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  • Author or Editor: Desmond R. Layne x
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White-fleshed peaches and nectarines are delicacies that have been enjoyed for centuries around the world. They are native to China and were introduced to the United States in the 1800s. Some white-fleshed peaches and nectarines are highly perishable and bruise easily, but are of very high eating quality. These are perhaps best suited for the local roadside market, where they can be sold and consumed more quickly. Others are much firmer at harvest, have a longer shelf life. and are suitable for long-distance transport to wholesale markets. White-fleshed peaches and nectarines may have some acidity or they may be very low acid with high sugar content (°Brix). Some novel flat (peento or donut) types also exist. Proximity to an urban market with a substantial Asian population is advantageous because Asians, in particular, often prefer the low-acid flavor and are willing to pay premium prices for high quality fruits. In our peach and nectarine cultivar evaluation program at Clemson University, we are currently evaluating 70 cultivars and advanced selections at four different locations in South Carolina. Several of these have been evaluated since 2000 and the “top performers” over the last six seasons by ripening date (earliest to latest) include the following: `Sugar May', `Scarletpearl', `Snowbrite', `Southernpearl', `White Lady', `Sugar Lady', `Summer Sweet', `Sugar Giant', `Stark's Summer Pearl', `Snow King', and `Snow Giant'. In general, most of the white nectarines and the flat/donut peaches and nectarines have serious problems with insect damage and brown rot. Complete details of our peach and nectarine (yellow- and white-flesh) evaluation work in South Carolina since 2000 will be noted by referring to my peach website (http://www.clemson.edu/hort/Peach/index.php).

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In 1993, PPF and KSU embarked on a joint venture to test within pawpaw's native range many of the commercially available named pawpaw cultivars and PPF's advanced selections. Orchards for RVT were planted in 17 locations from Fall 1995 through Fall 1996 (possibly into 1997) consisting of 300 trees each. At each RVT site, eight replicate trees of each of the 28 grafted scion varieties will be tested in a randomized complete-block design. Named varieties that are secured for testing include Middletown, Mitchell, NC-1, Overleese, PA-Golden, Sunflower, Taylor, Taytwo, Wells, and Wilson. The other 18 clones to be evaluated originated in PPF orchards at the Univ. of Maryland Experiment Stations at Wye and Keedysville. Seedling trees from local native sources were planted around the perimeter as a buffer against edge effects and to allow comparisons with local germplasm. Identical orchards of the RVT are located in the following states: Arizona, Indiana, Iowa, Kentucky (two sites), Louisiana, Maryland, Michigan, Nebraska, New York, North Carolina, Ohio, Oregon, South Carolina, Tennessee (two sites), and the Chinese Academy of Forestry, Beijing, China. An orchard of nonidentical design is located in Florida. Additional sites in Connecticut and Chile are contemplated. Variables being studied in the trial include climate, culture, pests, growth, flowering, yield, and fruit characteristics. Trees will be evaluated for several years for yield, year-to-year consistency, regional suitability, etc. At the end of the trial period, regional recommendations will be made. Scion–rootstock compatibility based on percent scion take, scion growth (scion height and cross-sectional area), and first year field data are presented and discussed.

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The source-sink ratio of l-year-old, potted `Montmorency' sour cherry (Prunus cerasus) trees was manipulated by partial defoliation (D) or continuous lighting (CL) to investigate the phenomenon of end-product inhibition of photosynthesis. Within 24 hours of D, net CO2 assimilation rate (A) of the most recently expanded source leaves of D plants was significantly higher than nondefoliated (control) plants throughout the diurnal photoperiod. Between 2 and 7 days after D, A was 30% to 50% higher and stomatal conductance rate (g,) was 50% to 100% higher than in controls. Estimated carboxylation efficiency(k) and ribulose-1,5-bisphosphate (RuBP) regeneration rate increased significantly within 2 days and remained consistently higher for up to 9 days after D. Leaf starch concentration and dark respiration rate decreased but sorbitol and sucrose concentration increased after D. The diurnal decline in A in the afternoon after D may have been due to feedback inhibition from accumulation of soluble carbohydrates (sucrose and sorbitol) in the cytosol. This diurnal decline indicated that trees were sink limited. By 9 days after D, photochemical efficiency was significantly higher than in control plants. In the long term, leaf senescence was delayed as indicated by higher A and gs in combination with higher chlorophyll content up to 32 days after D. CL resulted in a significant reduction of A, gs, k, variable chlorophyll fluorescence (Fv), photochemical efficiency, and estimated RuBP regeneration rate of the most recently expanded source leaves within 1 day. During the exposure to CL, A was reduced 2- to 3-fold and k was reduced up to 4-fold. The normal linear relationship between A and gs was uncoupled under CL indicating that A was not primarily limited by gs and since internal CO2 concentration was not significantly affected, the physical limitation to A imposed by the stomata was negligible. The decrease in Fv and photochemical efficiency indicated that leaves were photoinhibited within 1 day. The decrease in instantaneous chlorophyll fluorescence after at least 1 day of CL indicated that there was a reversible regulatory mechanism whereby the damage to photosystem II reaction centers was repaired. Leaf chlorophyll content was not altered by 1,2, or 3 days of exposure to CL, indicating that photooxidation of chlorophytl did not occur. The time to full photosynthetic recovery from CL increased as the duration of exposure increased. CL plants that were photoinhibited accumulated significant starch in the chloroplast in a companion study (Layne and Flore, 1993) and it is possible that an orthophosphate limitation in the chloroplast stroma was occurring. D plants that were continuously illuminated were not photosynthetically inhibited. After 7 days of CL, plants that were then partially defoliated yet remained in CL photosynthetically recovered within 5 days to pre-CL values. Under the conditions of this investigation, end-product inhibition of A occurred in young, potted sour cherry trees but the mechanism of action in D plants was different than in CL plants.

