Crop load, fruit quality and return bloom of ‘Galaxy’ and ‘Cripps Pink’ apples after NAA thinning at several physiological stages of fruit developmentAbstract:
Defining the optimal timing for naphthaleneacetic acid (NAA) application is critical for ensuring thinning efficacy, particularly due to genotype-specific response of apple trees. This study evaluated the effects of NAA application timing for thinning in ‘Galaxy’ and ‘Cripps Pink’ apples. NAA at 10 mg·L-1 was applied at different fruit diameters (4, 8, 12, 16 mm) over three seasons in Rio Negro, Argentina. NAA treatments efficacy was compared with an unthinned control and a manual thinning treatment. During spring, thinning response was determined as fruit set and fruit drop. At harvest, fruit number and production per tree, fruit size and colour were evaluated. At the following season, return bloom was determined. ‘Galaxy’ apples were more sensitive than ‘Cripps Pink’ apples but both cultivars showed maximum thinning response at 11 mm of fruit diameter. Late NAA application at 16 mm significantly reduced fruit set in ‘Galaxy’ apples but it was less effective for improving fruit size and colour than earlier ones. In ‘Galaxy’ apples, NAA treatments improved colour intensity as the same level as manual thinning. However, ‘Cripps Pink’ apples showed the best colour after manual thinning. Thinning treatments influenced fruit quality, with a lesser effect on return bloom.
Index terms
Malus domestica
; plant growth regulator; fruit abscission; biennial bearing; fruit quality
Resumo:
Definir o momento ideal para a aplicação do ácido naftaleno acético (ANA) é fundamental para garantir a eficácia do raleio, particularmente devido à resposta genótipo-específica das macieiras. Este estudo avaliou os efeitos do momento de aplicação do ácido naftalenoacético (ANA) para o raleio de maçãs ‘Galaxy’ e ‘Cripps Pink’. O ANA, na concentração de 10 mg·L-1, foi aplicado em diferentes diâmetros de frutos (4, 8, 12 e 16 mm) durante três coleitas em Río Negro, Argentina (2018/19, 2019/20 e 2020/21). A eficácia dos tratamentos com ANA foi comparada com do controle (sem raleio) e raleio manual. Os tratamentos foram dispostos em delineamento de blocos completos casualizados, com cinco repetiçõe, sendo cada repetição composta por uma planta. Na primavera, o vingamento de fruto e queda de frutos foram as respostas ao tratamiento do raleio. Na colheita, o número de frutos e a produção em kg por árvore, o tamanho e a cor dos frutos foram avaliados. Na temporada seguinte, determinou-se a floração de retorno por árvore. As maçãs ‘Galaxy’ mostrou-se mais sensível que a ‘Cripps Pink’, porém ambas apresentaram a resposta máxima de raleio com a aplicação de ANA no diâmetro de fruto de 11 mm. Curiosamente, a aplicação tardia de ANA no diâmetro de 16 mm reduziu significativamente o vingamento de frutos na cultivar ‘Galaxy’, porem esse tratamento foi menos eficaz para melhorar o tamanho e a cor do frutos em relação as aplicações mais precoces. Em maçãs ‘Galaxy’, os tratamentos com ANA melhoraram a in-tensidade da cor no mesmo nível que o raleio manual. No entanto, as maçãs ‘Cripps Pink’ mostraram a melhor cor após o raleio manual. Os tratamentos de raleio influenciaram a qualidade dos frutos, com um efeito menor no retorno da floração
Termos para indexação
Malus domestica
; regulador de crescimento de plantas; abscisão de frutos; alternância de produção; qualidade da fruta
Introduction
Fruit thinning is a usual agronomic practice for crop load reduction in apple trees, improving fruit quality, and enhancing return bloom (MUSACCHI; SERRA, 2018). This practice strongly affect fruit: leaf ratio, by increasing carbohydrate availability for sinks within the tree (COSTA et al. 2018).
