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Qingzhong Liu, Sarbagh Salih, and Freddi Hammerschlag

Factors influencing regeneration and ß-glucuronidase expression from apple (Malus × domestica Borkh.) stem internodes were studied as part of a program to develop transgenic `Royal Gala' apple with improved disease resistance. The early stages of the transformation process were monitored by counting the number of ß-glucuronidase (GUS) expressing zones immediately after co-cultivation of explants with Agrobacterium tumefaciens supervirulent strain EHA105 (p35SGUS_INT) and by counting the number of GUS-expressing calli developing on explants 2 weeks after co-cultivation. Etiolated shoots were produced from in vitro shoots cultured for 2 weeks in the light followed by 2 weeks in the dark and were compared with shoots cultured for 4 weeks in the light (green shoots). First internodes from etiolated shoots produced three, 10 and 100 times the number of shoots regenerated from second, third, and fourth internodal explants, respectively, and produced seven times the number of shoots compared with similar explants from green shoots. 100% of first internodes from etiolated shoots exhibited GUS-expressing zones and yielded twice as many GUS-expressing zones when compared with leaf explants from green shoots, which exhibited GUS-expressing zones in only 60% of the explants. An average of nine GUS-expressing calli per explant were produced on first internodes from etiolated shoots 2 weeks after co-cultivation.

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Carlos Miranda Jiménez and J. Bernardo Royo Díaz

Spring frosts are usual in many of Spain's fruit-growing areas, so it is common to insure crops against frost damage. After a frost, crop loss must be evaluated, by comparing what crop is left with the amount that would have been obtained under normal conditions. Potential crop must be evaluated quickly through the use of measurements obtainable at the beginning of the tree's growth cycle. During the years 1998 and 1999 and in 62 commercial plots of `Golden Delicious' and `Royal Gala' apple (Malus ×domestica Borkh.), the following measurements were obtained: trunk cross-sectional area (TCA, cm2), space allocated per tree (ST, m2) trunk cross-sectional area per hectare (TCA/ha), flower density (FD, number of flower buds/cm2 TCA), flower density per land area (FA, number of flower buds/m2 land area), cluster set (CS, number of fruit clusters/number of flower clusters, percent), crop density (CD, number of fruit/cm2 TCA), fruit clusters per trunk cross-sectional area (FCT, number of fruit clusters/cm2 TCA), fruit clusters per land area (FCA, number of fruit clusters/m2 land area), fruit number per cluster (FNC), average fruit weight (FW, g), average yield per fruit cluster (CY, g), yield efficiency (YE, fruit g·cm-2 TCA), and tree yield (Y, fruit kg/tree). FCT and average CY were related to the rest of the variables through the use of multiple regression models. The models which provided the best fit were FCT = FD - TCA/ha - FD and CY= -FCA - FCT. These models were significant, consistent, and appropriate for both years. Predicted yield per land area was obtained by multiplying TCA/ha × FCT × CY. The models' predictive ability was evaluated for 64 different plots in 2001 and 2002. Statistical analysis showed the models to be valid for the forecast of potential yields in apple, so that they represent a useful tool for early crop prediction and evaluation of losses due to late frosts.

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H.N. De Silva, D.S. Tustin, W.M. Cashmore, C.J. Stanley, G. Lupton, and S.J. McArtney

A number of mass—diameter equations were compared for their potential use in indirect measurement of fruit masses of `Royal Gala' apple (Malus ×domestica). The fruit fresh-mass—diameter relationship changed with time during the season, hence no single function fitted the data well. Smooth piecewise functions that assume different relationships for intervening segments of a curve bounded by knots on the x-axis are particularly useful for modeling such data. The curve is said to be smooth because the first derivative of the function is continuous on the interval, including the knots. Two such equations, a three-parameter piecewise power function and a five-parameter spline exponential function, provided good fits to data. For both equations, the estimated mean bias on individual fruit predictions was within 5% of predicted mass over the two validating data sets. As for the precision conditional on no bias, a sample size of 20 fruit gave standard errors within 2.5% of mean predicted mass. These precisions are adequate to meet the industry requirements for monitoring fruit mass through the growing season. There was evidence of a seasonal difference in the estimated bias, but we were unable to confirm that this variation resulted from seasonal differences in fruit shape. Application of these two equations to data from other regions suggested that divergence from the estimated functional form may in fact be greater under increasingly different climatic conditions. Hence, further investigations to identify possible sources of these differences are necessary before the proposed equations can be applied across climatically different regions.

