Bacterial canker of olives caused by of Pseudomonas syringae pv. syringae in Iran

Guilan is one of the main province olive oil producers in Iran. The culture of olive trees is of considerable importance. One problem that affects Iran olive orchards is a disease bacterial canker caused by Pseudomonas syringae pv. syringae. In this research, the bacterium was recovered from sunken brown stem lesions on 2-year-old olive trees cv. Marie, during survey from olive orchards in different areas of Guilan province. Samples were taken from infected tissues and were washed with sterile distilled water and crushed in peptone water. Then 50 μl of the extract was cultured on NA and King's B medium containing cyclohexamid antibiotic (50 μg/ml). After 48–72 h, bacterial colonies were selected. Based on morphological, physiological, biochemical, pathogenicity properties and PCR methods with specific primers the predominant pathogenic type was identified as P. s. pv. syringae. This is the first report of the existence of P. s. pv. syringae on olive trees in Iran.     

Compositional quality of virgin olive oils from cultivars introduced in Tunisian arid zones in comparison to Chemlali cultivars

This work is an evaluation of the quality of three introduced European olive cultivars Arbequina, Coratina, and Koroneiki grown under a biological agricultural system in the arid zone of Sfax in Tunisia compared to Chemlali cultivars. Various olive parameters were analyzed, such as ripening index, pulp/stone (P/S) ratio and oil content. We have considered the regulated physicochemical analytical parameters, fatty acids and minor compounds for olive oil. The most of the quality indices and fatty acid composition showed significant variations among olive cultivars. The Koroneiki, Coratina and Chemlali Zarzis cultivars had highest values of oleic acid (62.7%) (76.8%, 75.8% and 73.9%, respectively), Arbequina, had lowest value of oleic acid and similar to Chemlali Sfax. Coratina was noteworthy for its higher content of phenolic compounds (287.8 mg kg⁻¹), oil content (42.4%) and P/S ratio (4.7%). Although their low phenol contents, autochthonous cultivars presented high contents of α-tocopherol (577.8 and 434.6 mg kg⁻¹ for Chemlali Boughrara and Chemlali Zarzis, respectively) except for Chemlali Sfax. In conclusion, European cultivars seem to have adapted to the area studied according to their oil biochemical composition while the increase observed in their α-tocopherol levels. The cultivar Arbequina showed a similar composition of Chemlali Sfax.     

Effects of irrigation with treated wastewater on olive tree growth,yield and leaf mineral elements at short term

In arid regions, such as Tunisia, the reuse of treated wastewater (TWW) in agriculture can be a sustainable solution for water scarcity. A two-year field experiment was conducted in order to investigate the short-term effects of TWW on olive growth, yield and concentration of total nitrogen (N_t), potassium (K), phosphorous (P), and heavy metals (i.e. Zn, Mn, Pb and Cd) in olive leaves. Olive trees were subjected to the following irrigation treatments: (i) trees irrigated with well water (WW) and (ii) trees irrigated with treated wastewater (TWW). For both treatments, the TWW and WW were applied at a rate of 4.5 m³ day⁻¹ tree⁻¹ (5000 m³ ha⁻¹ year⁻¹). After two years, non-significant injuries caused by salts and/or heavy metals were observed on shoot growth of trees irrigated with TWW. The application of TWW significantly increased concentration of N_t, P and K in the leaves, whereas heavy metals (Zn and Mn) showed a significant increase only after the second year of irrigation.     