Chemical thinning is the most economical thinning method and reduces the need for hand thinning (GREENE; COSTA, 2013).
Nevertheless, chemical thinning effectiveness depends on weather conditions, especially sunlight and temperature (GREENE, 2002, 2019; ROBINSON; LAKSO, 2004) and there is an increasing concern about the environmental impact of some active ingredients (BOUND, 2019).
Chemical thinning requires environmentally- friendly strategies (CORTENS; CLINE, 2019). Since the auxin Naphthaleneacetic acid (NAA) is a non-toxic thinning compound, effective at low concentrations (5- 12 ppm) (COSTA et al., 2018; GREENE, 2019), it may constitute an interesting alternative for apples.
Fruitlets are susceptible to thinning compounds at specific times (GREENE, 2002).
Knowing vulnerability periods in which apples respond to different active ingredients allows growers to choose the optimum application times, avoiding ineffective treatments. However, late NAA applications may generate pygmy fruits in certain cultivars (BLACK et al., 1995; MARINI, 1996; ROBINSON, 2006; MILIĆ et al., 2017).
Research has focused on NAA efficacy at several phenological stages in ‘Delicious’, ‘McIntosh’, ‘Spy’, ‘Redchief’ and ‘Granny Smith’ cultivars (LEUTY, 1973, BLACK et al., 1995, FRUK et al., 2018) finding a genotype- dependant response. This calls for studies defining timely application windows in other cultivars like ‘Gala’ and ‘Cripps Pink’.
Chemical thinning response depends on apple cultivar (ROBINSON, 2006). ‘Gala’ and ‘Cripps Pink’ are cultivars that exhibit distinct physiological behaviors that could influence their response to chemical thinning.
“Gala” (a Kidd’s Orange Red × Golden Delicious cross) is an early-maturing cultivar characterized by small-sized fruits (LOMBARD, 2000). In contrast, ‘Cripps Pink’ (a Lady Williams × Golden Delicious cross) is a late-maturing cultivar with a vigorous growth habit and a more prolonged fruit growing season (CRIPPS et al., 1993).
Adequate bloom return and fruit quality improvement are fundamental for crop load regulation. Thinning soon after bloom is considered more effective than thinning later, especially in small-fruited cultivars such as ‘Gala’ (WÜNSCHE; FERGUSON, 2010; COSTA et al., 2018; BOUND, 2023), given an earlier relief in fruitlet competition (STOVER et al., 2001; COSTA et al., 2018). Considering few thinning trials have investigated fruit drop patterns after thinning application (MCARTNEY et al., 2004, GREENE; LAKSO, 2013; CURETTI et al., 2022), understanding fruit drop dynamics could clarify thinning influence on fruit quality and bloom return.
This study aimed to study NAA efficacy at different application times for ‘Galaxy’ and ‘Cripps Pink’ apples and evaluate the effect of NAA application times on bloom return and fruit quality considering fruit size and colour.
Materials and methods
Experimental orchards
The trials were conducted in two experimental orchards of apples (Malus × domestica Borkh) located in Rio Negro, Argentina (245 m above sea level) over three seasons (2018-19, 2019-20 and 2020-21). The climatic conditions of the region are 15°C of average temperature and 244 mm of rainfall per year. A strain of ‘Gala’, ‘Galaxy’/M7 apples were planted in 2004 at 4 x 1.8 m (1388 trees ha-1). The trees were trained in vertical axis and the tree height was 4.5 m.
This orchard was irrigated using a flooding system (39º01’47” S - 67º44’25” W). ‘Cripps Pink’/MM111 apples were planted in 2009 at 4 x 1.5 m (1666 trees. ha-1). The trees were trained in vertical axis and the tree height was 4.2 m. This orchard was irrigated using sprinklers (39º01’31” S - 67º44’17” W).
Both apple orchards were managed with conventional commercial practices in the region.
