Yield of sweet cherry (Prunus avium L.) is determined by fruit set, a developmental stage sensitive to variable spring environmental conditions. To sustain fruit production and enhance crop climate resilience, it is important to understand the impacts of abiotic stresses and the effectiveness of horticultural mitigations in the spring on the critical developmental processes during fruit set. In this study, flowering phenology, pistil browning and percent fruit set of ‘Lapins’ were monitored at five sites of different elevation and frost risk in the Okanagan Valley, British Columbia, Canada, in 2019 and 2022. At Site 1 in Summerland Research and Development Centre (“SuRDC1”), where a ‘Lapins’ on Krymsk 5 planting was located in a frost pocket where the crops were exposed to high risk of cold damage in the spring, a series of experiments were conducted to investigate the floral organ viability and percent fruit set under low temperatures, and under the effects of four spring horticultural mitigation measures. Installation of polyethylene sleeves and FAME spray (fatty acid methyl esters-based plant growth regulator, WAIKEN, SST Australia) were implemented in 2019; boric acid spray and postponed irrigation were tested in 2022. Low fruit set at SuRDC1 in both years was associated with severe pistil browning after night temperature dropped below −4 °C in late April. In 2019, the semi-enclosure of polyethylene sleeves led to an increase in the surface temperature (Tsurfae) of floral buds by 2–4 °C, which prolonged the stage of first bloom, delayed petal fall and prevented frost damage on pistils, but led to the decrease in percent fruit set by 77%, due to ovule abortion or cessation of fruitlet development. The early and late sprays of FAME had no significant influence on either abundance of germinated pollen tubes or percent fruit set; however, the potential of late spray in improving pollen abundance and reducing pistil browning requires further investigation. In 2022, the spray of 0.01% boric acid solution led to a decrease in fruit set by 6.95%. Six-week postponement of irrigation starting from full bloom decreased soil moisture, but increased soil temperature and improved fruit set by 7.61%. The results improved our understanding about the damages of adverse spring air temperatures on pistils and ovules, and suggested the potential of irrigation adjustment in regulating soil moisture and temperature and improving fruit set in the cool and moist spring.

High temperature deteriorates apple tree performance and fruit growth. In the summer of 2021, heat waves affected the apple production zones in the Pacific Northwest of North America. In a 'Buckeye Gala' experimental trial in Summerland, British Columbia, Canada, heat caused fruit skin sunburn browning (SB), and affected fruit mass, dry matter content, soluble solid content, titratable acidity, firmness and flesh water potential. We studied fruit skin δAbsorbance at 670–720 nm ( δA 670–720), cuticle characteristics, and key stress response compounds in peel and flesh, to elucidate the SB mechanism. SB was associated with thickened cuticle, chloroplast degradation, decreased anthocyanin content and increased firmness on the affected sunlit side of the fruit. As sunburn damage intensified, anthocyanin content in peel decreased, while the concentration of tartaric ester and total phenolics in flesh increased. The δA 670–720, measured using Delta Absorbance (DA) meter, was positively correlated with the total amount of phenolics and tartaric esters in peel and flesh. This suggested that the accumulation of the phenolics and tartaric ester triggered by heat stress contributed to the high δA 670–720 despite the degradation/absence of chloroplasts. The study improved our understanding of the impacts of sunburn severities on fruit quality and of the heat stress responses in 'Buckeye Gala' apple, and demonstrated the potential use of DA meter as a rapid, non-destructive tool in facilitating heat stress ecophysiology study and predicting SB severity in red apple varieties.

In the last decade, a sporadic tree health syndrome affecting high-density apple plantings in North America has become known as Rapid Apple Decline (RAD) or Sudden Apple Decline (SAD). The affected apple trees were typically grafted on small dwarfing rootstocks, often displayed necrosis at the graft union, and suffered from sudden mortality that occurred over 2–3 weeks amid the growing season or a gradual decline. In 2019 and 2020, we conducted a multi-site investigation in the south Okanagan, British Columbia, Canada, to assess the stem hydraulic characteristics, stomatal conductance, leaf δ13C‰, and fruit dry matter accumulation of the declining trees during disease progression. In trees that died, mortality appeared to be associated with severe disruption in xylem water transport at the damaged graft union, followed by abrupt hydraulic failure. In contrast, symptomatic trees that did not die exhibited the moderately declined plant water relations and a reduction in fruit dry matter accumulation followed by either further deterioration or eventual recovery. This pattern indicates the risk of carbohydrate depletion over gradual hydraulic decline and the importance of timely horticultural remedies. In the present study, we discuss potential horticultural practices to mitigate hydraulic dysfunctions and enhance crop tolerance.

