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In unstressed apple seedlings (Malus domestics Borkh.), concentrations of free abscisic acid (ABA) decreased in order from apical stem sections, immature expanding leaves, mature stem sections, and mature leaves. PEG-induced water stress stimulated a 2- to 10-fold increase in free ABA concentrations 1 day after treatment, depending on the amount of stress and the tissue. By the 3rd day of stress, free ABA concentrations were nearly the same as the unstressed treatment and remained low for the remainder of the 21-day stress period. Bound ABA concentrations were an order of magnitude lower than free ABA and were not influenced dramatically by water stress. Shoot growth rate, leaf expansion rate, and leaf emergence rate were reduced by water stress in relation to the severity of the stress; this reduction was associated with the initial increase in ABA. However, there was no increase in shoot or leaf growth rates associated with the decline in ABA concentrations by day 3 as growth rates remained depressed on water-stressed plants throughout the 21-day stress period. Water stress reduced evapotranspiration rate and midshoot leaf water potential (ψ_W)after 1 day, but leaf osmotic potential (ψ_S) adjusted more slowly, resulting in a loss of leaf turgor. The reduction in leaf turgor pressure (ψ_P) was highly correlated with decreased shoot growth rate and increased ABA concentrations on day 1 after treatment. By the 3rd day of water stress, ψ_P bad recovered even in the most severe treatment, and the recovery of turgor was associated with the drop in ABA concentrations. However, the increase in midshoot ψ_P and the decline in ABA were not associated with any increase in shoot growth rate. The continued inhibition of shoot growth was probably not related to ABA or turgor pressure of mature leaves but may have been related to turgor pressure in the growing tip.

Bases of orchard productivity were evaluated in four 10-year-old apple orchard systems (`Empire' and `Redchief Delicious' Malus domestics Borkh. on slender spindle/M.9, Y-trellis/M.26, central leader/M.9/MM.111, and central leader/M.7a). Trunk cross-sectional areas (TCA), canopy dimension and volume, and light interception were measured. Canopy dimension and canopy volume were found to be relatively poor estimators of orchard light interception or yield, especially for the restricted canopy of the Y-trellis. TCA was correlated to both percentage of photosynthetically active radiation (PAR) intercepted and yields. Total light interception during the 7th to the 10th years showed the best correlation with yields of the different systems and explained most of the yield variations among systems. Average light interception was highest with the Y-trellis/M.26 system of both cultivars and approached 70% of available PAR with `Empire'. The higher light interception of this system was the result of canopy architecture that allowed the tree canopy to grow over the tractor alleys. The central leader/M.7a had the lowest light interception with both cultivars. The efficiency of converting light energy into fruit (conversion efficiency = fruit yield/light intercepted) was significantly higher for the Y-trellis/M.26 system than for the slender spindle/M.9 or central leader/M.9/MM.111 systems. The central leader/M.7a system bad the lowest conversion efficiency. An index of partitioning was calculated as the kilograms of fruit per square centimeter increase in TCA. The slender spindle/M.9 system had significantly higher partitioning index than the Y-trellis/M.26 or central leader/M.9/MM.111. The central leader/M.7a system had the lowest partitioning index. The higher conversion efficiency of the Y/M.26 system was not due to increased partitioning to the fruit; however, the basis for the greater efficiency is unknown. The poor conversion efficiency of the central leader/M.7a was mostly due to low partitioning to the fruit. The Y-trellis/M.26 system was found to be the most efficient in both intercepting PAR and converting that energy into fruit.