Chemical thinning treatments
Each season, trees were selected according to their uniformity in terms of vigour and flowering intensity. The number of flower clusters per individual tree was counted, and five blocks were defined according to the initial flowering level. Additionally, the number of flower clusters per tree was included as a covariable for fruit number and production.
Treatments were assigned in a randomized complete block design with five single- tree replications with two border trees as guards. Four application times of naphthaleneacetic acid (NAA) at 10 mg·L-1 were compared with untreated control and manual thinning treatments. Table 1 shows information about fruitlet diameter and days after full bloom for each NAA application time. Since weather conditions influence thinning response (GREENE, 2002, 2019; ROBINSON; LAKSO, 2004), some weather variables as temperature and rainfall during full bloom and application times are shown in Figure 1. Hand thinning was performed in the first week of December leaving one fruit per cluster and 15 cm of separation between fruits.
The NAA formulation used was Nafusaku® 16% SP (S. ANDO y CÍA SACIIF, Argentina).
All the treatments were applied with a knapsack sprayer with a water volume of spray equivalent to 2000 L·ha-1 adjusted based on tree size (4.2-4.5 m of height and 1.6 m of width). The applications were made early in the morning when there was no dew on the trees. Since full bloom, fruit diameter was measured daily with digital callipers to define application times.
Weather conditions as rainfall (mm) (blue bars) and maximum daily temperatures in each season (°C- black line), 5-year average (°C- gray line) (2016-2020) registered at the INTA Alto Valle weather station. Dates are expressed as month-day. Flowers indicate the date of full bloom, and arrows indicate NAA application times. White figures for 'Galaxy' apples and black figures for 'Cripps Pink' apples.
Thinning efficacy
Early parameters of thinning efficacy were fruit set at the branch level and fruit drop at the fruitlet cluster level. Fruit drop was based on the number of persisting fruits in five tagged clusters per tree and was registered weekly until 60-70 days after full bloom (DAFB). Fruit drop was determined by the difference in persisting fruit number between consecutive weeks per cluster and tree. The percentage of cumulative fruit drop was calculated at three moments 30, 45 and 60 DAFB. Two branches (2.5-3 cm basal diameter) per tree were tagged, and the number of flower clusters was counted at bloom.
Average flower cluster per branch were 54.2 for ‘Galaxy’ and 39.2 for ‘Cripps Pink’. In December, clusters with different number of fruits were counted. The fruit set was calculated based on the fruit number per 100 flower clusters. Return bloom was assessed at the beginning of the following season based on the number of flower clusters per tree.
Fruit quality
At commercial harvest, in late January for ‘Galaxy’ and early April for ‘Cripps Pink’, total fruits per tree were harvested. The number of fruits per tree, fruit production and mean fruit weight were determined. All fruits of each tree were separately classified by seven categories of fruit weight with an automatic fruit size sorting machine. The percentage of commercial fruit per tree was calculated with a minimum weight of 140g for both cultivars. After the first season, this critical value was adjusted to 130 g for ‘Galaxy’ apples based on the fruit size characteristics of this cultivar.
A sample of 10 fruits was randomly selected per tree every season; thus, 50 fruits per thinning treatment were evaluated for quality parameters.
Colour and fruit size determine apples’ commercial value (IGLESIAS et al., 2008) therefore special attention was given to both fruit characteristics. Fruit weight, length, and diameter were measured individually. The percentage of fruit surface with red colour was visually assessed for each fruit. Fruits were classified according to CTIFL (Centre Technique Interprofessionnel des Fruits et Légumes) colour charts developed for each cultivar. This classification considered the colour intensity of the exposed part of the fruits. Additionally, fruit colour was measured objectively on the exposed fruit side with a Minolta CR-400 colorimeter (Konica Minolta Co., Japan) registering L, a* and b* parameters, according to CIE (Commission Internationale d’Eclairage), where L indicates lightness, a* indicates chromaticity on a green (-) to red (+) axis, and b* indicates chromaticity on a blue (-) to yellow (+) axis. The numerical values of a* and b* were converted into hue angles (H° = tan-1 b*/a*) and chroma [Chroma = (a*2 + b*2)1/2]. H° (hue) is an angle in a colour wheel of 360°, while chroma is the intensity or purity of the colour. Seed number and maturity indexes like starch degradation, flesh firmness and soluble solids were also evaluated (data not shown).