Suitable rootstock enhances apple tree resilience. In 2021, we studied "Buckeye Gala" apple (Malus domestica var. Buckeye Gala") on nine rootstocks with contrasting vigor in NS and BC, Canada. Rootstock effects on vigor, yield, and midday stem water potential were significant in BC. After sustained heat events, the large-dwarfing rootstocks Geneva 935, Geneva 4814, and Geneva 969 had lower ratio of sunburn fruits, resulting in higher projected damage-f ree yield. We discussed how higher stem water potential and larger canopy volume supported by vigorous rootstocks contributed to alleviate heat stress and improve apple resilience to global warming.

The 2021 summer heat waves 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.

Rootstocks with internal hydraulic limitations can effectively restrict scion growth, influence crop load, and improve yield efficiency in apple production. The characteristics of xylem vessels in rootstock and scion play essential roles in determining the hydraulic properties of the grafted tree; however, much remains unknown for commonly available rootstocks. In this study, we extracted secondary xylem using an increment borer from living Honeycrisp scion (Malus domestica cv. ‘Honeycrisp’), and two Malling rootstocks, one Budagovsky rootstock, and five Geneva rootstocks. The size and density of xylem vessels in rootstocks and scions were analyzed in relation to trunk cross-section area (TCSA), tree–water relations, and fruit dry matter of 2019, as well as with cumulative yield efficiency during 2014–2019. Honeycrisp scion exceeded most of the rootstocks in cross-section size and density of vessel elements. Scion vigor and cumulative yield were positively correlated with TCSA and total vessel cross-section area (VCSA) of the rootstock, with G.202 being the highest, and B.9 being the lowest with small xylem vessels in high density. In the rootstocks with the highest cumulative yield efficiency, the rootstock/scion ratio in VCSA was equal to or slightly higher than 1. Lower scion vessel density in G.214 was associated with lower fruit dry matter weight, more restricted water relations, and worsened leaf chlorosis. G.935 with larger rootstock vessels led to both high yield and high yield efficiency. This suggested that higher scion vessel density and larger rootstock vessel size can be advantageous characteristics for early-stage evaluation.

In recent years, awareness has been raised around the benefits of diversifying rootstocks, in order to enhance tree health and sustain apple fruit production under the influence of climate change. However, performances of many rootstocks under stresses remain unclear. This study aimed to set the first step towards a much-needed comprehensive evaluation on water relationships and stress responses of scion–rootstock systems for the popular apple cultivar AmbrosiaTM (Malus domestica var. Ambrosia), because its production and horticultural knowledge had been largely limited to the Malling 9 rootstock (M.9). Five rootstocks were evaluated in a greenhouse water deficit experiment and at the onset of heat stress in a field trial in Summerland, British Columbia, Canada. Multiple stress indicators of leaves and fruits were analyzed to elucidate water use strategies and drought resistance mechanisms. The rootstocks led to differences in scion vigor, and stomatal and photosynthetic characteristics. The largest semi-dwarfing Geneva 202 (G.202) demonstrated more water use and higher stress susceptibility. Large dwarfing Geneva 935 (G.935) and Malling 26 (M.26) showed more stringent stomatal control and reduced water use under stresses, typical of a drought-avoidance strategy. The smallest large dwarfing M.9NIC29® and the small dwarfing Budagovsky 9 (B.9) led to smaller and denser stomata. B.9 demonstrated the most stable water status and drought tolerance. The study suggested that scion stress responses were influenced by rootstock vigor and tree water use strategies. It implied the necessity of vigor-specific irrigation management for alleviating stresses and achieving production goals of different rootstocks.