Earthworm casts as an alternate carrier material for biofertilizers: Assessment of endurance and viability of Azotobacter chroococcum, Bacillus megaterium and Rhizobium leguminosarum

Vermicast was used with lignite in different combinations (0:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1 and 1:0) as carrier substrate for biofertilizers (Azotobacter chroococcum, Bacillus megaterium and Rhizobium leguminosarum). The viability count of biofertilizer organisms in stored carrier material was individually carried out once in 15 days for a total period of ten months. More than 1 × 10⁷ g⁻¹ viable cells of A. chroococcum, B. megaterium and R. leguminosarum were observed in 4:1, 5:1, 6:1 and 1:0 combination of carrier materials (vermicast:lignite) at the end of 10th month. In case of lignite carrier material, no viable cells were observed in 10⁷ g⁻¹ at the end of 6th month for A. chroococcum and R. leguminosarum and 5th month for B. megaterium. The correlation of viable cells of the biofertilizers was negative with reference to incubation period. The increase of vermicast proportion in carrier materials showed increase in the survival rate. The results of the present study suggest that the vermicasts can be used as an alternate carrier material for A. chroococcum, B. megaterium and R. leguminosarum.     

Can a post-harvest ripening treatment extend the longevity of Rhododendron L. seeds?

Pre-dehiscent capsules were collected from two Rhododendron griersonianum (Balf.f. & Forrest) trees and either immediately dried in a dry-room (15% relative humidity, 15 °C) or placed in a high humidity room (80% relative humidity, 15 °C) for 30, 60, or 90 d. Further capsules were also collected from the trees at 30 and 60 d, but seeds had been dispersed by 90 d. Seed ageing experiments (60% relative humidity, 45 °C) carried out on these seed-lots and on seeds from a further 10 Rhododendron (L.) species confirmed that short seed lifespans is a trait of the genus, with a mean P₅₀ value of ca. 20 d for this storage environment.     

Sap flow measurements in young olive trees (Olea europaea L.) cv. Chétoui under Tunisian conditions

Measurements of sap flux were carried out from May 2003 to March 2004 on 6 year-old irrigated olive trees of cultivar Chétoui cultivated at 6 m × 6 m spacing in Mornag (36.5°N, 10.2°E), Northern Tunisia. The aim of the research is to evaluate the sap flux technique for its applicability with young olive trees and to estimate their water consumption under field conditions. Three thermal sensors were implanted in the trunk of three olive trees following to North (N), South-East (SE) and South-West (SW) directions. Data were analyzed following to the procedure of Do and Rocheteau (2002b) that derives from Granier (1985). In this paper, data on probe calibration, wood conductive section estimation and sap flux spatial-variability are presented and discussed. Relationships between sap flux measurements, climate and soil water status have been investigated. Results show that sap flux values vary with sensor position, soil water content and climate demand. Good agreements between sap fluxes and global radiation and reference evapotranspiration measurements were observed. Some variations were recorded under water shortage conditions. Maximum and minimum daily fluxes of 4.5 l and 41.0 l per tree were found in February 2003 and in August 2003, respectively. Maximum transpiration represented only 53% of the crop evapotranspiration as determined by the F.A.O. method.     

Molecular characterization and analysis of somaclonal variation in chrysanthemum cultivars using RAPD markers

Molecular characterization using RAPD analysis was carried out in eight cut flowers and two pot plant cultivars of chrysanthemum. Three of them (‘Refocus’, ‘Red Reagan’, and ‘Sheena Select’) were established in vitro and the occurrence of somaclonal variation was studied using the same molecular technique. Two induction media (MS + 0.1 mg l⁻¹ NAA + 0.1 mg l⁻¹ BA, and MS + 2.0 mg l⁻¹ IAA + 0.5 mg l⁻¹ Kinetin), and two proliferation media (MS + 0.1 mg l⁻¹ NAA + 0.2 mg l⁻¹ BA, and MS + 4.0 mg l⁻¹ IAA + 2.0 mg l⁻¹ Kinetin) were employed in order to evaluate the effect of the medium composition in the shoots’ stability. Likewise, the effect of the culture age was considered in assessing genetic stability. Monthly subcultures were carried out, identifying the origin and history of the shoots, throughout a nine-month proliferation period followed by acclimatization. Molecular markers were obtained in every subculture cycle and from the acclimatized plants. Only one shoot from the 7th subculture of the cultivar ‘Refocus’ showed a different band pattern. The use of RAPD for chrysanthemum cultivar characterization and somaclonal variation detection is discussed.     

