Identification of superior late‐blooming apricot (Prunus armeniaca L.) genotypes among seedling‐originated trees

Abstract One of the major limiting factors in the intensive spread of apricot (Prunus armeniaca L.) in most of the countries is spring frost. Thus, the breeding efforts have concentrated on the use of late‐blooming genotypes as a means of frost avoidance. The aim of the present study was to identify late‐blooming genotypes with high fruit quality among seedling‐originated trees. Firstly, pre‐selections were done according to blooming time within 278 apricot seedling‐originated trees. Secondly, the late‐blooming selections were further evaluated according to their vegetative and fruit characteristics to determine superior types. Significant differences were observed among the late‐blooming genotypes in terms of the traits recorded. Fruit ground color was strongly variable, including white, yellow, yellow–green, light orange, orange, and dark orange. Fruit weight ranged from 27.37 to 33.99 g, fruit flesh thickness varied from 11.16 to 13.47 mm, and total soluble solids varied between 17.00% and 23.87%. Hierarchical cluster analysis (HCA) performed with Euclidean distance and Ward's method divided the genotypes into two main clusters based on morphological traits and in some cases, the genotypes belonging to an area were clustered into different clusters. All the 48 late‐blooming date genotypes selected could be useful as a parent to improve flowering season of cultivars. Also, among them, 10 genotypes were superior in terms of fruit quality‐related characters, such as fruit weight, fruit color, fruit taste, and TSS, and thus they can be singled out for cultivation.

the richest variation. Most of the cultivars are self-incompatible with small-to-medium-sized fruits which ripen over a long period and require high chilling (Uzun et al., 2010).
Apricot fruits are a great source of many antioxidants, including beta-carotene and vitamins A, C, and E. The main flavonoids in apricots are chlorogenic acids, catechins, and quercetin (Haciseferogullari et al., 2007). Apricot kernels are used as roasted and salted titbit. The kernel is a rich source of dietary protein, oil, and fiber. Kernels are a good source of fatty acids and phenolic compounds. The kernels are considered as nontraditional potential resources for oils. Large quantities of fruit kernels are usually discarded by the food processing industry (Özcan, 2000;Matthaus & Özcan, 2009;Matthaus et al., 2016;AL-Juhaimi et al., 2018).
The development of new fruit cultivars generally has been based on genetic resources. Germplasm collection and characterization are essential stages of breeding programs. Given the importance of apricot industry for this unique ecogeographic zone, characterization of germplasm collections and genetic diversity analysis are a prerequisite for any breeding program. Genetic resource management, including collection, precise characterization, and documentation of extant variability is of paramount importance for conservation, breeding, and commercialization of potential apricot genotypes.
Morphological characterization is timely for cultivar identification, selection, delineation, and germplasm management in start-up programs intended for selection of superior genotypes for breeding programs. Several studies have been undertaken on the variability of germplasm resources of European (Audergon et al., 1991;Badenes et al., 1998;Milosevic et al., 2010) and Irano-Caucasian eco-geographical groups Asma & Ozturk, 2005), resulting in the identification of interesting cultivars that have been used to generate new selections through breeding programs (Almeras et al., 2002;Forte, 1971;Krichen et al., 2014aKrichen et al., ,2014b. One of the major limiting factors in the intensive spread of apricot in most of the countries, including Iran, is spring frost which kills blossoms. Apricot cultivars with late flowering can be cultivated in mountain areas, where the late frosts are frequent. The breeding efforts have concentrated on the use of delayed flowering as a means of frost avoidance. The number of studies on Iranian apricot germplasm is limited (Khadivi-Khub & Khalili, 2017;Rezaei et al., 2020).
The aim of the present study was to identify late-blooming apricot genotypes with high fruit quality among seedling-originated trees in the Markazi province/Iran.

| Plant material
The present study was undertaken to assess the genetic diversity in seedling-originated apricot trees grown through morphological characters and to identify late-blooming genotypes with high fruit quality in the Markazi province/Iran. Firstly, pre-selections were done according to blooming time within 278 apricot seedling-originated trees from three areas which were near each other. The genotypes with early and middle blooming dates were eliminated and finally, 48 late-blooming trees were selected. Secondly, the late-blooming selections were further evaluated according to their vegetative and fruit characteristics to determine superior types. The selected genotypes were named based on their location, and these names were supplemented with numerical characters. The selected trees were mature (8-10 years old), healthy, and had a full crop. General orchard management, including irrigation, nutrition, pest, and disease control, was consistent with commercial practices.

| The characters evaluated
The selected late genotypes were evaluated using 47 morphological and pomological traits to select superior selections. Length, width, and thickness for leaf, fruit, stone, and kernel were measured using a digital caliper. Weight for fruit, stone, and kernel was measured using an electronic balance with 0.01 g precision. Total soluble solids (TSS) content was determined using a refractometer (pocket PAL-1 ATAGO Corporation, Tokyo, Japan), in •Brix. The remaining characters were qualitatively determined based on rating and coding according to the apricot descriptor (Guerriero & Watkins, 1984, IBPGR).

