1. Introduction
The Ambrosia™ apple (
Malus ×
domestica Borkh.) is the most promoted and rapidly growing cultivar in Canada, and is becoming popular on the global fruit market [
1]. The apple is easily recognized for its unique sweetness [
2], and is categorized as “a sweet, juicy and flavourful eating apple” in the fresh apple market (Pomiferous; website at
https://pomiferous.com/applebyname/ambrosia-id-176, accessed on 21 March 2021). Thus, the crisp and sweet characteristics are key to its marketability. Sensory studies demonstrated that the consumer acceptability of an apple can be further enhanced by improving its taste [
2,
3]. In contrast, although apple texture is a paramount attribute of quality, optimizations in controlled atmospheric storage have made this less of a forefront issue for the consumer market [
4,
5]. Customers now favor high taste intensity in fresh market products [
3,
6]. The total soluble solid contents (SSC) and titratable acidity (TA) play the most important roles in the perception of fruit taste [
7], and in creating the flavoring substances for organoleptic quality similar to those of fresh apples [
8]. A main determinant of SSC and TA is the dry-matter content (DMC), which includes carbohydrates, acids and proteins that directly determine taste attributes and flavor profiles [
9,
10,
11]. Although they are highly correlated with SSC, DMC serves as a better estimate of taste than SSC due to its broad and balanced contents [
10,
11,
12,
13]. Structural carbohydrates (pectin, fiber, etc.) are used to build and maintain texture and, together with sugars and organic acids, belong to the dry matter in apple fruit. A higher DMC means a higher total carbohydrate level, which translates into better quality and better quality retention [
3,
7,
14]. A post-harvest study showed that apples with better storability had greater DMC accumulation during fruit development compared to apples with lower DMC that rapidly softened post-harvest [
15]. In addition, the DMC in pre-harvest, harvest, and post-harvest are highly related [
14]. Thus, DMC can be used to predict fruit quality [
7,
12] and is a new quality metric for apples [
3,
13].
Ambrosia™ has the unique feature of being ready to eat on the tree with a full fruity taste, which endows this apple with a great advantage on the direct fresh market [
16]. However, the Ambrosia™ apple naturally has limited post-harvest quality retention [
17,
18], which is accompanied by and is synchronous with a relatively low DMC based on our investigation (
Table S1). The DMC generally depends on the apple cultivar [
14], and abiotic factors such as the weather (precipitation, temperature, wind) and orchard practices affect DMC as well [
14]. Apples with a higher DMC (DMC > 16%) lose starch more slowly during storage than those with a low DMC (DM < 13%) [
19]. Fruit firmness, both at-harvest and post-harvest, is positively correlated with the fruit DMC [
14,
19,
20]. In addition to inherited and abiotic (weather, location and climatic changes) factors, orchard practices can evidently affect the formation of DMC in apples [
14,
21,
22,
23,
24].
