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  • Author or Editor: Lee A. Kalcsits x
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‘Honeycrisp’ apple is susceptible to bitter pit, which is associated with fruit mineral nutrient composition. Rootstock genotypes can affect nutrient acquisition, distribution, and fruit yields, which all affect fruit nutrient composition and bitter pit susceptibility. However, the changes of these traits among different rootstock genotypes in response to abiotic stress under semiarid conditions are relatively unknown. The objective of this study was to evaluate the influence of different rootstocks and irrigation on nutrient uptake and partitioning with ‘Honeycrisp’ apple grown in an irrigated, semiarid environment. ‘Honeycrisp’ apple trees were grafted on four different rootstocks, Geneva 41 (‘G.41’), Geneva 890 (‘G.890’), M.9-T337 (‘M.9’), and Budagovsky 9 (‘B.9’), and these were planted at high density (3000 trees/ha). Irrigation was applied as either a water-limited treatment where volumetric soil water content was maintained near 50% field capacity (FC) and a well-watered control where soil water content was maintained near 100% FC. ‘G.890’, the most vigorous rootstock, had lower nitrogen and higher potassium content in leaves, while ‘B.9’, the least vigorous rootstock, had lower potassium and higher nitrogen content. Rootstock genotype did not affect calcium uptake. Interestingly, water-limited conditions increased the nutrient content in root and stems but not in leaves. Water-limited trees partitioned more nitrogen and calcium to roots, while well-watered trees in the control partitioned more nutrients to the stems. Fruit size was the largest for ‘G.890’ and smallest for ‘B.9’. Both ‘G.41’ and ‘G.890’ had higher bitter pit incidence, which was associated with higher potassium content in leaves and fruit. These results suggest that rootstock-induced vigor and irrigation can both contribute to nutrient imbalances in leaves and fruit that could affect the development of physiological disorders in ‘Honeycrisp’ apple.

Open Access

Complex changes in gene expression occur during postharvest storage of apple (Malus ×domestica) and often precede or accompany changes in ripening and disorder development. Targeted gene expression analysis fundamentally relies on previous knowledge of the targeted gene. Minimally, a substantial fragment of the gene sequence must be known with high accuracy so that primers and probes, which bind to their targets in a complimentary fashion, are highly specific. Here, we describe a workflow that leverages publicly available transcriptome data to discover apple cultivar–specific gene sequences to guide primer design for quantitative real-time polymerase chain reaction (qPCR). We find that problematic polymorphisms occur frequently in ‘Granny Smith’ and ‘Honeycrisp’ apple when candidate primer binding sites were selected using the ‘Golden Delicious’ genome. We attempted to validate qPCR-based gene expression measurements with RNA sequencing (RNA-Seq) analysis of the same RNA samples. However, we found that agreement between the two technologies was highly variable and positively correlated with the similarity between cultivar-specific genes and RNA-Seq reference genes. Thus, we offer insight that 1) improves the accuracy and efficiency of qPCR primer design in cultivars that lack sufficient sequence resources and 2) better guides the essential step of validation of RNA-Seq data with a subset of genes of interest examined via qPCR.

Free access

Globally, apple production often occurs in semiarid climates characterized by high summer temperatures and solar radiation. Heat stress events occur regularly during the growing season in these regions. For example, in the semiarid eastern half of Washington State, historic weather data show that, on average, 33% of the days during the growing season exceed 30 °C. To mediate some of the effects of heat stress, protective netting (PN) can be used to reduce the occurrence of fruit sunburn. However, the impacts of reduced solar radiation in a high light environment on light-use efficiency and photosynthesis are poorly understood. We sought to understand the ecophysiological response of apple (Malus domestica Borkh. cv. Honeycrisp) under blue photoselective PN during days with low (26.6 °C), moderate (33.7 °C), or high (38.1 °C) ambient temperatures. Two treatments were evaluated; an uncovered control and blue photoselective PN. Maximum photochemical efficiency of PSII, or photosystem II (Fv/Fm) was significantly greater at all measurement times under blue photoselective PN compared with the control on days with high ambient temperatures. Fv/Fm dropped below 0.79, which is considered the threshold for stress, at 1000 hr in the control and at 1200 hr under blue photoselective PN on a day with high ambient temperature. On days with low or moderate ambient temperatures, Fv/Fm was significantly greater under blue photoselective PN at 1400 hr, which coincided with the peak in solar radiation. ‘Honeycrisp’ apple exhibited dynamic photoinhibition as shown by the diurnal decline in Fv/Fm. Quantum photosynthetic yield of PSII (ΦPSII) was also generally greater under blue photoselective PN compared with the control for days with moderate or high ambient temperatures. Photochemical reflectance index (ΔPRI), the difference in reflectance between a stress-responsive and nonstress-responsive wavelength, was greater under PN compared with the control on the day with high ambient temperatures, with no differences observed under low or moderate ambient temperatures. Leaf gas exchange did not show noticeable improvement under blue photoselective netting when compared with the control despite the improvement in leaf-level photosynthetic light use efficiency. In conclusion, PN reduced incoming solar radiation, improved leaf-level photosynthetic light use efficiency, and reduced the symptoms of photoinhibition in a high-light, arid environment.

Free access

In semiarid apple (Malus domestica) growing regions, high temperatures and excessive solar radiation can increase the risk of sunburn development. Protective netting is increasingly used as a cultural practice under these conditions to mitigate fruit sunburn losses. However, fruit skin color development can be negatively affected under protective nets due to the reduction in light availability. Reflective groundcovers have been previously reported to increase fruit color development, particularly in the inner parts of the tree canopy. Here, we compared two types of reflective groundcover: a woven polyethylene fabric and a film material with a grassed control without reflective material under a protective netting installation that reduced photosynthetically active radiation (PAR) by 17%. The experiment was conducted in a semiarid climate on a 5-year-old ‘Cameron Select Honeycrisp’ apple orchard near Quincy, WA. Light penetration into the canopy was measured with a PAR sensor. At harvest, fruit quality, yield, and size were assessed. The use of reflective groundcover between the rows significantly increased reflected PAR into the lower canopy. Moreover, reflective groundcovers significantly increased the amount of fruit with greater than 25% skin red color compared with the control. Reflective groundcover did not affect fruit weight, yield, and fruit number. The use of reflective groundcover under protective netting can increase light penetration into the canopy, thereby improving fruit skin red coloration in apple.

Open Access