Preharvest Fruit Drop Research Articles

Regional differences in performance of Canadian-bred apple cultivars and implications for breeding

To evaluate their commercial potential in several major production regions of Canada, 10 Canadian breeding selections and two standard cultivars of apple (Malus × domestica Borkh.) were planted at four locations: one in British Columbia, two in Ontario and one in Nova Scotia. Subsidiary objectives of the trial were to determine whether wider testing of promising apple selections was warranted, and to accumulate information to help guide breeders in selecting apples for widespread adaptation. At each site the planting was arranged in a randomized complete block design with eight replicates. Individual trees were the unit of replication. Data were collected for 7 yr (5 cropping years). The measures of orchard performance recorded were: tree survival, trunk-cross sectional area, final tree height and canopy spread, precocity, times of bloom and harvest, yield, yield efficiency, and average fruit size. The effects of location, genotype and genotype × location interaction were statistically significant for all response variables. The two standard cultivars, Royal Gala and Summerland McIntosh, were intermediate to high in precocity, tree vigour, yield and fruit size. No single cultivar was superior in these characteristics at all locations. Among the test selections, S23-06-153 and 8S-27-43 had the most consistently good performance with regard to tree survival, tree vigour, yield, yield efficiency, pre-harvest fruit drop and fruit size. 8S6923 (Aurora Golden Gala™) and Silken also performed well, except that fruit size was smaller in eastern Canada. Many of the selections took fewer days to reach maturity in Nova Scotia and Ontario than they did in British Columbia. The results obtained suggest that regional testing is highly desirable for characteristics such as tree survival, yield, pre-harvest drop and climatic suitability (e.g. dates of bloom and harvest), all of which profoundly affect the commercial desirability of the cultivar. If the present results can be generalized, breeding and selection of new genotypes at one location are probably adequate for tree vigour, relative order of bloom and harvest, and precocity, despite the genotype × environment interaction demonstated. For these characteristics, the modest changes in rank among cultivars from location to location do not have great practical or economic consequences. Selection at one location may also be effective for fruit size and percentage of over colour, if the breeder takes location effects into account (smaller fruit in Ontario and Nova Scotia, less colour on early-harvest apples in British Columbia). Key words: Fruit breeding, cultivar testing

Gibberellins and N-(2-chloro-4-pyridyl)-N′-phenylurea improve retention force and reduce water core in pre-mature fruit of Japanese pear cv. Housui

A study of the effect of a gibberellin A3 + A4 mixture (GAs) on pre-harvest fruit drop of Japanese pear cv. Housui is reported. The GAs was applied alone or in combination with N-(2-Chloro-4-pyridyl)-N′-phenylurea (CPPU) in a lanolin paste to the abscission zone at the spur-end of the pedicel. The results showed that the GAs and CPPU combination treatment increased the pedicel–spur retention force relative to that of the untreated controls. Histological studies showed an accelerated rate of cambium division and the development of secondary xylem in the abscission zone near the spur-end of pedicels treated with the GAs plus CPPU paste, which delayed the formation of the abscission zone. Fruit quality (sugar, acid, firmness, color) was not adversely affected by the application of GAs plus CPPU, although the application of GAs alone promoted ripening. In contrast, the addition of CPPU to the two GAs delayed fruit ripening, which was measured as ethylene efflux. All treatments were without adverse effects on return bloom, measured as bud size. The CPPU plus GAs treatment also suppressed the incidence of water core, whereas the application of the GAs alone accelerated water core in this water core susceptible pear variety.

Ethylene and preharvest drop: the effect of AVG and NAA on fruit abscission in apple (Malus domestica L. Borkh)

l-Aminoethoxyvinylglycine (AVG) and 1-naphthylacetic acid (NAA) are known to affect preharvest fruit drop, fruit quality and fruit maturation in Golden Delicious apples (Malus × domestica Borkh). Experiments were carried out on GD/M9 trees treated at three different developmental stages (41, 28 and 17 days before the beginning of the commercial harvest) with AVG and NAA. Both chemicals significantly reduced fruit drop without significantly affecting the fruit weight. Background colour development and ripening were both delayed by AVG, whereas NAA significantly enhanced yellowing without affecting the evolution of ripening. Ethylene evolution and transcription profiles of genes involved in ethylene biosynthesis (MdACS1 and MdACO1) and action (MdETR1, MdERS1 and MdCTR1) were monitored in cortex from the date of the first treatment until the beginning of fruit drop in the control trees (21 days after the beginning of commercial harvest). AVG blocked or efficiently reduced the ethylene evolution. This effect was paralleled by a down-regulation of MdACS1, MdACO1, MdETR1 and MdERS1. NAA at the second and third date of application enhanced the onset of ethylene evolution, although, at the end of the experiment, no difference was found between control and treated fruits. The chemical applied in the first date significantly down-regulated the transcription of the genes at the end of the experiment. MdCTR1 expression, basically unaffected by AVG and NAA, appeared to be transiently down-regulated. The initial down-regulation is under developmental control, whereas the late regain of transcript accumulation paralleled the ethylene evolution.

