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Stephen M. Southwick and Kitren G. Weis

Selection and propagation of rootstocks for apricot (Prunus armeniaca L.) varies worldwide in response to local climate, soils, and cultivars. In this paper we review published research focused on these local selective practices. Additionally, we review the current development of apricot rootstocks and suggest new research avenues to satisfy the needs of commercial apricot growers. Rootstocks are identified by their responses to biotic and environmental stresses, with specific adaptive characteristics that enable establishment and production under unique zonal ecologies. Desirable characteristics include scion compatibility, adaptation for heavy or wet soils, pest and disease resistance, ease of propagation, control of vegetative vigor, effects on dormant season physiology of the scion, precocity, fruit quality, and productivity. Interstocks that can overcome incompatible rootstock-scion combinations are covered. As worldwide consumer demand for apricots increases with improved apricot cultivars, rootstock selections and propagation must be developed for niche fruit with specific characteristics, intensive production systems, mechanized harvest, and marginal site selection.

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Kitren G. Weis and Barbara D. Webster

The drought-adapted, disease-resistant tepary bean (Phaseolus acutifolius A. Gray. var. acutifolius) is of great value as a potential gene donor of useful traits to the common bean (P. vulgaris L.). Analyses of flowering and fruiting patterns of tepary indicate that anthesis and abscission of reproductive structures within a raceme follow well-defined spatial and temporal patterns. Flowering occurs acropetally, and the probability of flowering decreases with distance from the most basal bud of the raceme. The probability of bud or pod abortion increases with distance from the basal bud, and the rate of abortion is highest in buds and pods proximal to the apex. Buds that never reach anthesis abort in the green-bud stage of development and aborting fruits cease development within the first 25% increase in pod length. In nonaborting fruits, the rate of seed abortion is 6%. A marked increase in abscission of all buds and fruits at all raceme nodes occurs before cessation of flowering.

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Stephen M. Southwick, Kitren G. Weis, and James T. Yeager

Hand thinning fruit is required every season to ensure large fruit size of `Loadel' cling peach [Prunus persica (L.) Batsch] in California. Chemical thinning may lower costs of hand thinning. A surfactant, Armothin {[N,N-bis 2-(omega-hydroxypolyoxyethylene/polyoxypropylene) ethyl alkylamine]; AKZO-Nobel, Chicago; AR}, was sprayed at 80% of full bloom (FB), FB, and FB + 3 days. The spray volume was 935 liters/ha. Concentrations of AR were 1%, 3%, and 5% (v/v). An early hand thinning in late April, a normal hand thinning at 13 days before standard reference date (early May), and a nonthinned control were compared to bloom-thinned trees for set, yield, and fruit quality. AR resulted in no damage to fruit; however, slight leaf yellowing and burn and small shoot dieback were seen at the 5% concentration. Fruit set, and therefore, the number of fruit that had to be hand thinned, were reduced with 3% AR applied at 80% FB and 5% AR applied at all bloom phenophases (stages of bloom development). Thinning time was reduced by 37% (5% AR applied at 80% FB), 28% (5% applied at FB), and by 20% (3% applied at 80% of FB), compared to the normally hand-thinned control. Although AR resulted in early size (cross suture diameter and weight) advantages, at harvest there were no significant differences in fruit size among all AR treatments and the normally hand-thinned control. Total and salable yields of AR treatments and the normally hand-thinned control were equal. Armothin shows promise for chemical thinning of peach when used as a bloom thinner.