Dry matter accumulates as the fruit grows and matures on the tree [
22]. The tree’s physiological aspects hold great potential to affect the DMC in apples [
22,
24]. Orchard irrigation is one of the key factors in the determination of fruit growth and production [16, 23,25]. Deficit irrigation (DI) is increasingly implemented in apple orchards during growth seasons across the world [
26,
27]. DI involves supplying a tree with an amount of water that is less than 100% of the plant’s water needs. DI is a watering strategy that can be applied in different types of irrigation regimes, such as controlled, temporal, continual, or scheduled DI with different levels of water scarcity (mild, regulated, or drought) [
26,
27,
28,
29,
30]. The correct application of DI requires a thorough understanding of the yield response to water (crop sensitivity to drought stress). The amount of water to be supplied in DI can be calculated or planned based on several parameters, such as the water demand of the plants (the percentage of reference evapotranspiration of the plant), the measurement of plant parameters (e.g., stem water potential), or the water content, moisture, or water holding capacity in the soil. The amount of deficit also depends on the growth stage, and sometimes on the variety or cultivar of a particular species. Studies on irrigation suggest that applying DI during the period after fruit cell division is critical, as fruit growth is slow and shoot growth is rapid during this time. Practical trials have shown that mild water stress applied during this period controlled excessive vegetative growth while maintaining yields [
23,
26]. The proper timing of DI can not only improve efficiency of water usage but can also benefit fruit production in ways such as preventing the oversizing of fruit growth and enhancing fruit quality [
30,
31]. A regulated DI trial with Braeburn apples determined that DI applied between 40 and 70 days after full bloom (AFB) (the stage of peak cell expansion) resulted in apples with both the highest marketable yield and the highest red colour density in comparison to adequate irrigation, which was the “commercially irrigated control” [
29,
32]. There is increased interest in the use of periodic DI on apple trees to improve the fruit quality and enhance the sustainability of orchard production. However, excess or extended water scarcity has made Ambrosia™ apples more susceptible to soft scald disorder after subsequent exposure to chills [
1]. There is also a lack of knowledge regarding to the effects of different irrigations on the accumulations of DMC of apple fruit.
The major apple production areas in the northwest coastal regions of North America are semi-arid, and the apple industries in the regions such as the Okanagan–Similkameen Valley of British Columbia are heavily dependent on irrigation [
25]. Practically, commercial irrigation is implemented based on the Tree-Fruit Guide [
17], in which apple trees need to be irrigated to full water-holding capacity every 7–10 days during the summer. To date, the effect of deficit irrigation has not been well studied in Ambrosia™ apples. Furthermore, the critical timing of the implementation of DI in this cultivar still lacks detailed management guidelines for growers. This study aims to identify the impact of DI timing on Ambrosia™ apple fruit quality at harvest and post-harvest in a semi-arid region in consecutive years. DMC is the primary outcome measure, and the secondary outcome measures analyzed include the red blush color and compositional attributes.
4. Discussion and Suggestions
Irrigation is a substantial factor governing tree growth, fruit development and fruit quality [
23]. Excessive irrigation causes vigorous growth [
16] and the development of diseases in fruit, such as bitter pits [
39]; in contrast, water deficit led to the irregular growth of fruit (
Table 2 and
Table 3) or even to the death of the tree (6 out of 18 trees died in DD in 2018, as indicated in
Table S2). Because the climate in the Okanagan area was wet in the spring from the winter snow, with major rainfall for the whole growth season in the early summer (
Figure 1), an early reduction in irrigation did not cause adverse affects in the trees. That is why the ED practice works out for quality improvement with fewer flaws in the experimental site.
An SWP of −1.5 MPa is considered a general benchmark for plant water deficit, and is the wilting point for many crops [
22]. However, this did not seem to be the case in the present study, which suggested that dwarfing Ambrosia
TM can tolerate early-season short-term DI. As is further indicated in
Figure 6, the trees that experienced ED were subsequently able to resume a normal
SWP value. However, an SWP of −2 MPa seems to be the critical point of triggering DI stress such as “small” and “green” fruit (
Table 2 and
Table S2) and tree damage (there was no crop in the second year; data not show). These preliminary measurements only revealed the basic relation between
SWP and tree physiology tolerance; accurate
SWP measurement to indicate tree water demand needs further study. Additionally, in semi-arid regions, Ambrosia
TM apples seem highly sensitive to the water status of the tree during the late growth season, in which the SWP has to be over −1 MPa, otherwise the fruit would suffer SS disorder in the subsequent post-harvest stage (such as DD in 2019). Similar observations were documented previously in this apple in a row-cover trial in the 4 weeks up to harvest, in which the solid film preventing water penetration to the soil contributed to inducing heat and/or water stress, and caused SS disorder post-harvest [
1].