Effect of Sprayable 1-MCP, AVG, and NAA on Ethylene Biosynthesis, Preharvest Fruit Drop, Fruit Maturity, and Quality of ‘Delicious’ Apples

Effects of naphthaleneacetic acid (NAA), aminoethoxyvinylglycine (AVG), and sprayable 1-methylcyclopropene (1-MCP) alone or in combination on fruit ethylene production, preharvest fruit drop, fruit quality, and fruit maturation were examined in ‘Delicious’ apples (Malus ×domestica Borkh.). 1-MCP and AVG + NAA, when applied 15 days before anticipated harvest (DBAH) for untreated control trees, more effectively delayed preharvest fruit drop than AVG or NAA used alone. However, there was no significant difference in ethylene production between fruit treated with 1-MCP or AVG + NAA and those treated by AVG. Two applications of NAA increased fruit ethylene production and fruit softening, whereas AVG inhibited NAA-enhanced fruit ethylene production and fruit softening. There was no significant difference in fruit ethylene production, fruit firmness, and fruit drop control between one and two applications of 1-MCP. The concentrations of 1-MCP did not affect the efficacy of 1-MCP when applied 15 DBAH, but high concentration of 1-MCP more effectively delayed preharvest fruit drop than low concentration of 1-MCP when applied 7 DBAH. Both AVG and 1-MCP suppressed expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase gene MdACS1, ACC oxidase gene MdACO1, and polygalacturonase gene MdPG1 in fruit. Expression of ACS5A and MdACO1 but not MdACS1 in fruit abscission zones was decreased by AVG and 1-MCP. 1-MCP more effectively suppressed expression of MdPG2 in fruit abscission zones than AVG alone.

Eficácia do tratamento de avg no controle da queda e maturação dos frutos de maçã, cultivar imperial gala

O objetivo deste trabalho foi avaliar o efeito de concentrações e épocas de aplicação do AVG no controle da queda pré-colheita, no retardamento da maturação e na qualidade de maçãs cv. Imperial Gala. Testaram-se 4 épocas de aplicação (45; 30; 15 e 7 Dias Antes do início do Ponto de Colheita presumido da testemunha - DAPC) e 4 concentrações (0; 90; 124 e 62 + 62 g.ha-1), em dois anos, 2003 e 2004. A degradação do amido, a queda da firmeza da polpa, o aumento do teor de sólidos solúveis totais (SST) e as mudanças da cor dos frutos tratados com AVG foram retardados em relação aos frutos não-tratados. Conforme esperado, AVG retardou o ponto de colheita comercial em 7 a 16 dias e a queda pré-colheita dos frutos em até 30 dias. Os efeitos do AVG no retardamento da maturação foram dependentes da concentração e da época de aplicação. Aplicações de AVG mais próximas do ponto de colheita comercial, 7 DAPC, foram mais efetivas no controle de queda de frutos pré-colheita e no retardamento da perda de firmeza do que aplicações mais precoces (30 ou 45 DAPC). Doses de 124 e 90 g.ha-1 de AVG são igualmente efetivas quando aplicadas 7 ou 15 dias antes do ponto de colheita comercial. O retardamento da maturação e o controle da queda pré-colheita foi tanto maior quanto maior a dose e mais próximo do ponto de colheita comercial foi a aplicação do AVG. Os resultados demonstram que é possível manejar a intensidade de retardamento da colheita alterando a época de aplicação entre 7 e 30 DAPC e a dose de AVG entre 90 a 124 g.ha-1.