Evaluation of the effectiveness of soil-applied plant derivatives of Meliaceae species on nitrogen availability to peach trees

We assessed the effect of soil-applied derivatives of melia (Melia azedarach L.) and neem (Azadirachta indica A. Juss) on nitrogen (N) soil availability, root uptake and peach (Prunus persica L.) growth. First we evaluated the effectiveness of experimentally prepared amendments made with fresh ground melia leaves or commercial neem cake incorporated into the soil as nitrification inhibitors, then we evaluated the effect of fresh ground melia fruits and neem cake on growth and N root uptake of potted peach trees, and on soil microbial respiration. Soil-applied fresh ground melia leaves at 10 and 20 g kg⁻¹ of soil as well as commercial neem cake (10 g kg⁻¹) were ineffective in decreasing the level of mineral N after soil application of urea-N as a source of mineral N, rather they increased soil concentration of nitric N and ammonium N. The incorporation into the soil of fresh ground melia fruits (at 20 and 40 g kg⁻¹) and neem cake (at 10 and 20 g kg⁻¹) increased N concentration in leaves of GF677 peach × almond (Prunus amygdalus) hybrid rootstock alone or grafted with one-year-old variety Rome Star peach trees. An increase in microbial respiration, leaf green color and plant biomass compared to the control trees were also observed. The Meliaceae derivatives did not affect, in the short term (7 days), N root uptake efficiency, as demonstrated by the use of stable isotope ¹⁵N, rather they promoted in the long term an increase of soil N availability, N leaf concentration and plant growth.     

The influence of desiccation and rehydration on the survival of polyembryonic seed of Citrus suhuiensis cv. limau madu

A study was carried out to investigate the influence of desiccation and freezing followed by various presowing rehydration procedures on the desiccation sensitivity of the seed of Citrus suhuiensis cv. limau madu. The freshly harvested seeds of limau madu were desiccated under a broad range of relative humidity (RH) to various equilibrium water contents (g H₂O g⁻¹ dw). The desiccated and desiccated–frozen seeds were either directly sown under germination conditions or subjected to presowing rehydration procedures: seed preheating, prehumidification and osmoconditioning. The hydrated and desiccated seeds were sown in controlled germination conditions and the survival was evaluated 4–6 weeks after sowing. The results showed that desiccation progressively reduced the percentage of normal seedling of the seeds of limau madu and the viability is almost lost at water contents below 0.08 g H₂O g⁻¹ dw. The estimated desiccation sensitivity was substantially high (WC₅₀ = 0.143 g H₂O g⁻¹ dw) when the desiccated seeds were rapidly rehydrated (uncontrolled rehydration). In contrast, seed prehumidification, preheating and osmoconditioning (controlled rehydration procedures) markedly enhanced normal seedling percentages decreasing the estimated values of WC₅₀ (between 0.08 and 0.127 g H₂O g⁻¹ dw). While the rapidly rehydrated desiccated–frozen seeds were almost killed at water content of 0.15 g H₂O g⁻¹ dw, prehumidification and preheating have noticeably increased percentage of frozen seeds survival at the same water content. However, at water content of 0.21 g H₂O g⁻¹ dw preheating significantly (P < 0.05) increased percentage of normal seedling of the frozen seeds. Seed desiccation markedly reduced the percentages of germinated seeds with multiple seedlings. Seed controlled rehydration remarkably increased the survival of polyembryos. The beneficial effect of seed controlled rehydration on the survival of the desiccated seeds was pronounced at medium water contents (0.08–0.25 g H₂O g⁻¹ dw).     