The timing of the water deficit is the key to properly leveraging water management [
17,
23]. The growth cycle of the fruit tree is classified in five stages: stage 1—budburst and flowering (fruit growth by cell division); stage 2—beginning of rapid shoot growth while fruit grows slowly; stage 3—beginning of fruit filling (rapid fruit growth with cell expansion while the shoots grow slowly); stage 4—harvest; and stage 5—leaf fall [
23]. Amongst these, stage 1 occurs before “early summer.” Though water supplement is highly demanded during this stage, orchardists in the Okanagan–Similkameen region are not concerned about irrigation; there is naturally plenty of melted snow and rainfall, and temperatures are relatively low in this stage (
Farmwest.com, accessed on 21 March 2021). However, stage 2 and stage 3 are highly challenging times for water management due to the dry and hot weather. Conventionally, growers used to supply more water via frequent irrigation or extended irrigation (represented by CI in this study). However, this approach to irrigation can potentially negatively impact either fruit quality (overly large size, irregular shape, plain taste, etc.) or tree vigor. Therefore, the careful management of irrigation based on water demand in these two stages is important. This study demonstrates that the timing of the water deficit increased the DMC in Ambrosia
TM apples. How and why does this work? Dry matter mainly represents the carbohydrate levels in fruit [
22]. Water deficit in early summer is assumed to lead fruit to acquire a higher share of carbohydrate production via the suppression of vegetative growth [
22,
23]. ED allows the resumption of CI after the early-summer DI. This enables proper photosynthesis activity and fruit development during the subsequent period of rapid fruit expansion (stage 3) (
Table 2). Early studies on apple trees [
40] have demonstrated that fruiting spur leaves have two elevated photosynthetic rates correlated with the fruiting process during the growing season: the first period of increased photosynthetic rates was during the bloom period (stage 1 in the growth cycle classified by Boland et al. [
23], and the second was the rapid fruit growth from midsummer to harvest (stage 3). This suggests that ED treatment was rendered in a moderate period of photosynthetic activity while fruit development was not heavily impacted. Additionally, the Ambrosia
TM tree has a tendency for strong lateral branch development and upright growth [
18]; implementing DI in the stage of rapid vegetative growth (stage 2) would be a timely control for vigor growth. A pomology study found that the implementation of water deficit treatment between the 40 and 70 days AFB successfully controlled vegetative growth and produced apples achieving the highest red color density [
29]. After the DI regime of ED, the tree SWP attained the same level as AI (
Figure 6) and an equal rate of photosynthetic activity (data not shown) to those from AI, suggesting that the treatment retained the physiological ability to ensure fruit which were well balanced in size, quality and reproduction (
Table 3,
Table 4 and
Table 5). However, water deficit across the whole period of the fruits’ rapid growth (stage 3) caused water stress, which raised the DMC value simply through water loss from the fruit [
36] (
Table 2). This could explain the worse blush color profiles and size, as well as the difficulty in maturation of the fruit in the DD treatment in 2018 under heat and a dry climate (
Table 2). Regarding the effect of water deficit stress on increasing DMC, it may be possible that ED enables a physiological impact on fruit in high correlation with all of the quality parameters, but DD attained a higher DMC via a simply mechanical process (such as fruit dehydration and shrinkage), with poor association with quality attributes in consecutive years (
Table 3 and
Table 4). The details of the physiological and cytological mechanism need to be more precisely studied in the future.