Effects of NAA, AVG, and 1-MCP on Ethylene Biosynthesis, Preharvest Fruit Drop, Fruit Maturity, and Quality of ‘Golden Supreme’ and ‘Golden Delicious’ Apples

Effects of naphthaleneacetic acid (NAA), aminoethoxyvinylglycine (AVG), and 1-methylcyclopropene (1-MCP) alone or in combination on fruit ethylene production, preharvest fruit drop, fruit quality, and fruit maturation were examined in ‘Golden Supreme’ and ‘Golden Delicious’ apples (Malus ×domestica Borkh.). In ‘Golden Supreme’ apples, the combination of two applications of AVG and one application of NAA 3 and 1 week, respectively, before the anticipated optimum harvest date synergistically inhibited fruit ethylene production and delayed fruit drop and ripening. Compared with one or two applications of AVG, the combination of one application of AVG and two applications of NAA had much lower preharvest fruit drop, although there was no significant difference in fruit ethylene production among these treatments. In ‘Golden Delicious’ apples, 1-MCP at 396 mg·L−1 had a better effect in delaying fruit drop than did AVG at 125 mg·L−1 or NAA at 20 mg·L−1 when they were applied a week before the optimum harvest date. The combination of NAA and 1-MCP or AVG was more effective in delaying fruit drop than were NAA, 1-MCP, or AVG alone. Fruit ethylene production was inhibited by 1-MCP and AVG but not by NAA. 1-MCP and AVG delayed fruit ripening, whereas NAA increased fruit ripening as determined by fruit firmness and starch.

Foliar application of aminoethoxyvinylglycine (AVG) delays fruit ripening and reduces pre-harvest fruit drop and ethylene production of bagged “Kogetsu” apples

Covering apple fruits with double layer waterproof bags to enhance fruit quality and evenness of blush colour is typical on many cultivars in Korea and Japan. Aminoethoxyvinylglycine (AVG) applied to unbagged apple fruits at 3–4 weeks before commercial harvest reduces ethylene production in the fruit, delays fruit ripening and reduces pre-harvest fruit drop. Spray application of AVG to trees of bagged apples should have no effect on apple ripening as there is␣no direct contact with the fruit and the translocation of AVG in apple trees is regarded as negligible. However, preliminary experiments suggested that AVG applied to trees of bagged apples reduced pre-harvest fruit drop in “Kotgetsu” apples. This study investigated the effect of spray treatments of 125 ppm of AVG on fruit drop, fruit ripening (firmness, starch conversion and soluble solids) and ethylene production to whole trees with bagged or unbagged “Kogetsu” fruit, as well as sprays of only the bagged or unbagged fruit on trees on two orchards. AVG applied to whole trees with unbagged apples reduced fruit drop from an average of 58.9% to 10.4%, delayed starch conversion and decreased ethylene production. AVG applied to whole trees with bagged fruit was equally effective in reducing pre-harvest drop, delaying fruit ripening and reducing ethylene production. Application of AVG to unbagged fruit only was nearly as effective as application to whole trees with unbagged fruit but application to bagged fruit only had no effect on fruit ripening or ethylene production. Application of AVG to bagged fruit only did reduce fruit drop to an average of 42.5% but this was not as effective as spraying unbagged fruit only or whole trees with bagged fruit. Possible mechanisms for this effect are discussed.

Foliar application of aminoethoxyvinylglycine (AVG) delays fruit ripening and reduces pre-harvest fruit drop and ethylene production of bagged “Kogetsu” apples

The germination stimulants for root parasitic plants Striga and Orobanche produced by sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), and pearl millet (Pennisetum typhoideum Rich.) were examined. Characterization of strigolactones in the root exudates from the plants grown hydroponically was conducted by comparing retention times of germination stimulants on reverse phase high performance liquid chromatography (HPLC) with those of synthetic standards, and by using HPLC linked with tandem mass spectrometry (LC/MS/MS). All the plants tested, except for a sorghum cultivar Swarna, were found to exude two major stimulants, 5-deoxy-strigol, which is known as a branching factor for arbuscular mycorrhizal (AM) fungi, and an isomer of strigol, tentatively named sorghumol. Swarna was found to exude 5-deoxy-strigol and strigol. These results imply that 5-deoxy-strigol is one of major germination stimulants of gramineous plants and that major stimulants may differ even among cultivars within the same species.