Statistical analysis
The data were analysed by analysis of variance (ANOVA) with the statistical software Navure 2.2.0 and R 3.6.3. For the variables fruit number per tree and fruit production an ANCOVA was performed with flower cluster per tree as a covariable. For all the models, when the effects were significant (α=5%), comparisons between treatments were made using Tukey´s HSD test. The data were analysed as a season (3) x thinning treatment (6) factorial, including an interaction term. The Shapiro-Wilk test was used to test the normality of the residuals. When necessary (e.g. a*, b* and hue), a heteroscedastic model was fitted. Chi-square tests were performed to compare fruit distribution in CTIFL colour categories.
A regression model of relative fruit number per tree was fitted as a function of fruit diameter at application time in linear and quadratic terms. The relative fruit number for each application time was calculated with respect to the mean number of unthinned trees in each season. ‘Galaxy’ and ‘Cripps Pink’ data for all seasons were pooled together for the analysis. The cultivar factor was included as a dummy variable.
The optimum NAA application time was calculated from the quadratic regression equation. This is done by setting the first derivative of the equation equal to zero and solving for the fruitlet diameter.
Results and discussion
Fruit drop and fruit set
In ‘Galaxy’ apples, important fruit drops were registered before 60 DAFB (Table 2), with 73.8% fruit drop in control trees and 85.4-89.2% in all NAA treated trees. In the 2018-19 season, a cold and rainy period between 20 and 40 DAFB may have caused higher fruit drop at 60 DAFB (Figure 1).
NAA applications at 4 and 8 mm of fruit diameter resulted in early fruit drop differing from the control treatment from 30 DAFB, onward. Later NAA applications at 12 and 16 mm did not differ from control until 60 DAFB.
In ‘Cripps Pink’ lower fruit drop was registered in control trees than ‘Galaxy’ (Table 2). At 60 DAFB, NAA at 8, 12 and 16 mm presented higher fruit drop than control.
During the 2018-19 season, only NAA applications at 12 and 16 mm had significantly lower fruit set than control (Table 3). Greater fruit sets were observed in the 2019-20 season than in other seasons and NAA at 16 mm did not differ from control. NAA at 12 mm was the only application moment significantly reducing fruit set in all seasons.
In ‘Cripps Pink’ apples, NAA at 8 mm was more effective in 2020-21 (Table 3). This treatment was applied at the beginning of a hot period (Figure 1) of 14 days with a mean night temperature of 14.7°C in October 2020, while the 10-year average temperature for the same period is 12.2°C.
Cloud cover after application and high temperatures, especially at night, increase thinning effectiveness (BYERS, 2002; STOVER; GREENE, 2005). High temperatures during the night affect dark respiration and carbohydrate consumption of developing fruitlets (LAKSO et al., 2006; CLINE et al., 2022) favouring fruit drop (BYERS, 2002). González et al. (2020) also reported greater thinning efficacy when mean night temperature exceeded 14°C. Therefore, this long period with high night temperatures could explain the higher effectiveness after NAA application at 8 mm in this season.
Fruit drop presents two mutually related peaks during spring (COSTA et al., 2018). The first peak usually occurs before 30 DAFB.
The second peak occurs between 40 and 50 DAFB and is triggered by sink competition (RACSKÓ et al., 2007). ‘Galaxy’ apple was the only cultivar presenting significant differences in the first peak (Table 2), in NAA treatments at 4 and 8 mm. McArtney et al. (2004) also reported that near bloom NAA applications increased fruit drop before 20 DAFB in ‘Royal Gala’ apples. These early treatments reached c. 75% of fruit drop at 30 DAFB, between 13 and 22 days after NAA applications.