This study further confirms the proposition that fruit obtaining a higher DMC have better quality [
3,
15]. Consistently, the DMC level was highly correlated with the SSC value of fruit both at- and post-harvest in the two years (
Table 3 and
Figure 4). However, the DMC was not always positively correlated with the blush color profile. In this study, DD acquired a higher DMC in the two years in which the color attributes presented higher values in 2018 but were worse in 2019 (
Table 2). This suggests that the coloration of apple fruit is based on physiological activities which are highly water dependent [
22,
36]. Notably, this study suggests that the DMC level was not highly correlated with the flesh firmness (FF) and acidity recorded at harvest, but is was with their retention after storage (
Table 3). Based on the data collected from the treatments of AI and ED, the correlation analysis indicated that for the fruit DMC with the loss of FF, r = −0.87 in 2018 and −0.86 in 2019; with the loss of TA, r = −0.82 in 2018 and −0.76 in 2019. In comparison to the values at harvest, ED retained an FF value over 94% in 2018 and near 71% in 2019, while FF retention from AI pre-treatment was lower, at 85% in 2018 and 66% in 2019 (
Table 3). The retention ratio of TA was 83% and 77% in ED, and 71% and 63% in AI, in 2018 and 2019, respectively (
Table 4). The cell wall structures led to differences in the softening rates during the apple (
Malus ×
domestica) fruit growth [
14]. DMC is the primary material of the cell wall [
22]. This may be why the ED fruit that possessed a high DMC had a better retention of FF. DMC accumulation seems to be highly associated with seasonal temperature and precipitation (
Figure 1); therefore, the DMC accumulation was different between different years. The results obtained in this study highlight that both the mean (
Figure 3) and category (
Figure 4) fruit DMC values recorded near harvest were about one unit higher in 2018 than in 2019 across the treatments. The percentage unit in the most frequent categories was 16 in year 2018 and 15 in year 2019 (
Figure 4). Similar DMC differences between different years have been reported previously, such as a 4% gap in Royal Gala apples [
15]. This is not surprising because of two reasons. First, the weather was different between the two years; the accumulated degree days above 20 °C for early- and middle-summer in 2018 was about double that in 2019 (
Figure 1B). Horticultural studies [
34] have indicated that the mean fruit weight from warm post-bloom treatments is up to four times greater at harvest than that from cool-temperature treatments. Additionally, fruit from warm post-bloom temperature conditions have a higher soluble solids concentration and advanced maturation than fruit from cooler temperatures [
34]. On top of the temperature impact, differences in water statuses have a further effect on DMC. Secondly, there was less precipitation, and therefore more drought-like weather, in 2018, which amplified the impact from water stress and further increased the accumulation of DMC. In
Figure 3 showing DMC changes across the growth season of the fruit, the plot area delineated by the dashed line suggests that looking at the DMC accumulated over time may be more interesting than the amount of DMC attained at one time. In other words, maintaining DMC at a high level over a long period of time is more desirable than attaining the same level but for a shorter period of time. The method for the estimation of the accumulated DMC may mimic the formula of “degree d” as the sum of the DMC value in each recording × interval (days) between investigations × frequency of investigation in whole season (e.g., ∑
n DMC value × days of interval, where n = the frequency of investigation from fruit set to harvest), which may deserve future study.
The fruit maturations were highly impacted by the irrigation treatments. ED caused a rapid decline in the I
AD Index of Ambrosia
TM fruits in the late growing season in both years (
Figure 2). In 2018, the maturation based on I
AD values from ED was estimated to be completed 5 days earlier than that of the fruit from AI (
Figure 2A); similarly, in 2019, maturation was estimated to occur about 10 days ahead of harvest in fruit from ED compared to those from AI according to the I
AD decline ratio. The maturation progress of fruit treated with DD was quite different between 2018 and 2019, showing delayed maturation in the first year (
Figure 2A) but an early approach to the I
AD level of harvest in the second year (
Figure 2B). This suggests that water deficit to the proper extent, e.g., allowing photosynthetic activity to recover in stage 3, can lead to early fruit maturation; otherwise, extreme or irreversible drought caused delayed or disordered maturation. Ambrosia
TM is a relatively late-maturing apple with high susceptibility to chilling injury. The quick drop in night temperatures in the fall may increase the incidence of soft scald and core breakdown (personal communication with Dan Tayler, the apple industry operator in Cawston Cold Storage Ltd., Cawston, BC, Canada, 2018), and the earlier maturation of apples under ED allows an earlier and more flexible harvesting window to reduce or avoid chilling-related disorders [
5].