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Kitren G. Weis, Stephen M. Southwick, and George C. Martin

Gibberellic acid reduces return bloom in many fruit tree species. Reducing bloom may cut costs of hand thinning apricot, peach and plum fruit. Sprays of 250 ppm GA, during floral bud evocation (June 1993) resulted in bud death and abscission as determined by light microscopy sections in `Patterson' apricot (Prunus armeniaca L). GA treatment in May did not cause observable effects. August treatments, immediately prior to floral initiation, did not impede differentiation. Treatment of `Elegant Lady' peach (Prunus persica [L.] Batsch.) buds with 75-250 ppm GA, in late June, 1993 (evocation phase) did not have any discernable effects in that season with respect to abscission or differentiation. Treated peach buds differentiated simultaneously with untreated buds in early August. The patterns of response to GA treatment imply `windows of opportunity' with respect to effectiveness of GA treatments. The specific response suggests that apricot buds possess differing levels of sensitivity to GA treatment and probably reflect distinct phases in transition to flowering. In August buds were already `determined' and were in a potentially floral state that was irreversible.

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Stephen M. Southwick, James T. Yeager, and Kitren G. Weis

`Loadel' cling peach [Prunus persica (L.) Batsch] trees were sprayed with Release® LC (Abbott Laboratories, North Chicago, Ill.) in 1993. Preharvest (harvested 16 July) sprays of 50, 75, 100, and 120 ppm applied on 15 June improved fruit firmness without altering fruit maturity (flesh color by commercial standards) in 1993. In the following 1994 season, flower number per centimeter of shoot length was reduced by sprays ranging from 50 to 120 ppm applied on 15 June and 9 July. No hand-thinning was required on trees treated on 15 June. Trees treated 9 July had 50% fewer fruit removed than on untreated trees, where more than 3000 fruit were removed by hand-thinning. Salable yield was higher than untreated control trees where Release® LC had been applied at 50 ppm on 15 June and 9 July. Fruit size equaled those of hand-thinned controls. As concentration increased on 15 June, salable yield decreased linearly. Fruit size (diameter and individual weight) increased with reductions in salable yield. Interestingly, fruit were evenly distributed along shoots after Release® LC treatment, similar to those found after hand-thinning. Release® LC will be available for commercial chemical thinning of stone fruit in California during 1995. Additional results from peach and other stone fruit will be presented.

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Tjasa Burnik-Tiefengraber, Kitren G. Weis, Barbara D. Webster, George C. Martin, and Hisashi Yamada

When continuously stem-fed with 75 mm NaH2PO4, `Manzanillo' olive explants showed significant leaf abscission after 48 hours; by that time 1.042 mg·g-1 fresh weight P had accumulated in the abscission zone (AZ). The potential contribution of ethylene to phosphate-enhanced abscission was investigated using aminooxyacetic acid (AOA), an ethylene-synthesis inhibitor, and by measuring ethylene evolution in phosphate-treated explants. In combination with NaH2PO4, AOA did not affect leaf abscission. Though ethylene evolution from explants increased as leaf abscission was initiated, it was about two orders of magnitude less than the concentration necessary to induce leaf abscission as judged by exogenous treatments. Based on leaf-abscission kinetics, we have concluded that the mechanism of P-induced abscission is independent of gross measurement of evolved ethylene, but we cannot rule out ethylene confined to the AZ itself. When evaluated for P-induced leaf abscission, leaves of `Manzanillo' and `Sevillano' abscised earlier than `Ascolano' and `Mission'.

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Tjasa B. Tiefengraber, Kitren G. Weis, George C. Martin, and Barbara D. Webster

Ethephon reduces olive fruit removal force but also results in leaf loss when used as a harvest fruit loosening agent and in reduction of flower production in the subsequent year. Phosphorus (P) has been implicated in the fruit loosening process. Nuclear magnetic resonance analyses indicate that P accumulates rapidly in olive leaf explant abscission zones. P also causes ethylene evolution prior to abscission; this effect appears to be direct. In combination with AOA an ethylene synthesis inhibitor, P accelerates `Manzanillo' leaf explants abscission, inducing significant abscission 3 days after treatment. These results will assist in development of a P use strategy that leads to fruit but not leaf abscission.