(98) Molecular Analysis of Abscission Layer Activation in Apple Fruit Pedicels

Abscission of leaves, floral organs, and fruit is a developmentally and environmentally regulated process initiated in specialized thin layers of cells within abscission zones (AZs). Very little is known about early molecular events that drive abscission, especially of fruit. Commercial apple production relies on the use of flower and fruit abscission-promoting and -inhibiting compounds to enhance fruit quality, control preharvest fruit drop, and maintain consistent annual bearing. The success of chemical treatments is strongly influenced by numerous factors, including environment, genotype, developmental stage of the fruit, and physiological state of the tree. Toward developing improved strategies for regulating fruit abscission, we carried out transcriptional profiling of competent-quiescent and activated abscission layers. We found that a decisive event in the sequential process of abscission layer development is the transcriptional activation of the MdPEL1 gene, encoding a plant pectate lyase protein and potentially involved in the degradation of the middle lamella of adjacent abscission layer cells. Additionally, regulatory elements of at least 12 homologous pectate lyase genes in Arabidopsis thaliana were found to direct expression in floral AZs and in dehiscence zones along valve margins, suggesting that these genes have evolutionary conserved function. This work identifies a novel role for pectate lyases in plants. Furthermore, many abscission-related genes identified in this study are being used to track biochemical and regulatory pathways that participate in abscission in response to chemical treatments or environmental effects.

Pre-harvest drop control of ‘Red Delicious’ and ‘Jonathan’ apple (Malus domestica) as affected by the synthetic auxin 3,5,6-TPA

Summary‘Red Delicious’, the main apple cultivar grown in Israel, and ‘Jonathan’, grown on a smaller scale, are prone to substantial pre-harvest drop. Spraying both cultivars with the synthetic auxin 3,5,6-trichloro-2-pyridyl-oxyacetic acid (3,5,6-TPA) reduced pre-harvest fruit drop and increased yield. Application of 40 mg l–1 3,5,6-TPA 2 weeks before anticipated harvest (WBH), or 30 mg l–1 3,5,6-TPA at 4 and 2 WBH gave the best results, and were as effective as 2,4,5-trichloro-phenoxypropionic acid (2,4,5-TP) which was the commercial product commonly in use until being withdrawn. There were no negative effects on fruit quality at harvest, or during 8 months of storage. Therefore 3,5,6-TPA can be recommended for routine use on ‘Red Delicious’ and ‘Jonathan’ apples in Israel.

EFFECTS OF 2,4-D, 2,4-DP, TRICLOPIR AND GA3 ON PRE-HARVEST FRUIT DROP AND SENESCENCE OF ´TAROCCO COMUNE´ BLOOD ORANGES IN SICILIAN ORCHARDS

'Tarocco comune' blood oranges [Citrus sinensis (L.) Osb.] suffer from a heavy natural abscission and senescence of mature fruit. The present experiments were performed to reduce this problem and consequently to expand the harvest season and to improve fruit marketing. During the period of 2000-2004 we have studied in four field experiments the effects of several plant growth regulators (PGRs) on senescence and incidence of pre-harvest fruit drop of blood oranges. The best effects of 2,4-dichlorophenoxyacetic acid (2,4-D), 2-(2,4-dichlorophenoxy) propanoic acid (2,4-DP), 3,5,6-trichloro-2-pyridyloxytacetic acid (triclopir) (45 to 10 mg L -1 ) and gibberellic acid (GA 3 ) (9.4 mg L -1 ) in delaying and reducing the undesired fruit abscission and senescence of 'Tarocco' blood oranges were achieved when sprays have been applied in October through December. GA 3 was found to delay changes in rind colour, an effect that can be considered either desirable or undesirable, depending on the harvest season. At harvest no residues of the PGRs were detected. However, currently only GA 3 and triclopir are authorized for pre-harvest use with citrus crops in Italy.

A pulverização pré-colheita com ácido giberélico (GA3) e aminoetoxivinilglicina (AVG) retarda a maturação e reduz as perdas de frutos na cultura do pessegueiro

