Dwarf tree shoots were distinct with few leaves (134 per tree) and a large LAI of 76. Compact trees grew more higher-order lateral roots than pillar and standard trees. More second-order lateral (SOL) roots were produced by compact than standard trees (1.2 vs. 0.8 SOL roots/cm first-order lateral root). Pillar trees had higher shoot-to-root dry weight ratios (2.4) than compact and standard trees (1.7 for both) due to smaller root dry weights. The results indicate fundamental differences in root characteristics among the peach tree growth habits. Compact trees had more higher order lateral roots in roots originating near the root collar (i.e., more fibrous roots), and this correlated with more lateral branches in the canopy. Shoot weights were the same among pillar, compact, and standard trees but root weights were less in pillar trees, resulting in greater shoot-to-root dry weight ratios. These results indicate significant differences in root as well as shoot architecture among growth habits that can affect their use as scion or rootstock varieties.

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F.A. Hammerschlag and R. Scorza

Four peach [Prunus persica (L.) Batsch] scion cultivars, `Jerseyqueen', `Redskin', `Suncrest', and `Sunhigh', that were propagated by tissue culture techniques and by bud-grafting onto `Lovell' seedlings, were compared at Kearneysville, W.Va., and at Beltsville, Md. At Kearneysville, total fruit production was higher for tissue-cultured (TC) trees when compared with budded trees in the first 3 years of fruiting, whereas trunk diameter increases were generally larger for budded trees. In the following year, fruit production was similar for both TC and budded trees, although trunk diameter increases continued to be larger for budded trees. At Beltsville, fruit production was significantly higher for TC trees in 1987, the first fruiting season, but the same for both in the second season. Trunk diameter increases were larger for budded trees both years. Differences in tree growth and productivity in the early years of orchard establishment appeared to be related to the size of plants that were planted. Budded trees, which were smaller than TC trees at planting, increased in size faster than TC trees but were less productive. Crop efficiency was cultivar-specific, but differences among cultivars was less if trees were TC propagated. These results suggested that based on yield and growth, own-rooted TC trees should be an acceptable tree type for commercial orchards.

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D.M. Glenn and R. Scorza

In reciprocal grafts of tall (`Elberta' and `Loring') and dwarf (`Empress' and `Juseito') peach (Prunus persica Batsch.) phenotypes, we measured dry-matter partitioning, resistance to root system water flow, and phytohormone content of xylem exudate. Scion characteristics determined the phenotype and growth characteristics of the tree irrespective of the rootstock. Tall phenotypes had higher dry weight and lower root resistance to water flow than dwarf phenotypes. Cytokinin-like activity and auxin levels in xylem sap were higher in dwarf than in tall phenotypes; whereas gibberellin-like activity was unaffected by either rootstock or scion. The scion of peach influenced phytohormone levels and resistance to water flow in the root system in addition to root and shoot growth.

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D. Bassi, A. Dima and R. Scorza

The response of young, nonbearing peach [Prunus persica (L.) Batsch] trees to pruning was studied in six distinct growth forms including semidwarf, spur-type, upright, columnar or pillar, weeping, and standard. Two years after field planting, pillar and upright trees were trained to slender spindle. Semidwarf, spur-type, and standard trees were trained to the open or delayed vase form. Weeping trees were pruned in a manner similar to the Lepage hedge for pear. Branch density before pruning was highest in semidwarf, spur-type, and upright trees and lowest in pillar trees. Standard, semidwarf, and spur-type trees reacted similarly to pruning, but semidwarf trees produced as much wood in the following season as had been pruned off, and produced large numbers of fruiting branches. The small size of semidwarf trees suggested their use for medium-density plantings (MDPs). Pillar trees needed only light pruning. No major cuts were necessary and many fruiting branches were produced even on nonpruned trees. The pillar canopy was 60% thinner and required 50% fewer pruning cuts than the standard canopy and may be particularly suited to high-density plantings (HDPs). The upper canopy of weeping trees grew more than most other forms. They were intermediate in branch density and required an intermediate amount of pruning. Most striking was the unique canopy form of weeping trees, which may be used in developing new training systems. The results of this study suggest that new growth forms have the potential to reduce pruning and training requirements for peach, particularly in MDPs and HDPs. This potential suggests further investigation and exploitation of alternate peach tree growth forms.

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D. Michael Glenn, R. Scorza and C. Bassett

A chimeric, willow-leaf mutation of a standard peach [Prunus persica (L.) Batsch.] phenotype was evaluated for its water use efficiency (WUE). The willow-leaf phenotype had greater WUE than its standard-leaf parent under both nonstressed and well-watered conditions, and this was supported by isotopic carbon discrimination. Under water-stress conditions, willow-leaf trees developed less water potential gradient from the roots to the leaves. The mechanisms associated with increased WUE by the willow-leaf phenotype include a reduced water potential gradient within the plant and uncoupling of the leaf from the aerial environment. Willow-leaf peach trees in seedling populations, descended from a different willow-leaf parent, also had reduced carbon isotopic discrimination than did sibling standard-leaf seedlings.

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D. Giovannini, D.M. Glenn and R. Scorza

The objective was to study selected physiological characteristics of the canopy and examine changes in dry matter partitioning between the root and shoot in two genetically reduced size growth types (dwarf and pillar) relative to the standard growth type. The dwarf phenotype had reduced leaf/root ratio, less allocation of dry matter to woody tissue and more to leaf tissue, high net photosynthesis, and lower leaf respiration compared to the standard and pillar phenotypes. The dwarf and pillar types had greater resistance to water flow than the standard type. Genetic changes in growth habit significantly alter many physiological parameters of peach tree growth and structure.