Crop Load Influences Fruit Quality, Nutritional Balance, and Return Bloom in ‘Honeycrisp’ Apple

By S. Serra, R. Leisso, L. Giordani, L. Kalcsits, S. Musacchi. 2016

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The apple variety, ‘Honeycrisp’ has been extensively planted in North America during the last two decades. However, it suffers from several agronomic problems that limit productivity and postharvest quality. To reduce losses, new information is needed to better describe the impact of crop load on productivity and postharvest fruit quality in a desert environment and the major region where ‘Honeycrisp’ expansion is occurring. Here, 7-year-old ‘Honeycrisp’ trees on the M9Nic29 rootstock (2.53 0.9 m) were hand thinned to five different crop loads [from 4.7 to 16.0 fruit/cm of trunk cross-sectional area (TCSA)] to compare fruit quality, maturity, fruit size, elemental concentration, and return bloom. Fruit size distribution was affected by crop load. Trees with the highest crop load (16 fruit/cm) produced smaller fruit. Index of absorbance difference (IAD) measurements (absorption difference between 670 and 720 nm), a proxy indicator of the chlorophyll content below the skin of fruit measured by a DA-meter, were made shortly after harvest (T0) and after 6 months of storage (T1). Fruit from the trees with the lowest crop load had lower IAD values indicating advanced fruit ripeness. The comparison between the IAD classes at T0 and T1 showed that fruit belonging to the lowest IAD class had significantly higher redblushed overcolor percentage, firmness, dry matter, and soluble solid content than those in the ‘‘most unripe’’ class (highest IAD readings) regardless of crop load. The percentage of blushed color, firmness, titratable acidity (TA), soluble solids content, and dry matter were all higher in the lowest crop loads at both T0 and T1. Fruit calcium (Ca) concentration was lowest at the lowest crop load. The (K + Mg + N):Ca ratio decreased as crop load increased until a crop load of 11.3 fruit/cm, which was not significantly different from higher crop loads. For return bloom, the highest number of flower clusters per tree was reported for 4.7 fruit/cm crop load, and generally it decreased as crop load increased. Here, we highlight the corresponding changes in fruit quality, storability, and elemental balance with tree crop load. To maintain high fruit quality and consistency in yield, careful crop load management is required to minimize bienniality and improve fruit quality and storability. The apple variety, ‘Honeycrisp’, is a premium fresh fruit marketplace variety (Embree et al., 2007; Rosenberger et al., 2004) selected in Minnesota. As such, it is well adapted to northern climatic conditions (Luby and Bedford, 1990). ‘Honeycrisp’ is a profitable apple for growers and, for this reason, the acreage is expanding from cold to warmer areas (Nichols and Wright, 2003) including Washington State. Orchard management and the production of high quality for ‘Honeycrisp’ fruit can be challenging, especially in hot climate areas like Washington State. Washington’s climate varies greatly from west to east. In the Columbia Basin, one of the main area for apple production, the climate is semiarid (http://www.wrcc.dri. edu). Early fruit production before the trees have developed a complete canopy can permanently limit the final tree size and consequently, can impact future productivity (Castro et al., 2015). Crop load management is particularly important for ‘Honeycrisp’ because it can limit inconsistent annual flowering and fruit set (Embree et al., 2007), improve fruit quality (Baugher and Schupp, 2010), and decrease susceptibility to storage disorders (Robinson and Watkins, 2009). ‘Honeycrisp’ has exhibited an extreme predisposition to overcropping, therefore, is susceptible to biennial bearing (Crassweller et al., 2005; Embree and Nichols, 2005; Robinson and Watkins, 2003; Wright et al., 2006), especially when thinning is completed too late (Pellerin et al., 2011, 2012). Thinning is a common agronomical practice to optimize crop load to improve fruit size and quality as well as reducing biennial bearing (Dennis, 2000; Link, 2000; Wertheim, 2000). Thinning affects the cell number and fruit size (Bain and Robertson, 1951), and is considered effective within 4 to 12 weeks after full bloom, while its efficacy is cultivar dependent (Bergh, 1990; Denne, 1960). Both crop load and warmer climate conditions can affect the balance of nutrients in the fruits and leaves and, subsequently, affects postharvest fruit quality and postharvest susceptibility to storage disorders. Young ‘Honeycrisp’ orchards are susceptible to bitter pit and symptoms can appear before harvest or during storage (Rosenberger et al., 2004). High vegetative vigor can lead to transpiration imbalances and less Ca being allocated to the developing fruit (W€unsche and Ferguson, 2005). These effects may be further magnified in a hot, desert climate where transpiration pressures are greater. The amount of affected fruit can increase dramatically in situations of excessive vigor and/or low crop loads. Bitter