1. Introduction

Walnut (Juglans regia L.) is a very important source of nutrition for the human body and a valuable plant for the ecological cycle as it usually has a long lifespan. It is a hard-shelled fruit species and is cultivated in various regions of the world. Walnut production in the world has increased significantly since 1985 and has reached nearly 5 million tons a year ^[1]. Among various factors affecting walnut production is the increase in public awareness about the positive effects of nutritional elements in walnut, along with a gradual increase in demand for walnut as a result of population growth ^{[2][3][4][5][6][7]}.

Global climate change, diseases, and pests, as well as a decrease in the area of agricultural lands have affected plant cultivation. In addition, increasing demand for walnut and other negative factors make it necessary for walnut producers to grow productive and high-quality walnut cultivars that can tolerate different biotic and abiotic factors, thereby indicating the significance of walnut breeding programs. In today’s world, walnut-growing countries such as the US, China, France, Turkey, and Iran have recently focused on walnut crossbreeding programs to develop new walnut cultivars with superior traits.

One of the most critical ecological factors that limit walnut cultivation is the late-spring frost. High yield, early harvest date, and tolerance to plant diseases/pests are favorable traits in a good walnut cultivar. In addition, high nut quality directly boosts the market value of walnuts ^{[8][9][10][11][12][13][14]}. The main objective of walnut breeding programs is to obtain new walnut cultivars with these traits. Thanks to walnut breeding studies in different countries, several walnut cultivars such as ‘Chandler’, ‘Franquette’, ‘Fernor’, ‘Howard’, ‘Serr’, and ‘Pedro’ are currently entrenched in the global market ^[15][16]. Similar to current trends in the world, walnut breeding programs have become popular in Turkey over the past recent years. For instance, two patents were obtained for two important walnut cultivars, i.e., ‘Maraş 18’ and ‘Sütyemez-1’ ^[17]. The first walnut crossbreeding program was initiated by Prof. Mehmet Sütyemez in 2005 ^[18]. Within the scope of this crossbreeding program, the first crossbred walnut cultivar (‘Diriliş’) in Turkey was released in 2016 ^[19]. Likewise, ‘Helete Güneşi’ is now a crossbred walnut cultivar with superior traits after being selected among different genotypes resulting from the cross combinations of ‘Maraş 18’ x ‘Chandler’. A patent was obtained for ‘Helete Güneşi’ from the Ministry of Agriculture and Forestry in 2021 ^[20].

The present study aimed to compare ‘Helete Güneşi’ with its parents (‘Maraş 18’ and ‘Chandler’) in terms of their phenological and pomological traits.

2. Current Insights into‘Helete Güneşi’

The present study mainly aimed to offer insights into the details of ‘Helete Güneşi’ and its performance. Comparisons were made with previous crossbreeding research and also with the parents of ‘Helete Güneşi’. Phenological and pomological characteristics of ‘Helete Güneşi’ were evaluated in contrast to the features of its parents (Tables 3 and 4). As can be seen in Table 3, the first leafing date in ‘Helete Güneşi’ was 22 April, which was 2 days later than ‘Chandler’ and 10 days later than ‘Maraş 18’ (Figure 1). A later leafing date in ‘Helete Güneşi’, compared to ‘Chandler’, is a promising phenological trait.

Many studies in the existing literature have listed ‘Chandler’ having a later leafing date then many walnut genotypes ^{[16][23][24][25][26][27]}. Various studies have underlined the importance of a late leafing date that can help protect the tree from late-spring frost ^{[11][15][17][28][29][30]}. ‘Helete Güneşi’ can be seen as an important genetic resource for walnut cultivation because of the said trait. 

Walnut is a monoic fruit species and, because of its hereditary nature, is characterized by a high dichogamy tendency. Therefore, the determination of active periods in male and female flowering times is of vital importance in terms of fertilization biology.

The mean values of phenological traits in walnut cultivars through the three consecutive years are given in Table 3. In ‘Helete Güneşi’, the male bloom period lasted for 9 days between 23 April and 2 May, while the female bloom period lasted for 10 days between 29 April and 9 May. In ‘Chandler’, however, the male bloom period lasted for 9 days between 21 and 30 April, and the female bloom period lasted for 7 days between 1 and 8 May. Meanwhile, regarding ‘Maraş 18’, the male bloom period lasted for 7 days between 13 and 20 April, whereas the female bloom period lasted for 7 days between 21 and 28 April (Figure 1, Table 3).

The dichogamy status of a cultivar is determined as the overlap between male and female bloom dates. All walnut cultivars analyzed in the present study displayed a protandrous flowering trait (Table 3). This was not a surprising result because previous studies in the existing literature pointed to the fact that ‘Chandler’ has a protandrous flowering trait ^[12][27][31]. Another study by Sütyemez ^[17] demonstrated that ‘Maraş 18’ is also protandrous in flowering. It can be thus stated that the findings in the present study are in agreement with previous research in the available literature. Pollinators of ‘Helete Güneşi’ can benefit walnut orchards if they are planted in the right combination of male–female flowering overlap.

Walnut breeding programs usually aim at higher rates of lateral bud flowering and a high abundance of female flowers, which can be considered as two important parameters in assessing yield potential in a given cultivar ^{[12][16][28][32]}.

Phenological observations in the present study demonstrated that ‘Helete Güneşi’ had a fairly high lateral bud flowering rate (98.8%). Furthermore, this cultivar had a high level of female flower abundance. Therefore, the suitability of these traits has made ‘Helete Güneşi’ superior to ‘Chandler’ in some respects, since the latter is also known to have a high percentage of lateral bud flowering (90.7%). On the other hand, lateral bud flowering and female flower abundance in ‘Maraş 18’ were calculated as 70.2% and intermediate, respectively. In sum, the level of male flower abundance in ‘Helete Güneşi’, ‘Chandler’, and ‘Maraş 18’ were intermediate, high, and intermediate, respectively.