Ethychlozate reduces acidity of loquat (Eriobotrya japonica) fruit

Ethychlozate (ethyl 5-chloro-1H-3-indazolylacetate, Figaron) was applied to 12-year-old ‘Jiefangzhong’ (a high-acid cultivar) loquat (Eriobotrya japonica) trees at 0 (control), 75, 150 and 250 mg L⁻¹ on March 19th, 90 days after flowering. To determine pulp titratable acidity (TA), organic acid concentration and acid-metabolizing enzyme activity, fruits were collected at 0, 13, 25, 42 and 49 days after treatments (DAT), when fruits were ripe. Malate was the major organic acid determining loquat pulp acidity. Ethychlozate was very effective in reducing pulp malate concentration, thus decreasing pulp acidity. The best result for reducing acidity (malate) was obtained when ethychlozate was applied at 250 mg L⁻¹. The ethychlozate-treated pulp showed lower phosphoenolpyruvate carboxylase (PEPC) and NAD-malate dehydrogenase (NAD-MDH) activities through the experimental period, but higher or similar NADP-malic enzyme (NADP-ME) from 25 to 49 DAT compared to non-sprayed control. The reduction in malate concentration in the ethychlozate-treated pulp was probably caused by decreased malate biosynthesis and increased malate degradation.     

Effect of irrigation frequency on cashew nut yield in Northeast Brazil

The study was carried out in Ceará, Brazil, with the goal of evaluating the response of three dwarf cashew clones to different irrigation frequencies. Cashew trees were evaluated during seven consecutive seasons, from the 2nd to the 8th year after planting. Four irrigation treatments were used: no irrigation, and irrigation when accumulated evaporation on a class “A” pan equaled 10 mm (T10), 30 mm (T30), and 50 mm (T50). The sub plot treatments were three cashew clones (CCP 09, CCP 76, and CCP 1001). Seasonal irrigation rates were the same for all irrigated treatments, and ranged from 400 to 500 mm per year for fully developed plants (6–8 years old). Response to irrigation varied among cashew genotypes. Irrigation had a significant effect (P < 0.05) on nut yield of cashew clones CCP 76 and CCP 09. The highest nut yield was obtained with cashew clone CCP 09, irrigated when accumulated pan evaporation reached 10 mm. Cashew trees trended to alternate years of high nut yield and years of low yield, even when irrigation was applied. Irrigation did not influence nut weight of clones CCP 76 and CCP 09.     

Isotopically-labelled nitrogen uptake and partitioning in sweet cherry as influenced by timing of fertilizer application

Sweet cherry (Prunus avium L.) is a fruit of increasing economic importance though it is less significant than other stone fruit species such as peach. Cherry has received little attention concerning nitrogen (N) uptake and dynamics in mature trees. The aim of this work was to determine N uptake and partitioning as influenced by the timing of fertilizer application in 7-year-old sweet cherry trees cultivated in a cold region (Los Antiguos, Santa Cruz, Argentina; 71°38′ W, 46°32′ S). Nitrogen (95 kg ha⁻¹) was applied as ammonium nitrate to a soil with ‘Bing’ sweet cherry trees grafted onto Prunusmahaleb rootstocks. Fertilization was split into two equal applications per treatment, involving either the commercial fertilizer ammonium nitrate or the same fertilizer labelled with ¹⁵N isotope (10% atom.). Treatments consisted of one early spring (full bloom, October 2005) or one summer (late January 2006, 15 days after harvest) application of ¹⁵N ammonium nitrate to three replicate trees. Fruit were harvested in early January and leaves were collected at both full canopy and leaf fall. All trees were excavated in winter (August, 2006). Trees were partitioned into their components: trunk, branches (current-season shoots, 1-year-old and over-1-year-old branches), buds of the same age, small roots (less than 1 mm thick), large roots, leaves (sampled in February and April), and fruit (collected at harvest). Those components were dried and analysed for total N and ¹⁵N content. Total N per tree and N content derived from the fertilizer did not differ between treatments. Summer postharvest ¹⁵N application partitioned not only to structural components (trunk and roots) but also to buds and leaves. Uptake efficiency was significantly (p = 0.0113) higher in the spring than in the summer application (65.7% vs. 37.44%). Nevertheless, 52.5% of N applied in spring was lost due to harvest and summer pruning. This emphasizes the importance of the postharvest N fertilization which increases N accumulation in both reserve organs and buds though, according to our data, it is less efficiently used. The extent of nitrogen uptake, efficiency of use and partitioning in the following growing seasons are still open questions that deserve further research.     