Este trabalho objetivou avaliar os efeitos da pulverização pré-colheita com ácido giberélico (GA3) e aminoetoxivinilglicina (AVG) na queda pré-colheita, maturação e qualidade de pêssegos, da cultivar Rubidoux. O delineamento experimental foi em blocos inteiramente casualizados, com quatro repetições, consistindo de seis tratamentos, resultantes da combinação de duas doses de GA3 (0 e 100 mg L-1) e três doses de AVG (0; 75 e 150 mg L-1). O GA3 e o AVG foram pulverizados cerca de seis e três semanas antes do início da colheita comercial dos frutos, respectivamente. Tratamentos envolvendo a combinação de GA3 (100 mg L-1) e AVG (75 e 150 mg L-1) retardaram a maturação dos frutos na colheita e durante o armazenamento refrigerado (4 semanas a 0-2ºC/90-95% UR), ocasionando maior retenção de cor verde da casca, menor redução da firmeza de polpa, menor aumento no teor de sólidos solúveis totais e menor redução na acidez titulável. De forma geral, os efeitos mais expressivos foram observados para tratamentos com GA3 100 mg L-1 do que para tratamentos com AVG 75 e 150 mg L-1. O tratamento com GA3 100 mg L-1 também reduziu o número de frutos com rachaduras e podridões, aumentou o peso médio de frutos na colheita e reduziu a incidência de escurecimento da polpa após armazenamento refrigerado. O AVG aumentou a incidência de frutos rachados.

Effect of Aminoethoxyvinylglycine (AVG) on Preharvest Drop, Fruit Quality, and Maturation of `McIntosh' Apples. I. Concentration and Timing of Dilute Applications of AVG

To compare the effects of growth regulators on preharvest fruit drop and fruit maturity, aminoethoxyvinylglycine (AVG) was applied to `McIntosh' apple trees at 75, 150, or 225 mg·L-1, at 8, 4, or 2 weeks before harvest (WBH). These treatments were compared to NAA, daminozide, and to an untreated control. All AVG treatments and timings except 75 mg·L-1 applied 8 WBH delayed preharvest drop and fruit maturity. AVG applied at 225 mg·L-1was more effective in delaying drop and development of maturity than other rates when applied 8 or 2 WBH, but at 4 WBH, 150 mg·L-1 gave equivalent results to 225 mg·L-1. AVG at 150 mg·L-1 was superior to NAA or daminozide as a stop-drop agent. No concentration, or time of application of AVG influenced fruit size at harvest. AVG reduced internal ethylene concentration (IEC) in `McIntosh' apples linearly with increasing AVG concentration. There was a linear relationship between time of AVG application (8, 4, or 2 WBH) and IEC in the fruit after harvest, and the time required for harvested fruit to enter the ethylene climacteric. Development of red color was delayed by AVG. This was attributed to a delay in ripening as determined by a slower increase in IEC and starch hydrolysis. In general, earlier application of AVG resulted in reduced effectiveness of lowering IEC following harvest. Chemical names used: aminoethoxyvinylglycine (AVG), naphthaleneacetic acid (NAA), succinic acid-2,2-dimethylhydrazide (daminozide).

Involvement of Ethylene in the Pedicel in Preharvest Abscission of 'Tsugaru' Apple

The involvement of ethylene in the pedicel in preharvest fruit drop and the relationship between ethylene in the pedicel and its production in the flesh and the seed were investigated in 'Tsugaru' apple. The induction of pedicel abscission was achieved by the injection of ethylene with a syringe, whereas it was inhibited by 2, 5-norbornadiene (NBD), an antagonist of ethylene action. The injection of ethephon into the fruit core, or the spray on the bourse resulted in increasing ethylene content in the pedicel and subsequent fruit abscission. The treatment with aminoetoxy-vinylglicine (AVG) of the fruit core, the inhibitor of ethylene synthesis, reduced the fruit abscission and ethylene content in the pedicel. Ethylene production of the seed and the flesh, and its content in the pedicel in 'Tsugaru' greatly increased before the rapid increase in fruit drop. Fruit of 'Jonagold' usually do not exhibit preharvest drop, the ethylene production and ethylene content are low; they increased little before harvest. Therefore, ethylene likely originates in the flesh and seeds and is translocated to the pedicel where it acts to induce fruit abscission.

Effect of aminoethoxyvinilglycine (AVG) on preharvest fruit drop and maturity of apples

Apple trees cultivars Gala and Fuji were sprayed four weeks before commercial harvest with aminoethoxyvinilglycine (AVG), at doses of 0, 125, or 250 mg L-1, and assessed for preharvest fruit drop, fruit growth, and maturation on tree. In 'Gala', 64 days after AVG spraying, fruit drop for control treatment was 85%, and AVG (at 125 and 250 mg L-1) reduced it to 10%. In 'Fuji', 64 days after AVG spraying, fruit drop for control was 6%, while treatments with AVG (at 125 and 250 mg L-1) increased fruit drop to 10%. AVG was a powerful retardant of fruit maturation for 'Gala' but not for 'Fuji'. In 'Gala', the most affected attribute was the skin background color, followed, in decreasing order, by soluble solids content, the starch index, skin red color, the flesh firmness, and titratable acidity. In 'Gala', only flesh firmness retention was improved by increasing AVG dose from 125 mg L-1 to 250 mg L-1. The AVG at 250 mg L-1 inhibited "Gala" late fruit growth but not 'Fuji'.