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Stephen M. Southwick, Kitren G. Weis, James T. Yeager, and Hong Zhou

Whole-tree sprays of Release LC [predominantly gibberellic acid] (GA,) were applied in a commercial peach [Prunus perisca (L.) Batsch.] orchard in the California Central Valley on three dates from mid-June (about 90 days after full bloom = 28 days before harvest) to late July (14 days postharvest) 1993 at 50, 75, 100, and 120 mg·liter-1. Gibberellin (GA) reduced the number of flowers differentiated in 1993, thereby reducing fruit density in 1994, when sprays were applied by early July 1993. Sprays in late July did not reduce flowering and fruiting density in the following year. In 1994, there were fewer fruit located on the proximal third of the shoot after GA sprays of 75,100, and 120 mg-liter' applied on 15 June compared to hand-thinned controls, and reduction was linear with increase in GA rate. Fruit numbers in the middle and distal sections of shoots were reduced by all 15 June and some 9 July GA sprays, with fewer fruit as concentration increased. However, the distribution of fruit within shoot sections, after GA treatments during floral differentiation, expressed as a percentage of the total number of fruit along fruiting shoots, showed even fruiting compared with hand thinning. Due to reduced flowering in response to GA treatments in June and early July 1993, the hand-thinning requirement was significantly reduced, with no thinning required in 1994 from 15 June 1993 GA sprays. All sprays applied in early July resulted in 40% to 60% fewer fruit removed during thinning than the nontreated controls. Sprays in late July were ineffective. Sprays of GA applied in mid-June at 50,75, 100, and 120 mg·liter and sprays of 120 mg·liter-1 GA applied in early July (4 days preharvest) increased the firmness of `Loadel' cling peach (about 26% improvement in June sprays) in 1993. The salable yield of fruit (after removal of the undersized fruit) was the same on hand thinned and on non-hand thinned trees treated with GA on 15 June at 50 mg·liter-1. The salable yield of fruit was increased by GA sprays of 50 and 75 mg·liter applied on 9 July 1993 compared to controls. There were no differences in fruit size (by weight or diameter) among the aforementioned treatments and hand thinning. GA sprays of 75,100, and 120 mg-liter' applied on 15 June 1993 tended to reduce salable yield, but fruit size increased with decreased yield. Based on the results obtained in 1993 and 1994, we believe that Release LC has good potential for chemically thinning peaches in California.

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Stephen M. Southwick, Kitren G. Weis, James T. Yeager, Michael E. Rupert, and Janine K. Hasey

In 1994, we established that a surfactant, Armothin (AR), reduced fruit set when applied as 3% and 5% AR at 100 gal/acre with a Stihl mistblower to `Loadel' clingstone peach [Prunus persica (L.) Batsch]. In 1995 we compared 3% AR at volumes of 100 and 200 gal/acre (935 and 1870 L.ha-1, the volumes most commonly used by tree fruit growers in California) applied with commercial airblast sprayer; overthinning resulted with the latter. In 1996, we applied 3% AR at 100 gal/acre and 1% AR at 200 gal/acre. In 1995, differential applications of 3% AR at 100 gal/acre (two-thirds of the material applied to either the upper or lower canopy) reduced fruit set in the upper canopy in proportion to the amount of chemical applied (twice as much fruit set reduction with twice as much chemical); fruit set in the lower canopy was reduced by an equal amount regardless of amount of chemical used. Salable yields, equivalent to those obtained by hand thinning, and improved fruit size were achieved with all treatments of 3% AR at 100 gal/acre in 1995 with a 76% reduction in hand thinning. Following a low-chill winter (1995-96) with a protracted bloom, flower bud density (return bloom) was significantly greater in 1995 AR-treated trees. In 1996, treatment with AR did not result in fruit set reduction due to the protracted bloom and poor weather conditions before and after bloom. Nonetheless, 1% AR at 200 gal/acre applied in 1996 increased salable yield and increased final fruit mass. Return bloom in 1997 was equal among 1996 treatments.