Similarly, Greene and Lakso (2013) reported fruit drop between 14 and 21-23 days after NAA application on ‘McIntosh’ apples. A late fruit drop was observed in NAA treatments at 12 and 16 mm in ‘Galaxy’ apples, with significant differences only after 45 DAFB.
Thinning efficacy and bloom return
Harvest results and bloom return of ‘Galaxy’ apples are shown in Table 4. Bifactorial analysis determined that the 2020-21 season showed higher crop loads and lower bloom returns than other seasons. NAA applied at petal fall (4 mm) was less effective than later applications and did not differ with respect to control in any measured variable.
This lower efficacy registered after early NAA treatments differs from previous results on ‘Brookfield Gala’ in Chile (REGINATO et al., 2017). These authors tested NAA plus benzyladenine (6-BA) and reported consistent effects immediately after petal fall.
Following NAA applications at 8, 12 and 16 mm were effective and significantly reduced fruit number per tree with respect to control, not differing from hand thinning.
Surprisingly, the late NAA application at 16 mm fruit diameter significantly reduced fruit set (Table 3) and fruit number per tree; suggesting that the window for NAA application in this cultivar may be broader than originally believed (GREENE, 2002).
Regarding ‘Cripps Pink’ apples, the most effective treatment was NAA at 12 mm, with similar crop load levels as hand thinning (Table 5); while NAA at 4 mm resulted also ineffective. In this cultivar, the late NAA application at 16 mm resulted in lower fruit set in two of the three seasons (Table 3) but did not reduce significantly fruit number per tree. Therefore, the interval for effective NAA applications seems to be broader for ‘Galaxy’ than for ‘Cripps Pink’ apples.
At certain fruitlet diameter and application time, final fruit number per tree is minimal, indicating a critical stage in which trees are most sensitive to NAA. Fruit number per tree provides a more precise measurement of overall response than fruit set on selected branches (CLINE et al., 2022). The fitted quadratic model predicted the greatest thinning response for NAA at 11.2 mm of fruitlet diameter (Figure 2) in both cultivars.
However, ‘Galaxy’ apples were more sensitive to NAA than ‘Cripps Pink’ apples.
Robinson and Lakso (2004) also evaluated different application times of NAA plus Carbaryl in several cultivars and reported greatest response with 12 -13 mm in ‘Gala’ apples. Based on our results, optimal thinning responses to NAA were obtained at 11.2 mm fruitlet diameter for both cultivars under local conditions.
Although improving return bloom is one objective of crop load regulation (WÜNSCHE; FERGUSON, 2010), in our study effective thinning treatments only increased return bloom with respect to control in ‘Galaxy’ apples (Tables 4 and 5). Non-significant differences in return bloom in ‘Cripps Pink’ apples may be cultivar-dependent. In fact, Bound (2023) mentioned that the effect of crop load in return bloom varied between cultivars and it is reduced once a particular level is reached. This research established a threshold level of 8 fruit.cm-2 of trunk cross area for these cultivars and control trees in our trials presented a lower crop load level.
Nevertheless, this expression may not accurately describe crop load in vigorous trees that have already occupied space in plantations (WÜNSCHE; FERGUSON, 2010), like the evaluated trees in the present study.
Mean fruit number per tree relative to control unthinned trees in each season as a function of fruit diameter (mm) at NAA application.
Fruit quality
In both cultivars, NAA applications at 8- and 12-mm improved fruit size (Tables 4 and 5).
NAA application at 12 mm increased mean fruit weight of ‘Galaxy’ apples by 16.2%, in accordance with previous findings (LARSON et al., 2023). Even though late NAA application at 16 mm effectively reduced crop load in this cultivar, it was less effective in improving fruit size, probably due to the late fruit drop generated (Table 2). NAA applications at 8 and 12 mm significantly increased fruit weight and percentage of commercial-size fruits of ‘Cripps Pink’ apples. Even though NAA at 8 mm was less effective than NAA at 12 mm in reducing fruit number per tree, both presented similar fruit size at harvest.
