Characteristics of Sunburn Browning Fruit and Rootstock-Dependent Damage-Free Yield of Ambrosia™ Apple after Sustained Summer Heat Events

Abstract

The 2021 summer heatwaves experienced in the Pacific Northwest led to considerable fruit damage in many apple production zones. Sunburn browning (SB) was a particularly evident symptom. To understand the mechanism underlying the damage and to facilitate the early assessment of compromised fruit quality, we conducted a study on external characteristics and internal quality attributes of SB ‘Ambrosia’ apple (Malus domestica var. Ambrosia) and evaluated the fruit loss on five rootstocks. The cell integrity of the epidermal and hypodermal layers of fruit skins in the SB patch was compromised. Specifically, the number of chloroplasts and anthocyanin decreased in damaged cells, while autofluorescent stress-related compounds accumulated in dead cells. Consequently, the affected sun-exposed skin demonstrated a significant increase in differential absorbance between 670 nm and 720 nm, measured using a handheld apple DA meter, highlighting the potential of using this method as a non-destructive early indicator for sunburn damage. Sunburn browning eventually led to lower fruit weight, an increase in average dry matter content, soluble solids content, acidity, deteriorated weight retention, quicker loss of firmness, and accelerated ethylene emission during ripening. Significant inconsistency was found between the sun-exposed and shaded sides in SB apples regarding dry matter content, firmness, and tissue water potential, which implied preharvest water deficit in damaged tissues and the risk of quicker decline of postharvest quality. Geneva 935 (G.935), a large-dwarfing rootstock with more vigor and higher water transport capacity, led to a lower ratio of heat-damaged fruits and a higher yield of disorder-free fruits, suggesting rootstock selection as a long-term horticultural measure to mitigate summer heat stress.

Hydrogel Improved Growth and Productive Performance of Mango Trees under Semi-Arid Condition

“…These results concur with those of Awad et al [ 24 ] in the apple seedling and those of Cavalcante et al [ 25 ] in passion fruit. In addition, by increasing the ratio of hydrogel in the substrate’s culture, Hüttermann et al [ 26 ], Akhter et al [ 27 ], and Chirino et al [ 28 ] found increases in shoot height. In this study, as expected, the highest vegetative growth parameter values were found in the hydrogel at a concentration of 750 g hydrogel∙tree −1 .…”

“…Tommy Atkins maintained higher values with respect to deficit irrigation. There was a significant relationship between the water deficit and the reduction of cell size, and the reduction of water in plant tissues [ 28 ]. These results reflect that the highest average of some fruit physical parameters, such as fruit weight and fruit volume, recorded high values with an increase in hydrogel [ 2 , 42 ].…”

Seasonal Changes in Amylose and Starch Compositions in ‘Ambrosia’ Apples Associated with Rootstocks and Orchard Climatic Conditions

“…This suggests that the orchards had consistently warm but not adversely hot temperatures in 2016. Importantly, there were no extreme high-temperature events, often referred as "heat waves," experienced in the orchards during 2016 [16,17]. This indicates that the temperatures in 2016 remained within an optimal range for apple production, which contributed to a higher fruit DMC (Figure 2) and higher fruit mass (See Section 3.1) than 2017.…”

“…Similarly, lower night-temperature accumulations and a higher day-night temperature difference (DTR accumulation) were related to dry matter accumulation, relative starch contents, and onset of starch clearing [10,13]. In addition to the above-mentioned temperature variables, recent studies have demonstrated the significant impacts of rootstocks on DMC accumulation during the growing season, and on fruit mass, DMC, SSC, and color coverage and intensity at maturation [14][15][16][17]. These impacts could be attributed to the alteration of scion hydraulics and photosynthetic capacity by rootstocks [18,19].…”