| Statistical analysis
Analysis of variance (ANOVA) was performed to evaluate the variation among the genotypes based on the traits measured using SAS software (SAS Institute, Cary, NC, USA, 1990). Simple correlations between traits were determined using Pearson correlation coefficients (SPSS Inc., Chicago, IL, USA, Norusis, 1998). Principal component analysis (PCA) was used to investigate the relationship between genotypes and determine the main traits effective in genotype segregation using SPSS software. Hierarchical cluster analysis (HCA) was performed using Ward's method and Euclidean coefficient using PAST software (Hammer et al., 2001). The first and second principal components (PC1/PC2) were used to create a scatter plot with PAST software.  (Blasse & Hofmann, 1993). Temperatures ranging from 7 to 9˚C determine the start of the phenophase "beginning of blossoming" (Vachun, 2003). The difference in the flowering time between the genotypes is 2-4 days under favorable environmental conditions or 6-8 days under less favorable ones (Milosevic, 1997). Late blooming is an important factor to protect damages caused by spring frosts in continental climates (Unal et al., 1999). Therefore, finding late-blooming trees is one of the main goals of apricot breeding program. Thus, secondly, the 48 late-blooming genotypes selected were evaluated according to their vegetative and fruit characteristics to determine superior types.

| RE SULTS AND D ISCUSS I ON
There were significant differences among the late-blooming genotypes selected in terms of the traits recorded. The CV ranged from 1.59 (spur leaf width) to 90.18% (ripening date). The CV was more than 20.00% in 28 out of 47 characters recorded (Table 1).
Fruit shape and size determine market value and are important physical attributes in grading, sorting, packaging, and transportation of fruits (Erdogan et al., 2003).
Fruit ground color was strongly variable, including white (11), yellow (15), yellow-green (7), light orange (6), orange (6), and dark orange (3). Also, there was significant diversity among the genotypes in terms of fruit flesh color, ranging from white to orange (  (Ayour et al., 2016). Ruiz et al. (2005) reported that carotenoid content in apricot fruit showed significant correlations with skin and flesh color, with apricots having orange-colored flesh containing higher levels of carotenoids than those having white-colored flesh. It has been shown that the orange color was closely correlated with the carotenoid content (Marty et al., 2005). Previous studies focused on chlorophylls degradation and showed that this degradation was accompanied by the formation of chromo-plastids during fruit ripening (Abaci & Asma, 2013). Chlorophyll degradation during maturation occurs in parallel with the development and accumulation of other pigments, such as carotenoids. It has been reported that beta-carotene is the main pigment quantified in apricot fruit (Ayour et al., 2016;Munzuroglu et al., 2003;Ruiz et al., 2005;Zeb & Mehmood, 2004 (24), and sweet (7). Recently, sweet kernels of apricots have been used for direct consumption as a snack food like almond. Also, bitter kernels are used in the pharmaceutical and cosmetics industries (Yilmaz et al., 2012). The values of the most important fruit-related traits for the selected superior late-blooming genotypes are presented in Table 3.
The PCA classified the characters into 18 PCs which justified 81.64% of the total variance (not shown). The PC1 accounted for 6.24% of the total variance and was significantly correlated with tree growth habit, fruit ground color, and fruit flesh color. Three F I G U R E 2 Ward cluster analysis of the studied late-blooming apricot genotypes based on morphological traits using Euclidean distances characters, including kernel color, kernel shriveling, and kernel taste, were placed into PC2 and accounted for 6.12% of the total variance.
The scatter plot created using PC1/PC2 showed phenotypic variations among the genotypes (Figure 1). The genotypes were distributed into four sides of the plot and showed high differences for most of the characters. Also, the HCA performed with Euclidean distance and Ward's method divided the genotypes into two main clusters based on morphological traits (Figure 2). The first cluster (I) included 16 genotypes, while the second cluster (II) consisted of the rest 32 genotypes. In some cases, the genotypes belonging to an area were clustered into different clusters. Differences in morphological characters under the same environmental and geographical conditions are probably the result of genetic effects (Karadeniz, 2002). The nuclear genome contains the majority of the genes related to different characters and also has high rate of mutation (Provan et al., 2001). Thus, the mutation increases the variation in the population (Khadivi-Khub et al., 2016).
Late blooming is one of the most important factors in preventing spring frost damages to fruit trees in continental climates. Thus, one of the most important aims in the first phase of apricot breeding programs is to identify and introduce late-blooming genotypes.
Reduction or elimination of damages caused by spring frost is possible by cultivating late-blooming genotypes or new cultivars having this trait. Besides, because the blooming occurs after the rainy season in such genotypes/cultivars, pollination and effective use of pollinators such as insects significantly increase (Rezaei et al., 2020).

| CON CLUS IONS
In many parts of the world, including Iran, apricot production is limited by late spring frost and thus, late blooming is the most important selection criteria. The knowledge of blooming date and fruit attributes of the apricot genotypes studied here could be useful to choose the appropriate ones to be grown or used as parents in future breeding programs. The promising genotypes were selected through blooming date and then fruit quality-related characteristics. Thus, after pre-selections among many genotypes, all the 48 late-blooming date genotypes selected could be useful as a parent to improve flowering season of cultivars. Furthermore, among them, 10 genotypes, including Shazand-7, Khondab-17, Marzigharan-8, Shazand-3, Shazand-15, Marzigharan-1,  and Khondab-20, were superior in terms of fruit quality-related characters, such as fruit weight, fruit color, fruit taste, and TSS, and thus they can be singled out for cultivation.

ACK N OWLED G M ENT
None.

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest.

R E S E A RCH I N VO LV I N G H U M A N PA RTI CI PA NTS A N D/ O R A N I M A L S
None.

I N FO R M E D CO N S E NT
None.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.