pit is associated with low Ca concentration in fruit and the elemental balance between Ca, potassium (K), magnesium (Mg), and nitrogen (N). High concentrations of K, Mg, and N are associated with an increase in the percentage of fruit lost to bitter pit. Crop load has a strong effect on fruit size (Embree et al., 2007). Fruit size can also be influenced by low soil pH, boron deficiency, and environmental stress such as drought (Faust and Shear, 1968; Ferguson and Watkins, 1989; Rosenberger et al., 2004). Bangerth (1979) and W€unsche and Ferguson (2005) reported that larger fruit have lower Ca concentrations suggesting that high fruit growth rates in late stages of fruit development can lead to fruit Ca deficiencies. High crop loads can limit tree vigor and fruit size and can decrease the incidence of this physiological disorder. However, it is well known that a high crop load can reduce size, color appearance, lower flesh firmness, lower sugars, and acidity of fruits (Embree et al., 2007; Hampson and Kemp, 2003) in addition to return bloom the following year. Crop load can also affect the photosynthesis in apple tree (Palmer et al., 1997) and dry matter Received for publication 2 Oct. 2015. Accepted for publication 13 Jan. 2016. We thank Celeste Wheeler for their helpful technical assistance. Corresponding author. E-mail: stefano.musacchi@ wsu.edu. 236 HORTSCIENCE VOL. 51(3) MARCH 2016 production and carbon allocation (Avery, 1975; Heim et al., 1979; Palmer, 1992; Palmer et al., 2010). The effect of crop load on ‘Honeycrisp’ postharvest quality has not been widely reported in Washington State. Here, the main objective was to determine the impact of different crop loads on fruit quality, storability, nutritional balance, and return of bloom in ‘Honeycrisp’ in a desert environment. Our hypothesis was that lower crop loads would promote more return bloom and increase fruit quality for ‘Honeycrisp’ grown in warmer climates. Additionally, our preliminary work indicated that a nondestructive approach using the DA-meter (Ziosi et al., 2008) can be used to predict quality attributes of pome fruit in storage. Additionally, we sought to further test the application of DA-meter for ‘Honeycrisp’ fruit maturity and storability assessments. Understanding the impact of tree crop load on mineral nutritional balance in combination with fruit quality, ripening, and storability can help to optimize tree management and better identify optimum crop loads suitable for ‘Honeycrisp’ grown in warmer climates. Materials and Methods Field conditions. The experimental field was located in Naches, WA (46 43#23.01##N, 120 40#26.07##W). Seven-year-old apple trees cv. Honeycrisp grafted on M9-NIC29 were trained to a vertical axis with a planting density of 2.5 · 0.9 m (4444 trees/ha). The orchard was watered with drip irrigation and mulched along the rows with coarse bark chips. Soil in the trial location was collected (at 20 cm from the trunk under the mulch) at two depths (0–30 and 30–60 cm) pooling together three rows. Soil was classified as a loam (U.S. Department of Agriculture texture) with 50% sand, 12% clay, and 38% silt in the first 30 cm of soil sampling, while 40% sand, 14% clay, and 46% silt in 30–60 cm depth. pH was 7.7 and 7.2 at the two depths, respectively, which is a little higher than conventional apple orchards in the same region of Washington (Glover et al., 2000). Soil analysis did not indicate abnormal nutrient or pH imbalances and fertility was managed with a combination of fertigation applied with drip irrigation and foliar treatments. Tree size was variable within the orchard. To limit the effect of tree-to-tree variation on fruit productivity and quality during tree sample selection, trunk cross-sectional area was measured for 40 trees that ranged from 5.2 to 10.0 cm (Supplemental Fig. 1). Fifteen homogeneous trees (average height 314 cm) were selected with a trunk cross-sectional area between 7 and 9 cm (measured at 15 cm from the grafting point). In 2014, full bloom in the experimental block occurred during the last week of April. At the end of June, sample trees were hand thinned to five different crop load targets; 30–40, 50–65, 75–85, 90–100, and 125–135 fruit/tree, which at harvest averaged 34, 56, 84, 95, and 130 fruit/tree, respectively. Converting to fruit/TCSA cm crop was equal to 4.7, 7.5, 11.3, 12.5, and 16.0 fruit/TCSA cm, from low to high crop loads described above, respectively. These were the actual number of fruit remaining on the tree at harvest and, therefore, crop loads were not evenly distributed between treatments. Fruit analysis. Fruit were harvested in a single picking (1 week before commercial harvest) on 28 Aug. 2014. The number of fruits and yield (kg/tree) were measured for each tree. Yield per tree and total number of fruit per tree were combined with trunk crosssectional area to calculate yield efficiency (kg/cm) and crop load (fruit/cm), respectively. Fruit from each tree was categorized into fruit size classes (<55, 55–59, 60–64, 65–69, 70

Subfields: Molecular Biology, Plant Science

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