NAA and 3,5,6-TPA Control Mature Fruit Drop in California Citrus

The synthetic auxins NAA and 3,5,6-TPA were investigated for reducing abscission of mature citrus fruit in California (CA). NAA was investigated on navel orange trees in San Joaquin Valley and southern CA locations. Of the seven NAA experiments presented, five had substantial fruit drop. In these five experiments, a treatment of NAA reduced drop by 31% to 88% compared to the untreated control. Although NAA treatments as low as 25 mg·L-1 (acid equivalent) reduced drop, the greatest reductions in drop were obtained by spray concentrations in the 100 to 400 mg·L-1 range. 3,5,6-TPA was investigated for fruit drop control properties on navel orange and grapefruit grown in various CA locations. The untreated control in seven of the ten 3,5,6-TPA experiments had substantial fruit drop. In each of these cases, a treatment of 10, 15 or 20 mg·L-1 (acid equivalent) of 3,5,6-TPA reduced drop 69% to 96% compared to the untreated control. A strong linear response from 3,5,6-TPA in these seven experiments indicates maximum fruit drop reduction from the highest rate investigated. On an acid equivalent basis 3,5,6-TPA seems to be comparable to 2,4-D. Both NAA and 3,5,6-TPA were effective in controlling preharvest fruit drop in citrus under CA conditions. Both materials provided fruit holding late into the harvest season. NAA, and in particular 3,5,6-TPA, offer the potential to provide a substitute for 2,4-D which is commonly used for controlling fruit drop in many countries. Chemical names used: naphthaleneacetic acid (NAA); 3,5,6-trichloro-2-pyridinyloxyacetic acid (3,5,6-TPA, triclopyr); 2,4-dichlorophenoxyacetic acid (2,4-D).

Relationships Among Day of Year of Drop, Seed Number, and Weight of Mature Apple Fruit

Preharvest fruit drop of apple [Malu×domestica (L.) Borkh.] can cause significant crop losses, but factors controlling date of individual fruit drop are unknown. In three types of experiments, we investigated the relationships among seed number/fruit, fruit weight, and day of year of drop. By shading in mid-May and stigma excision before bloom, we induced variability in seed number. Dropped fruit were weighed, and their seeds were counted daily from late August until all fruit had dropped. Nontreated trees were studied similarly. Regression analyses were used to assess relationships among day of drop, fruit weight, and seed number/fruit. Substantial variation in day of drop of individual fruit was not explained by seed number of the fruit in these experiments with `Smoothee Golden Delicious', `Redchief Delicious', and `Commander York'.

662 Molecular Mechanisms and Potential Control of Abscission in Horticultural Crops

The process of abscission results in shedding of plant parts such as leaves, fruit, flowers, and in citrus, shoot tips and entire shoots. Growers must successfully manage abscission in their operations to avoid unnecessary defoliation or loss of yield due to floral abscission or preharvest fruit drop. Conversely, abscission enhancement may be desired during harvest. Yet despite its importance to horticulture, little is known about mechanisms that control abscission. We know that abscission can be induced by ethylene and altered to some extent by auxin. Over the years, many physiological and anatomical events of abscission have been described. For example, cellulase, polygalacturonase and pectin methylesterase genes are induced during abscission, and they are thought to have a role in alteration and depolymerization of middle lamella polysaccharides located in the abscission zone area. Other genes, such as those associated with the process of pathogen resistance, are also induced during abscission. We are interested in using tools of molecular biology to examine abscission-related gene expression prior to organ separation in Florida field-grown Valencia orange (Citrus sinensis L. Osbeck) and greenhouse-grown calamondin (Citrus madurensis Loureiro) citrus trees. Subtractive cDNA library screening and differential display were used to examine gene expression in fruit, leaf and floral abscission zones 6, 24 and 48 h after induction of abscission with 5-chloro-3-methyl-4-nitro-1H-pyrazole or Ethrel® (Rhone-Poulenc, [2-chloroethyl] phosphoric acid). Some isolated cDNAs encoded polypeptides with no significant matches in the database or share significant similarities with unknown proteins isolated from Arabidopsis. Other cDNAs encoded polypeptides with similarity to cell wall modifying proteins such as polygalacturonases and expansin, PR proteins such as chitinase, proteins associated with secondary and xenobiotic metabolism such as amine oxidase, benzoquinone reductase, caffeic acid methyltransferase, phenylalanine ammonia lyase and squalene synthase, and proteins associated with signal transduction such as several serine/threonine kinases. Temporal and spatial expression of these genes and others will be presented. Use of this information to target potential points of abscission control will be discussed.