Robinson (2006) reported that NAA negatively affected fruit size in ‘Delicious’ and ‘Fuji’ apples. In our study, fruit size resulting from effective NAA treatments was similar to hand thinning, meaning that NAA did not negatively affect fruit size in these cultivars.
The appearance of pigmy fruit after NAA application is a concern, especially for late applications. Robinson (2006) indicated that NAA, and particularly the combination of NAA and 6-BA, causes the retention of pigmy fruits in certain apple cultivars like ‘Delicious’ and ‘Fuji’, with low incidence in ‘Gala’. However, Cortens et al. (2019) found pigmy fruits after a late application of NAA plus 6-BA at 17 mm in ‘Gala’. In our trials, pigmy fruits were observed with a minor incidence and without commercial relevance.
NAA treatments had similar effects on colour in ‘Galaxy’ apples regardless of season (non-significant interactions), despite the highly significant differences observed among seasons (Table 6). Hot and dry conditions delay colour development (IGLESIAS et al., 2008). Habitual hot Januarys produce poor colour development in ‘Gala’ apples in Alto Valle and low percentage of red surface was observed (47-54%).
Minimum required red colour is 60% for ‘Gala’ apples (GONZÁLEZ et al., 2020). Similar percentage of red surface was observed between treatments, but some significant differences were detected in colour intensity. NAA applied at 4, 8 and 12 mm resulted in greater chroma values than control. A lower effect was observed with late NAA application at 16 mm. Chemical treatments and hand thinning showed similar fruit distribution for the CTIFL colour categories (Figure 3), with an increase of 10-18% fruits within highest categories of colour coverage.
Distribution of fruits in different colour intensity categories (CTIFL charts) for each thinning treatment in 'Galaxy' (left) and 'Cripps Pink' apples (right).
Minimum required red colour for ‘Pink Lady’® apples is 40% (MUSACCHI; SERRA, 2018) and percentage of red surface observed was between 54% and 66% (Table 7). Hand thinning resulted in maximum red intensity (chroma) of ‘Cripps Pink’ apples and best fruit distribution considering CTIFL colour categories (Figure 3).
Intriguingly, a better distribution was observed with late NAA applications (12-16 mm) than with earlier ones (4-8 mm) in this cultivar (X2 p= 0.0001). However, hand thinning presented the best colour of ‘Cripps Pink’ apples, probably because hand thinned fruits were better exposed to sunlight than NAA treated fruits.
Conclusions
The maximum response to naphthaleneacetic acid (NAA) was determined at 11 mm fruitlet diameter for ‘Galaxy’ and ‘Cripps Pink’ apples. ‘Galaxy’ was more sensitive than ‘Cripps Pink’ and responded to NAA until 16 mm of fruit diameter without relevant appearance of pigmy fruits. To the best of our knowledge, this is the first study reporting NAA high efficacy of late applications in ‘Galaxy’ apples.
Thinning majorly affected fruit quality, while return bloom was less responsive.
Effective thinning treatments, generally at 8-12 mm of fruitlet diameter, improved fruit size at harvest. Late NAA application was less effective for improving fruit size and colour. In ‘Galaxy’, chemical thinning with NAA improved colour intensity with similar efficacy as manual thinning. In ‘Cripps Pink’ apples, no chemical treatments achieved colour improvement as hand thinning, but late NAA applications resulted in better-coloured fruits than early ones.
Acknowledgements
Financial support for this research was received by the INTA under Grant 2019-PE-E1- I010-001 and thinning compound donation from S. ANDO y Cía S.A. Fernando Roma for the technical assistance and Sebastian Izaguirre for the graphic design of figures.
Data Availability:
References
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