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Paul J.R. Cronjé, Gerard Jacobs, and Nigel C. Cook

Two-year-old apple branches, ≈50 cm long, were selected from a commercial `Royal Gala' orchard in the Ceres (Koue Bokkeveld) region of the Western Cape, South Africa [33 °S, 945 m, 1500 Utah model chilling units (CU)]. In 2000, the branches received either cold storage at 5 to 7 °C or natural chilling in the field. In 2001, the trial was repeated, but only with field chilling. The branches received five dormant pruning treatments: control (not pruned); pruning back to the fourth lateral shoot (heading) before or after chilling; and removal of the second and third lateral shoots (thinning) before or after chilling. After pruning and chilling, the branches were removed from the orchard or cold room every 2 weeks and forced in a growth chamber at 25 °C. The rate of budburst (1/days to budburst) was determined for the terminal buds of the lateral shoots. Lateral shoots on the 2-year-old branches were categorized according to position: the most distal extension shoot, and all other laterals grouped. Removing distal tissue by pruning (heading more than thinning) enhanced the effect of chilling on the terminal buds on the lateral shoots and promoted budburst. Pruning was more effective before than after chilling. Pruning enhanced the growth potential of the terminal buds on proximal shoots on 2-year-old branches.

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M.D. White, D.S. Tustin, K.F. Foote, R.K. Volz, J. Stokes, J. Campbell, R. Marshall, and C. Howard

ReTain™ is a plant bioregulator containing the active ingredient aminoethoxyvinylglycine (AVG), which inhibits the ethylene biosynthesis pathway. In 1997, the first efficacy studies on `Royal Gala' apple with ReTain™ were conducted under New Zealand conditions in Hawkes Bay. ReTain™ was applied 4 weeks before the anticipated start of harvest on `Royal Gala' at 850 and 1700 g·ha–1 with or without adjuvants. ReTain™ application delayed the onset of `Royal Gala' fruit maturation between 1 and 2 weeks, resulting in enhanced fruit size and fruit flesh firmness at harvest. The optimum response for delaying the onset of fruit maturation was achieved using ReTain™ at 850 g·ha–1 if applied in combination with a wetter. Fruit were also graded for fruit quality and air-stored at 0.5 °C. Fruit after 10 weeks of storage showed no difference in fruit flesh firmness, but all ReTain™ treatments had fruit with less yellow background colour compared with untreated fruit. In 1998, efficacy studies were undertaken in three geographical locations on `Royal Gala'. ReTain™ was applied at a rate of 830 g·ha–1 in combination with Silwet L-77 at 0.1%. All trees with the exception of `Royal Gala' grown in the Hawkes Bay had not received any ReTain™ previously. In all regions, seasonal changes in background color and starch pattern index were delayed by ReTain™ treatment. A concurrent delay of an increase in soluble solids concentration and retention of higher flesh firmness were also induced by ReTain™ treatment.

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Cheryl R. Hampson, Harvey A. Quamme, and Robert T. Brownlee

In 1993, a planting of virus-free 'Royal Gala' apple (Malu×domestica Borkh.) on 'M.9' rootstock was established at Summerland, B.C., Canada, to determine whether angled-canopy training systems could improve orchard tree performance relative to slender spindles. The trees were trained in one of five ways: slender spindle (SS), Geneva Y-trellis (GY), a modified Solen training we called 'Solen Y-trellis' (SY), or V-trellis (LDV), all at the same spacing (1.2 m × 2.8 m), giving a planting density of 2976 trees/ha. In addition, a higher density (7143 trees/ha) version of the V-trellis (HDV) was planted to gauge the performance of this system at densities approaching those of local super spindle orchards. The plots were drip-irrigated and hand-thinned. No summer pruning was done. After 8 years, differences among training systems at the same density and spacing were small and few. The two Y-shaped training systems had 11% to 14% greater cumulative yield/ha than the SS, but did not intercept significantly more light at maturity. No consistent differences occurred in fruit size or the percentage of fruit with delayed color development among the four training systems at the same density. Relative to the LDV, the HDV yielded less per tree, but far more per hectare, particularly in the first 3 years. After 8 years, the cumulative yield/ha was still 65% greater than with LDV. Yield efficiency was unaffected by tree density. Fruit size on HDV ranked lowest among the systems nearly every year, but was still commercially acceptable. The HDV intercepted more light (73%) than SS (53%). The percentage of fruit with delayed color development in HDV was not significantly different from that for LDV in most years. The trees in HDV were difficult to contain within their allotted space without summer pruning. The substantially similar performance of all the training systems (at a given density, and with minimal pruning) suggests that cost and ease of management should be the decisive factors when choosing a tree training method.