Yield and fruit quality of ‘Gala’ and ‘Fuji’ apple trees protected by white anti-hail net

Covering apple orchards with nets protects them from hailstorms but this changes the amount and quality of the light supplied to the trees. This study was carried out to assess the effects of shade provided by white protection net on yield and fruit quality of apple orchards in Southern Brazil, along three growing seasons (2002/2003, 2003/2004, and 2004/2005). ‘Gala’ and ‘Fuji’ apple trees (on MM111 rootstock, raised as slender spindles, with a spacing of 5.5 m × 3 m) were left uncovered (control trees) or covered with white net. The white net reduced the photosynthetically active radiation (λ = 400–700 nm) accumulated over the trees’ canopy along a day by 18.4%, leading to a higher leaf chlorophyll content in both cultivars, and a higher specific leaf area in ‘Gala’. The shade provided by the net did not affect yield and increased fruit average weight in both cultivars. At harvest, the number of seeds per fruit and severity of russet were not affected by netting in both cultivars, as well as the incidence of watercore in ‘Fuji’. The main beneficial effects of the white net were the reduced incidence of sunburn on ‘Gala’, reduced incidence of bitter pit during cold storage in both cultivars, and reduced incidence of apple scab at harvest and of decay after cold storage in ‘Fuji’. Fruit fly damage in ‘Gala’ (assessed in one season) was reduced by netting. However, netting affected negatively some fruit quality attributes. ‘Gala’ and ‘Fuji’ apples from trees protected by the net had a poor skin color (pale blush and a more intense green background color) at harvest. The net reduced flesh firmness and SSC, and increased the starch index of ‘Gala’ apples at harvest, and reduced flesh firmness and SSC, assessed after cold storage and shelf life, in fruits of both cultivars.     

In vitro proliferation from axillary buds and ex vitro protocol for effective propagation of Syringa × hyacinthiflora ‘Luo Lan Zi’

As a precondition for lilac mass propagation, the optimal shoot-multiplication medium for Syringa × hyacinthiflora ‘Luo Lan Zi’ was ascertained mainly based on clustered microshoot inducement and large leaf area establishment in 6-benzyladenine (BAP) (1.00 mg L⁻¹) and zeatin (Z) (0.10 mg L⁻¹) combination. Medium supplied with lower level of BAP (0.50 mg L⁻¹) and auxin (IAA) (0.25 mg L⁻¹) was not suitable for lilac shoot proliferation, but it could be competent for long-term preservation of the un-rooted shoots so that subsequent proliferation culture could be carried out at anytime. In addition, excess height growth which resulted in low transplanting survival rate was effectively controlled by decrease in node number when paclobutrazol (PBZ) was applied in rooting medium at a concentration of 1.00 mg L⁻¹ after taking into account the effects on shoot height, rooting, persistent leaf area and PBZ carry-over. An important overwintering treatment was to use a plastic chamber covering for plants in the greenhouse prior to field planting to ensure adequate biomass of stem and underground parts not only in the current growing season but also in the subsequent years.     