116 Preharvest Fruit Drop Studies, Harvest Quality, and Cold Storage of `Golden Delicious' and `Rome' Apples

Combinations of aminoethoxyvinylglycine (AVG, ReTain) and NAA gave better control of fruit drop of `Golden Delicious' than either alone. When the full rate of ReTain (50 g/A) was compared to a reduced rate of ReTain (86 g/ha) plus NAA, equivalent control of fruit drop of `Golden Delicious' resulted. ReTain delayed softening and starch depletion of `Golden Delicious' fruit. NAA in some cases promoted earlier fruit maturity; but when used in combination with ReTain, maturity was similar to ReTain-treated fruit. Fruit with the highest firmness and starch came out of cold storage in the best condition. Neither 1-methylcyclopropene (MCP, EthylBloc) or NAA inhibited fruit drop of `Golden Delicious' fruit when applied at harvest; but previous ReTain and NAA data indicate that late applications are frequently much less effective than if applied 4 weeks before harvest. Ethephon spray treatments caused more rapid and extensive fruit drop than the control. Trees gassed or sprayed with EthylBloc before ethephon sprays also dropped rapidly. `Golden Delicious' fruit on the tree were dramatically maintained firmer by the EthylBloc gas, and to a lesser extent by EthylBloc sprays by 19.1 N and 10.2 N firmness, respectively, tested on 28 Oct. Starch was maintained by the EthylBloc gas, but not by the sprays. These data indicated that EthylBloc applied as a gas or spray did have a physiological affect but did not control fruit drop. Fruit diameter, soluble solids and color did not appear to be affected. Further study of earlier applications of EthylBloc or combinations with fruit drop control agents may be needed to get fruit drop control. NAA plus Silwet L-77 inhibited fruit drop of `Law Rome', but none of the EthylBloc sprays inhibited fruit drop when applied at harvest. Previous data with ReTain and NAA indicated that late applications are frequently much less effective than if applied 4 weeks before harvest. EthylBloc sprays applied 21 Oct. dramatically maintained fruit firmness tested on 3 Nov. Starch was not maintained by the EthylBloc gas, but starch had almost disappeared by the application time on 21 Oct. Fruit diameter, soluble solids, and color did not appear to be affected. Further study of earlier applications of EthylBloc may be needed to demonstrate fruit drop control. Shading trees with 92% polypropylene shade material for 3 or 7 days caused more rapid fruit abscission at 7 days than 3 days and both were greater than the control.

576 Effects of NAA-800 and Retain on Fruit Retention and Quality of `Rome Beauty' and `Delicious' Apples

Effects of various combinations of NAA-800 and Retain on fruit retention, yield, and harvest and post-storage fruit quality of `Rome Beauty' and `Delicious' apples were studied over one to three seasons. Retain and NAA-800 often reduced preharvest fruit drop as compared to control. Fruit from trees that received Retain at 123.6 g a.i./ha, or 61.8 g a.i./ha plus NAA-800 showed lower starch degradation pattern (SDP) at harvest and higher firmness. Retain treated fruit had lower evolved ethylene and respiration. Application of Retain at 61.8 g a.i./ha plus NAA-800 delayed fruit maturity, and the effects on fruit quality at harvest was comparable to the effects of this chemical at 123.6 g. a.i./ha. However, after storage, fruits from trees receiving Retain at 123.6 g a.i./ha often were firmer. Split applications of NAA-800 did not show major improvement in delaying fruit maturity over a single application. Application of NAA-800 at 585 mL/ha tended to reduce fruit firmness and increase fruit SDP. These fruits some times tended to have better color. Results on fruit color varied from year to year.