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The leaf surface area of l-year-old, potted `Montmorency' sour cherry (Prunus cerasus L.) trees was reduced by punching disks from some or all leaves to determine the threshold level of leaf area removal (LAR) necessary to reduce net CO2 assimilation (A) and whole-plant growth. Removal of 30% of the leaf area of individual leaves reduced A on a whole-leaf basis between 1 and 3 weeks following LAR. Less than 30% LAR was compensated for by higher estimated carboxylation efficiency and ribulose-l,5-bisphosphate (RuBP) regeneration capacity. The threshold level of LAR based on gas exchange of individual leaves was 20%. Although whole-plant dry weight accumulation was reduced at all levels of LAR, a disproportionately large decrease in dry weight occurred as LAR increased from 20% to 30%. This result indicates that 30% LAR exceeded the threshold LAR level that was noted for A (20% LAR). Wound ethylene production induced by leaf-punching ceased after 24 hours, which indicated that wounds had healed and that ethylene, therefore, did not influence A significantly. The observed threshoId of 20% LAR represents a significant compensation ability for sour cherry, but this threshold may change with crop load, environment, or both.

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The influence of increasing levels of trunk damage on vegetative and reproductive capacity of 3- to 5-year-old `Montmorency' sour cherry (Prunus cerasus L.) trees was determined for three seasons. Removal of or damage to bark up to halfway around the trunk circumference minimally affected growth and productivity. The total wound callus produced per tree was related to wound size. Wound repair was variable depending on the type or extent of injury. Removal of damaged bark greatly reduced wound repair. Girdling 75% or 100% of the trunk circumference resulted in no tree mortality at one site and 17% and 50% mortality, respectively, at another. Differentiated phloem in wound callus of trees with 100% bark removal and survival 4 years following injury indicated that vascular reconnection occurred across wounds.

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In 2003, a replicated long-term research trial was established on a commercial peach replant site with a history of Armillaria root rot and other soilborne diseases. The objectives of the trial were to determine the short- and long-term effects of preplant fumigation, rootstock, and preplant root dipping with mycorrhizal fungi and beneficial bacteria on tree growth, productivity, and survival. Preplant fumigants included none (control), methyl bromide, Telone II, or Enzone. Rootstocks tested included Guardian, Lovell, and Halford. Root dipping (or not) was with MycorTree. The scion cultivar was Big Red. There were a total of 24 experimental treatment combinations and the trial site comprised more than 1500 trees on 11.5 acres. By 2 years after planting, fumigation with Enzone was disadvantageous when compared with no treatment at all. Enzone-treated blocks had higher tree mortality or were significantly reduced in growth compared to other treatments. Preplant fumigation with Telone II or methyl bromide, however, resulted in reduced tree stunting and phytotoxicity and increased tree growth when compared to the untreated control. After 2 years, 10% of the total trees planted were dead. Half of these were from the Enzone treatment. Enzone does not appear to be a viable preplant fumigation product for South Carolina peach growers, based on this preliminary data. Both Guardian and Halford rootstocks had performance superior to Lovell during the first 2 years. Although Guardian trees were smaller than Halford at the time of planting, by the end of the second growing season, their TCA was not significantly different. There was no benefit to preplant root dipping with MycorTree. Experimental results were not influenced by the location of trees on the site.

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This experiment was designed to determine the optimal light level for growing pawpaw seedlings in the greenhouse. In addition, we wanted to determine if modifying the root-zone would positively impact pawpaw seedling growth and development. Experimental treatments were imposed from seed sowing until the plants were destructively harvested. The experimental design was a split-plot, where blocking was done by position in the greenhouse. The main plot of the experiment was shade. This was accomplished by growing seedlings under a wooden frame covered with shadecloth to reduce incident light intensity received by the plant by 30%, 55%, 80%, or 95%. The control treatment was 0% shade or ambient greenhouse light level. The split-plot was root-zone modification. Half of all growing containers were untreated (control) while the other half were painted with SpinOut™, a commercially available product used to reduce root spiraling in nursery containers. There were 40 replicate seedlings per experimental treatment combination per block. Seedling shoot length and unfolded leaf number was recorded twice a week from seedling emergence until destructive harvest. Whole-plant leaf area was also determined. Leaves, stems, and tap and lateral roots were separated and dried to determine biomass partitioned to the respective organs. Up to 55% shade did not significantly reduce whole-plant biomass, while plants at 80% and 95% shade were stunted. Shade in the greenhouse is not required as was previously thought. Specific leaf mass and lateral root mass decreased as shade increased. Neither tap or lateral root dry weights were significantly affected by root-zone modification. New recommendations for container production of pawpaws in the greenhouse will be discussed.

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