Spatial and temporal oxygen distribution measured with oxygen microsensors in growing media with different levels of compaction

Oxygen microsensors were used to determine oxygen profiles in situ from the top to the bottom layer of the growing medium for potted plants of Rosa sp. ‘Dior’. The growing medium was peat- based and compacted uniformly to 3 different bulk densities of 0.14, 0.18 and 0.23 g cm⁻³ (0, 20 and 40% compacted, respectively). The water distribution in the pot was determined as water content (g cm⁻³) in the top, middle and bottom layers of the peat. Oxygen content was also determined after a standard subirrigation cycle and after excessive irrigation where the bottom of the pots were left waterlogged for 24 h. Measurements were carried out at 5.5 weeks during the production phase and at 12 weeks at the end of the production. The results showed that with increasing compaction and density, more water was transported to the upper layers of the pot. After a standard irrigation cycle there was no effect of the level of medium compaction on the oxygen distribution, whereas after excessive irrigation, the oxygen contents at the bottom of the pots were strongly reduced and the level of compaction significantly affected oxygen availability. The most compacted medium had the lowest oxygen content at 5.5 weeks, with anoxic conditions in the bottom 30 mm. Plant quality measured as fresh weight, dry weight, height and number of shoots with flowers and buds was not affected by the different levels of compaction. The use of oxygen microsensors provided a new insight into the spatial and temporal distribution of oxygen in growing media and how this was affected by the physical characteristics of the growing media.     

Cold storage of oil rose (Rosa damascena Mill.) flowers

Oil rose flowers were stored at 0 °C in four different packaging materials [plastic box + stretch film (PB + SF), Xtend^®, Smartbag^® and polyethylene (PE)] for 60 days. During storage, weight loss, O₂ and CO₂ concentrations in the packages, petal color and sensorial attributes were investigated besides essential oil content and composition. Storage duration and packages had significant (p < 0.01) effects on weight loss. At the end of storage, the lowest weight loss was in PE package (1.696%) whereas the highest weight loss was in Xtend^® (10.081%). The essential oil content was significantly (p < 0.01) affected by storage duration and packages. In addition, the essential oil contents obtained from all packages for a storage period of 10 days and the essential oil contents obtained from unstored (control) petals were included in the same group. At the end of storage, the essential oil contents decreased by 91.3, 57.7, 80.0 and 64.3% in PB + SF, Xtend^®, Smartbag^® and PE packages, respectively as compared to control. In addition, storage duration and package types significantly (p < 0.01) affected petal color, O₂ and CO₂ concentrations in the packages and sensorial scores. The concentration of citronellol, a main component of rose oil, increased in all packages during storage of 10 days in comparison to the control group while it varied in other storage durations and package types. However, nerol and geraniol were lower than the control group during storage while concentrations of nonadecane, heneicosane and eicosane were higher. In conclusion, loss of oil yield and quality, due to various reasons and particularly due to fermentation in oil rose from the harvest of petals to their distillation, can be minimized with storage of petals in all package types for up to 10 days.     

Effect of genotype,explant size,position, and culture medium on shoot generation of Gerbera jamesonii by receptacle transverse thin cell layer culture

An unique procedure for the mass shoot propagation of Gerbera using receptacle transverse thin cell layer (tTCL) culture procedure was developed. Genotype, flower bud age, explant size, position of receptacle tTCLs and culture media were found to affect the success of culture. Ten interspecific crosses of Gerbera showed different shoot regeneration rates and callus induction via receptacle tTCL culture, all of which had shoot regeneration rates higher than 57%. Flower buds collected on the 10th day resulted in 91% shoot regeneration after 6 weeks of culture on basal MS medium [Murashige, T., Skoog, F., 1962. A revised medium for rapid growth and bioassay with tobacco tissue cultures. Physiol. Plant. 15, 475–497] supplemented with 0.02 mg l⁻¹ thidiazuron (TDZ), 0.8 mg l⁻¹ adenine and 10% (v/v) coconut water (CW). This was significantly higher than those from flower buds on the 7th and 14th days (22% and 54%), respectively. Shoot regeneration rate was the highest (94–100%) in the middle layers of the receptacle. For mass shoot propagation, shoot clusters were subcultured on half-strength MS medium supplemented with 0.5 mg l⁻¹ indole-3-butyric acid (IBA), 0.5 mg l⁻¹ 6-benzyladenine (BA) and 2.0 mg l⁻¹ kinetin after every 4 weeks. Plantlets formed when single shoots were cultured on half-strength MS medium containing 1 mg l⁻¹ IBA. All plantlets acclimatized well in the greenhouse.     