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Q. Liu, S. Salih, J. Ingersoll, R. Meng, L. Owens, and F. Hammerschlag

Transgenic `Royal Gala' apple (Malus × domestica Borkh.) shoots were obtained by Agrobacterium-mediated gene transfer using the plasmid binary vector pGV-osm-AC with a T-DNA encoding a chimeric gene consisting of a secretory sequence from barley-amylase joined to the modified cecropin MB39 coding sequence. Shoots were placed under the control of a wound-inducible, osmotin promoter from tobacco. The integration of the cecropin MB39 gene into apple was confirmed by Southern blot analysis. The transformation efficiency was 1.5% when internodes from etiolated shoots were used as explants and 2% when leaf explants were used. Both non- and transgenic tetraploid plants were produced by treatment of leaf explants with colchicine at 25 mg·L-1, and polyploidy was confirmed by flow cytometry. Of the diploid transgenics, three of seven were significantly more resistant to Erwinia amylovora than the non-transgenic `Royal Gala' control. Also, in one instance, a tetraploid transgenic was significantly more resistant than the diploid shoot from which it was derived.

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Cheryl R. Hampson, Harvey A. Quamme, Frank Kappel, and Robert T. Brownlee

The effect of increasing planting density at constant rectangularity on the fruit yield, fruit size, and fruit color of apple [Malus ×sylvestris (L) var. domestica (Borkh.) Mansf.] in three training systems (slender spindle, tall spindle, and Geneva Y trellis) was assessed for 10 years. Five tree densities (from 1125 to 3226 trees/ha) and two cultivars (Royal Gala and Summerland McIntosh) were tested in a fully guarded split-split plot design. Density was the most influential factor. As tree density increased, per-tree yield decreased, but yield per unit area increased. The relation between cumulative yield per ha and tree density was linear at the outset of the trial, but soon became curvilinear, as incremental yield diminished with increasing tree density. The chief advantage of high density planting was a large increase in early fruit yield. In later years, reductions in cumulative yield efficiency, and in fruit color for `Summerland McIntosh', began to appear at the highest density. Training system had no influence on productivity for the first 5 years. During the second half of the trial, fruit yield per tree was greater for the Y trellis than for either spindle form at lower densities but not at higher densities. The slender and tall spindles were similar in nearly all aspects of performance, including yield. `Summerland McIntosh' yielded almost 40% less than `Royal Gala' and seemed more sensitive to the adverse effects of high tree density on fruit color.

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Cheryl R. Hampson, Harvey A. Quamme, Frank Kappel, and Robert T. Brownlee

The effect of increasing planting density at constant rectangularity on the vegetative growth and light interception of apple [Malus ×sylvestris (L) var. domestica (Borkh.) Mansf.] trees in three training systems (slender spindle, tall spindle, and Geneva Y trellis) was assessed for 10 years. Five tree densities (from 1125 to 3226 trees/ha) and two cultivars (Royal Gala and Summerland McIntosh) were tested in a fully guarded split-split plot design. Planting density was the most influential factor. As tree density increased, tree size decreased, and leaf area index and light interception increased. A planting density between 1800 and 2200 trees/ha (depending on training system) was needed to achieve at least 50% light interception under the conditions of this trial. Training system altered tree height and canopy diameter, but not total scion weight. Training system began to influence light interception in the sixth leaf, when the Y trellis system intercepted more light than either spindle form. Trees trained to the Y trellis tended to have more spurs and a lower proportion of total leaf area in shoot leaves than the other two systems. The slender and tall spindles were similar in most aspects of performance. Tall spindles did not intercept more light than slender spindles. `Royal Gala' and `Summerland McIntosh' trees intercepted about the same amount of light. `Royal Gala' had greater spur leaf area per tree than `Summerland McIntosh', but the cultivars were similar in shoot leaf area per tree and spur density.

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W.D. Lane, D.-L. McKenzie, and M. Meheriuk

Contribution no. 2018. We thank Dr. J. Hall for help with the statistical analysis, Dr. G. Neilsen and others for access to fruit samples of `Royal Gala' and `Regal Gala' from experimental plots, and M. Weis for the digital imaging. The cost