A field planting of `Empire' and `Redchief Delicious' apple trees (Malus domestics Borkh.) was established in 1978 to evaluate four planting systems: 1) slender spindle/M.9, 2) Y-trellis/M.26, 3) central leader/M.9/MM.111, and 4) central leader/M.7a. During the first 5 years, yields per hectare for `Empire' were positively correlated with tree density. In the second 5 years, the Y-trellis/M.26 trees produced the highest yields while yields of the other systems continued to be related to tree density. Cumulative yields were highest with the Y-trellis/M.26 trees. With `Delicious', the Y-trellis/M.26 yields were greatest during all 10 years despite lower tree density than the slender spindle/M.9. Yields of `Delicious' with the other three systems were a function of tree density during the 10 years. At maturity, canopy volume per tree was greatest on the central leader/M.7a trees and smallest on the slender spindle/M.9 trees; however, there were no significant differences in canopy volume per hectare between the systems despite large differences in yield. Trunk cross sectional area (TCA) per hectare was greatest with the Y-trellis/M.26 trees and smallest with the central leader/M.7 trees. Yield was highly correlated to TCA/ha. Yield efficiency with `Empire' was greatest for the slender spindle/M.9 system, followed by the Y-trellis/M.26, central leader/M.9/MM.111, respectively. With both cultivars, the central leader/M.7a system had the lowest yield efficiency. With `Delicious', there were no differences in yield efficiency for the other three systems. The greater yield of the Y-trellis/M.26 system was the result of greater TCA/ha and not greater efficiency. `Empire' fruit size was largest on the central leader/M.7a and the central leader/M.9/MM.111 trees and smallest on the slender spindle/M.9 and the Y-trellis/M.26 trees. With `Delicious', fruit size was larger with the Y-trellis/M.26 trees than the other systems. When fruit size was adjusted for crop density, there were no significant differences due to system with `Empire', but with `Delicious' the Y-trellis/M.26 trees had larger adjusted fruit size than the other systems. Crop density calculated using TCA correlated better to fruit size than did crop density calculated using annual increase in TCA, canopy volume, or land area. Fruit color and quality with `Redchief Delicious' were not influenced by system. With `Empire', average fruit color and soluble solids content were lower for the Y-trellis/M.26 and slender spindle/M.9 in some years when canopy density was allowed to become. excessive.

Abstract

Limbs of ‘Miller Sturdeespur Delicious’ apple trees (Malus domestica Borkh.) with bearing spurs ranging in age from 2 to 11 years were provided with differing solar exposure levels ranging from 5% to 95% of full sunlight (400 to 700 nm) from 55 days postbloom until harvest. As the exposure level of the limb canopy was reduced, fruit length, width, weight, soluble solids, starch content, and total solids were reduced while fruit firmness and total acidity were increased. Visual fruit red color of this high-coloring strain was not affected. As spur age increased, fruit length, width, weight, and soluble solids decreased while fruit firmness and total acidity increased. Spur age did not influence fruit red color, starch content, or total solids. Light exposure level accounted for a relatively large portion of the variation in fruit size and quality between limbs while spur age accounted for only a small portion of the variation within each limb.

The ability of certain apple rootstocks to dwarf their scions has been known for centuries and their use revolutionized apple (Malus ×domestica) production systems. In this investigation, several apple rootstock breeding populations, planted in multiple replicated field and pot experiments, were used to ascertain the degree of dwarfing when grafted with multiple scions. A previous genetic map of a breeding population derived from parents ‘Ottawa 3’ (O.3) and ‘Robusta 5’ (R5) was used for quantitative trait locus (QTL) analysis of traits related to scion vigor suppression, induction of early bearing, and other tree size measurements on own-rooted and grafted trees. The analysis confirmed a previously reported QTL that imparts vigor control [Dw1, log of odds (LOD) = 7.2] on linkage group (LG) 5 and a new QTL named Dw2 (LOD = 6.4) on LG11 that has a similar effect on vigor. The data from this population were used to study the interaction of these two loci. To validate these findings, a new genetic map comprised of 1841 single-nucleotide polymorphisms was constructed from a cross of the dwarfing, precocious rootstocks ‘Geneva 935’ (G.935) and ‘Budagovsky 9’ (B.9), resulting in the confirmation and modeling of the effect of Dw1 and Dw2 on vigor control of apple scions. Flower density and fruit yield data allowed the identification of genetic factors Eb1 (LOD = 7.1) and Eb2 (LOD = 7.6) that cause early bearing of scions, roughly colocated with the dwarfing factors. The major QTL for mean number of fruit produced per tree colocated with Dw2 (LOD = 7.0) and a minor QTL was located on LG16 (LOD = 3.5). These findings will aid the development of a marker-assisted breeding strategy, and the discovery of additional sources for dwarfing and predictive modeling of new apple rootstocks in the Geneva^® apple rootstock breeding program.