In 10 same-age plants (mean values of 7th, 8^th, and 9th years), the mean values of yield among the three cultivars under the same ecological conditions were calculated as 9.36 kg/per tree in ‘Helete Güneşi’, 7.03 kg/per tree in ‘Chandler’, and 4.14 kg/per tree in ‘Maraş 18’ (Table 3). It was evident that ‘Helete Güneşi’ displayed a higher yield value compared to the credible yield of ‘Chandler’. Statistical differences between lateral bud flowering and yield per tree in each walnut cultivar are shown in Figure 2. Thus, it can be suggested that ‘Helete Güneşi’ is superior to ‘Chandler’ and ‘Maraş 18’ cultivars in terms of lateral bud flowering and total yield.

Another important phenological parameter that determines the economic potential of a cultivar is “harvest date”, which directly affects the spot value of marketed products. While it applies to walnut as well, an early harvest date can be seen as a protective measure against early-autumn frost events. Sütyemez ^[17] reported that the harvest date for ‘Maraş 18’ was earlier compared to ‘Chandler’. In the current research, the harvest dates of ‘Helete Güneşi’ and ‘Maraş 18’ cultivars were 17 and 15 September, respectively, whereas the harvest date of ‘Chandler’ was 5 October, i.e., 18 days later than ‘Helete Güneşi’ (Table 1). Therefore, an early harvest date in ‘Helete Güneşi’ can be seen as another promising trait in the phenological features of this cultivar.

Despite its lesser importance than late-spring frost, early-autumn frost can affect the success rate of walnut cultivation and the sustainability of orchard management. Due to its late defoliation date, ‘Chandler’ is not wholly suitable for agricultural lands exposed to frequent, early-autumn frost. In contrast, ‘Maraş 18’ is a walnut cultivar with an early defoliation date. Specifically, defoliation dates in ‘Helete Güneşi’, ‘Maraş 18’, and ‘Chandler’ cultivars were 7 November, 3 November, and 6 December, respectively. In other words, the defoliation date in ‘Helete Güneşi’ occurred 30 days earlier than ‘Chandler’ and 4 days later than ‘Maraş 18’ (Table 1).

Table 1. Phenological traits of ‘Helete Güneşi’ and its parents.

Pomological analysis helps determine fruit quality and has been widely used in many breeding studies to identify fruit cultivars with superior traits. The most important traits in a high-quality walnut cultivar are nut weight, kernel weight, kernel percentage, kernel color, kernel fill, and ease of removal of kernel halves ^{[12][16][23][28][33][34]}.

Twelve different traits were analyzed for the purpose of characterizing pomological traits among the three different walnut cultivars in the present study. The findings indicated that ‘Helete Güneşi’ had a broad elliptic nut shape, a very smooth shell texture, and a light shell color, thereby indicating remarkable differences in comparison with its parents in terms of nut shape. Shell thickness in ‘Helete Güneşi’, ‘Chandler’, and ‘Maraş 18’ cultivars were 1.10 mm, 1.19 mm, and 1.37 mm, respectively. In other words, the shell strength of ‘Helete Güneşi’ was comparatively weak (Table 4).

Nut weight is not the sole determiner of walnut quality since kernel weight is also a crucial criterion. In the available literature, there are various reports on kernel weight in different walnut genotypes, ranging from 3.62 to 27.00 g ^{[17] [33][34][35][36][37][38]}. In addition, it is reported that a high-quality cultivar should have a kernel percentage of over 50% ^[39].

In the present study, the nut weight and kernel weight of ‘Helete Güneşi’ were 13.41 g and 7.16 g, respectively, while its kernel percentage was 53.39%. According to standards on walnut quality, thus ‘Helete Güneşi’ can be seen as a high-quality cultivar in terms of kernel percentage. Nut weight, kernel weight, and kernel percentage were 14.62 g, 7.86 g, and 53.76% in ‘Maraş 18’, whereas in ‘Chandler’ they were 12.73 g, 6.13 g, and 48.23%, respectively. With these descriptions, the ‘Helete Güneşi’ cultivar struck a position between the two parent cultivars in terms of kernel weight, although it was closer to ‘Maraş 18’ in terms of kernel percentage (Table 2).

The statistical analysis of nut weight, kernel weights, kernel percentage, and shell thickness of ‘Helete Güneşi’ and its parents indicated statistically significant differences among these three cultivars (Figure 3). Previous studies on the ‘Chandler’ cultivar showed that its nut weight and kernel weight ranged from 7.7 g to 14.0 g and from 3.32 g to 7.00 g, respectively ^{[17][27][31][40]}. Similarly, Sütyemez ^[17] reported that the kernel weight and kernel percentage of ‘Maraş 18’ ranged from 13 to 15 g and from 52% to 57%, respectively.

A suitable degree of kernel fill was observed in the three cultivars of this study. Meanwhile, kernel color is another important indicator of market value for a walnut cultivar. It was found that the three cultivars in the present study were similar to each other in this respect since they had a light kernel color. The current findings describe the kernel color of ‘Chandler’ and ‘Maraş 18’ as the same as those in the available literature ^[17][19][27]. Finally, ease of removal of kernel halves was easy in the case of ‘Helete Güneşi’ and ‘Maraş 18’ cultivars, while it was very easy in the ‘Chandler’ cultivar. Kernel flavor in the three cultivars was tested by a group of 20 people, as a result of which, all three cultivars had kernels with satisfactory flavors. For a more tangible description of ‘Helete Güneşi’, a pictorial overview of its kernels and the tree is depicted in Figure 4.