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D.S. Tustin, T. Fulton, and H. Brown

Growth of apple fruit can be described as an initial exponential phase lasting the 40+ days of fruit cell division followed by a more-or-less linear phase where growth is by cell expansion. Temperature is a major influence on fruit growth rate during the cell division phase, thereby affecting fruit size at maturity. However it is generally thought that temperature has less-direct impact on fruit development during the fruit expansion phase. Our observations of apple growth among regions and seasons of considerable climatic variability led us to speculate that temperature may impact directly on fruit development during fruit expansion but that responses may be interactive with carbon balance (crop load) influences. Controlled environment studies are being used to examine this hypothesis. Potted `Royal Gala' trees set to three levels of crop (one fruit per 250, 500, or 1000 cm2 leaf area) were grown from 56 to 112 DAFB in day/night temperature regimes of 18/6, 24/12, and 30/18 °C. All trees grew in field conditions prior to and following the controlled environment treatments. Treatments were harvested when 20% to 25% of fruit on trees showed the visual indicators used commercially to indicate harvest maturity. Fruit were evaluated using attributes that determine quality and that may have implications for fruit post harvest behaviour. Temperature and crop load influences on time to maturity, fruit fresh and dry weight, fruit DM content, fruit firmness, fruit airspace content and estimated fruit cortical cell size will be presented and implications discussed.

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C.J. Stanley and D.S. Tustin

Many factors contribute to final apple fruit size. Researchers have studied these factors and have developed models, some very complex. Results from many New Zealand regions over several years suggest that early season temperature along with crop load are the key factors driving final fruit size. Accumulated growing degree days from full bloom to 50 days after full bloom (DAFB), accounted for 90% of the variance in fruit weight of `Royal Gala' apples at 50 DAFB under nonlimiting low-crop-load conditions. In turn, fruit weight at 50 DAFB accounted for 90% of the variance in final fruit size at harvest under the low-crop-load conditions. We hypothesise that a potential maximum fruit size is set by 50 DAFB, determined by total fruit cell number, resulting from a temperature-responsive cell division phase. Under conditions of no limitations after the cell division phase, we suggest that all cells would expand to their optimum size to provide the maximum fruit size achievable for that cell number. Factors which affect growth partitioning among fruits, e.g., higher crop loads, would reduce final fruit size, for any given cell number, when grown in the same environment. In Oct. 1999, four different crop loads were established at full bloom on `Royal Gala' trees (M9 rootstock) in four climatically different regions. In Hawkes Bay, similar crop loads were established at 50 DAFB on additional trees. Hourly temperatures were recorded over the season. Fruit size was measured at 50 DAFB and fruit will be harvested in Feb. 2000. These data should provide fresh insight and discussion into the respective roles of temperature and competition during the cell division fruit growth phase on apple fruit size.

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Emily Hoover, S. McArtney, S. Tustin, M. White, and P. Hirst

Experiments were initiated to document the effect of cultivar, GA4+7, and number of fruit/spur on appendage number and flower bud initiation in apple. `Pacific Rose' is strongly biennial, `Braeburn' and `Fuji' are moderately biennial, and `Royal Gala' is not biennial. In the cultivar study, buds were sampled every 18 days starting at 50 days after full bloom and continuing through until leaf fall to determine the rate of appendage formation and appendage number in relation to doming. Because of the tendency for `Pacific Rose' to exhibit biennial bearing, the rate of appendage formation and the timing of doming were compared on nonfruiting trees, trees carrying a commercial crop, and trees sprayed with 300 PPM GA4+7 applied 14 days after full bloom. Number of appendages for the treatments were similar up to 100 days after full bloom. Presence of fruit on a spur has been demonstrated to inhibit flowering of apple. Spurs of `Pacific Rose', `Splendor', and `Royal Gala' were labeled with zero, one, two, and three fruit per spur and sampled three times during the season. As buds were harvested to count appendage number, the number of fruit per spur and the number of total seeds per spur were recorded. Correlation between number of seeds per spur and rate of appendage formation were done.

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R.L. Rusholme Pilcher, J-M. Celton, S.E. Gardiner, and D.S. Tustin

Little is known of the precise physiological or genetic basis of the phenomenon of rootstock-induced dwarfing in apple (Malus × domestica Borkh.). Phenotypic assessment and genetic marker analysis of a segregating population of apple rootstocks derived from a cross between the dwarfing rootstock ‘Malling 9’ (‘M.9’) and the vigorous rootstock ‘Robusta 5’ (‘R5’) was undertaken over a 5-year growth period. Clear segregation of the appearance of the ‘Braeburn’ scions permitted expert visual classification of each tree into four growth habit classes: dwarfed, semidwarfed, intermediate, or vigorous. Measurements of the cross-sectional area of the trunk of each tree correlated clearly with the expert-assigned phenotypic class. This clear segregation facilitated bulked segregant analysis (BSA) of the ‘M.9’ × ‘R5’ population, and the Dwarfing 1 (Dw1) gene was located in a 2.5-cM region at the top of the linkage group 5 of ‘M.9’. The gene Dw1 was mapped between the random amplified polymorphic DNA marker NZraAM18_700 and the microsatellite marker CH03a09, identified by BSA and the genome scanning approach, respectively. Dw1 is the first reported mapped locus controlling the dwarfing ability of the apple rootstock and maps to a chromosome region not previously known to contain quantitative trait loci or major genes that contribute to tree architecture and development. The identification of a group of trees in the ‘M.9’ × ‘R5’ population exhibiting a vigorous phenotype although their rootstocks possess alleles from the Dw1 region of ‘M.9’ suggests that more loci may be involved in conditioning the dwarfing phenotype. Dw1 is a major component of dwarfing, as most of the dwarfing and semidwarfing rootstocks carried the dwarfing allele of this locus. Genetically mapping Dw1 is a major step in unraveling the mechanism behind the dwarfing ability of apple rootstocks.

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D.C. Ferree, J.R. Schupp, D.S. Tustin, and W. Cashmore

Flower and spur characteristics of eight apple cultivars were determined at bloom and following cell division and related to fruit growth over the season. Flower number per spur was higher in `Jonagold', `Fiesta', `Southern Snap', `Royal Gala', than in `Red Chief Delicious', `Pacific Rose' and `Fuji', and the latter three cultivars also had the lowest total flower dry weight per spur. Generally, pedicel length of the king flower was shorter than the lateral, with `Fuji' having the longest king flower pedicel and `Red Chief' and `Pacific Rose' the shortest. At full bloom, `Jonagold' and `Fiesta' had the most leaves/spur, followed by `Fuji', `Southern Snap', `Royal Gala', `Pacific Rose', with `Red Chief' having fewer leaves/spur than all other cultivars. Leaf area/spur was highest in `Jonagold', `Fiesta', `Royal Gala', and `Fuji', followed by `Southern Snap', with `Red Chief' and `Pacific Rose' having lower leaf areas and fruit set than all other cultivars. At the end of cell division, `Fuji' and `Jonagold' had the highest leaf area per spur and `Fuji' and `Royal Gala' the highest bourse leaf area. `Braeburn' and `Red Chief' had lower bourse leaf areas than all other cultivars. `Royal Gala', `Southern Snap', and `Fuji' had the longest bourse shoots and `Red Chief' the shortest.

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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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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.