Long-term effects of managed grass competition and two pruning methods on growth and yield of peach trees

Ground cover competition and tree training strongly affect development of newly planted peach trees and eventual productivity of peach orchards. This experiment characterized the long-term interactive effects of three levels of competition and two pruning criteria on yield, fruit size, and marketable yield efficiency. Trees of two cultivars (’Jersey Dawn’, JD, and ‘Redskin’, RS, on Lovell) of peach (Prunuspersica (L.) Batsch) were planted in an orchard in 1993 and grown for 14 years in a vegetation free area (VFA) width of 0.6 or 2.4 m. A separate group of trees that were in the 2.4 m VFA had grass seeded beneath them in 1998 to obtain 0 m VFA. All trees were pruned to maintain canopy size with wide-angled scaffold limbs and intense pruning (IP) or upright branch form with reduced pruning (RP). In general, RS had greater yield than JD and yield was greatest in the 2.4 m VFA with RP and least in the 0.6 m VFA with IP. Cumulative marketable (≥6.35 cm) and average annual total yield of both cultivars was similar for RP trees in 0 m VFA and IP trees in 2.4 m VFA's although more of the fruit were in the largest size class (>6.98 cm) in the IP trees. Reduced pruning increased crop load. Fruit weight decreased with increased crop load more in RS than JD and this response was similar for all VFA's within each cultivar. Grass competition tended to reduce both the number and weight of fruit per tree but the average weight of individual marketable fruit was reduced only in the 0.6 m VFA of RS. Tree size was reduced by grass competition and pruning times measured from 1995 to 2000 were less in RP than IP. Consequently, marketable yield efficiency of marketable fruit (grams fruit ≥6.35 cm/cm² trunk cross-sectional area, TCSA) measured from 2004 to 2007 was generally greater in trees with RP than IP and in the 0.6 than the 0 and 2.4 m VFA. The results indicate that persistent competition will reduce total annual yield per tree but with reduced pruning the concomitant increased crop load can help maintain marketable yield.     

Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent. I: GA3 increases the yield of commercially valuable ‘Nules’ Clementine Mandarin fruit only in the off-crop year of an alternate bearing orchard

Worldwide, gibberellic acid (GA₃) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA₃ concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2 mm) of ‘Nules’ Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard. GA₃ applied during the period of intense flower abscission failed to reduce the total number of abscised flowers in both the light off- and heavy on-bloom years. No GA₃ treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA₃ in the high yield on-crop year had fewer abscised fruit than untreated control trees (P = 0.0188) and GA₃ applied 10 days after 75% petal fall and in July increased the number of fruit retained on tagged branches >20% compared to control trees (P = 0.0005). Maximum air temperature was not related to flower or fruit abscission. In the off-crop year (548 fruit per untreated control tree), it was necessary to apply 15 or 25 mg L⁻¹ GA₃ at 60% bloom, 90% bloom, 75% petal fall and 10 days after 75% petal fall to significantly increase the number of fruit per tree and yield of commercially valuable fruit (kilograms and number per tree) (P < 0.0001) above that of control trees, with no reduction in total kilograms per tree. In the following on-crop year, it was better not to apply GA₃: no treatment increased total yield or fruit size and five of seven GA₃ treatments tested reduced total yield as kilograms and number of fruit per tree (P = 0.0003). The results provide strong evidence that GA₃ efficacy is crop load-dependent and dictate that crop load should be considered when using GA₃ to increase fruit set or fruit size of mandarins.