The influence of rootstock on average fruit weight was evaluated for a subset of data from a multilocation NC-140 apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] rootstock trial. Data for eight dwarf rootstocks were collected at four locations for 2 years. Analysis of covariance was used to evaluate the effect of rootstock on average fruit weight when crop density or number of fruit per tree was included in the linear model as a covariate. When number of fruit harvested per tree was used as a covariate, average fruit weight was not affected by rootstock in either year in Ontario. In Michigan and Virginia, rootstock and number of fruit per tree, but not the rootstock × number of fruit interaction, were significant, so common slopes models were used to estimate least squares means for average fruit weight. In general, trees on M.27 and P.1 produced the smallest fruit, and trees on B.9, M.9 EMLA, and Mac.39 produced the largest fruit. In New York the interaction of rootstock × number of fruit was significant, so least squares means were estimated at three levels of number of fruit per tree. Both years, at all levels of number of fruit, trees on M.26 EMLA produced the smallest fruit and trees on M.27 EMLA produced the largest fruit. Average fruit weight was most affected by number of fruit per tree when Mark was the rootstock. In general, results were similar when crop density was used as the covariate, except that trees on M.27 EMLA did not produce small fruit in Michigan and Ontario.

Although apple (Malus domestica Borkh.) system yield differences are generally related to whole-canopy light interception, this study tested the hypothesis that these orchard yields are related primarily to total light intercepted by the spur canopy. Seasonal leaf area development of different shoot types, exposed bourse shoot leaf net photosynthesis, fruit growth, whole canopy light interception (by image analysis of fisheye photographs) and relative light interception by different shoot types (by a laser assisted canopy scanning device) were estimated within four 14-year-old `Empire' apple production systems (slender spindle/M.9, central leader/M.7, central leader/M.9/MM.111 and Y-trellis/M.26). The final LAI values were CL/M.7 = 1.8, CL/9/111 = 2.3, SS/M.9 = 2.6 and Y/M.26 = 3.6. Exposed leaf net photosynthesis showed few differences and was not dependent upon the production system. Yields of the pyramidal shaped tree forms were 40 to 42 t·ha^-1 while Y-trellis produced 59 t·ha^-1, with similar fruit sizes. Again, yields were primarily related to the percentage of light intercepted by the whole canopy, 48% to 53% for conic forms versus 62% for the Y-trellis system. Laser analyses showed that the Y-trellis system intercepted about 20% to 30% more light with the spur canopy than the conic tree forms, supporting the hypothesis. Yields were better correlated with spur canopy LAI and spur canopy light interception than with extension shoot canopy LAI and light interception.

Mature `Empire' and `Redchief Delicious' apple trees (Malus domestica Borkh.) trained to a Y-shaped trellis (Y/M.26) or trained as pyramid-shaped central leaders (CL/M.7) were mechanically harvested with the Cornell trunk recoil-impact shaker during 4 years. With `Empire', fruit removal from the Y/M.26 trees (85% to 90%) was significantly less than from the CL/M.7 trees (95% to 97%). With `Delicious' there were no differences in fruit removal (90% to 95%) between the two tree forms in any year. When the catching pad was on the ground, fruit grade based on damage was only slightly better for the Y/M.26 trees than for the CL/M.7 trees. When the catching pad was raised up near the Y/M.26 canopy, fruit grade was significantly improved for the Y/M.26 trees and was better than the CL/M.7 trees. Fruit grade for both cultivars ranged from 83% to 94% Extra Fancy with 5% to 16% culls for the Y/M.26 trees and from 74% to 88% Extra Fancy and 11% to 21% culls for the CL/M.7 trees. Skin punctures, skin breaks, and number of large and small bruises were lower and the percentage of nondamaged fruit was higher with the Y/M.26 trees when the pads were close to the canopy than when the pads were on the ground. The CL/M.7 trees had higher levels of all types of fruit damage than did the Y/M.26 trees. Damaged fruit from the CL/M.7 trees was mainly from the top half of the tree, while fruit from lower-tier scaffold branches had low levels of damage. Mechanically harvested fruit from the Y/M.26 trees had lower incidences of fruit rot and flesh breakdown after a 6-month storage period than did fruit from the CL/M.7 trees. Stem pulling was high with both systems and averaged 60% for `Delicious' and 30% for `Empire'. The advantage of the single plane Y-trellis system for mechanical harvesting appears to be that the catching pads can be placed close to the fruit, thereby reducing fruit damage.