Abstract

Sweet cherry fruit is a tasty and valuable product for consumers. In order to increase the export share of cherry, which is also very important in export, it is beneficial to grow with cherry varieties that mature at different times. The cherries offered to the market in the early period will be more attractive. In this study, morphological and biological features of pistils of early-maturing ‘Cristalina’, ‘Early Lory’, ‘Prime Giant’, fruit set rates and pollen germination status and some chemical applications were investigated. As a result, fruit sets of cultivars were 17.6-28.6% in two years. Significant differences were observed in pistil morphology of the cultivars and ‘Cristalina’ had shorter pistil (14.35-14.51 mm) and style (11.47-11.65 mm) lengths than the other cultivars. Greater deformation was observed in primary ovules of ‘Early Lory’ right after anthesis. There were not significant differences in pollen germination ratios of the cultivars, but boric acid treatments improved pollen germination ratios of all cultivars. Boric acid application increased pollen germination with 21%. This was followed by IAA (8%), GA₃ (5%), KNO₃ (4%). It was concluded based on present findings that in orchard establishment with the early cultivars, flower biology should momentously be assessed.

Keywords:
Prunus avium L; anthesis; pistil; style; ovule; fruit set

HIGHLIGHTS

Flower biology of early sweet cherry cultivars.

Fruit set and ovule structure.

Pistil morphology of cherries.

Pollen and fruits.

INTRODUCTION

Horticulture plants are adding value of earth's diversity and fundamental to all life. They include high content of non-nutritive, nutritive, and bioactive compounds such as flavonoids, phenolics, anthocyanins, phenolic acids, and as well as nutritive compounds such as sugars, essential oils, carotenoids, vitamins, and minerals [¹1 Senica M, Stampar F, Mikulic-Petkovsek M. Different extraction processes affect the metabolites in blue honeysuckle (Lonicera caerulea L. subsp. edulis) food products. Turk J Agric For. 2019;43:576-85.,²2 Gecer MK, Kan T, Gundogdu M, Ercisli S, Ilhan G, Sagbas HI. Physicochemical characteristics of wild and cultivated apricots (Prunus armeniaca L.) from Aras valley in Turkey. Genet Resour Crop Ev. 2020;67:935-45.]. Worldwide, annually 2.443.407 tons cherry are produced and about 627.132 tons of that production come from Turkey [³3 Food and Agriculture Organization of the United Nations. (2017): Statistical Database. http://faostat3.fao.org/download/Q/QC/E.
http://faostat3.fao.org/download/Q/QC/E... ]. With such a production quantity, Turkey is the leading cherry producer of the world. However, recent decreases in unit-area yields and exports have fallen Turkey behind Chili (118.316 tons) and China (81.627 tons) in cherry exports. According the recent data, Turkey was able to export 79.789 tons cherry [⁴4 Food and Agriculture Organization of the United Nations. (2016): Statistical Database. http://faostat3.fao.org/download/Q/QC/E.
http://faostat3.fao.org/download/Q/QC/E... ]. To improve exports, it is quite significant to work with cherry cultivar ripening at different periods. Early-harvested ones may find buyer at quite high prices since the quantity of production is low. These cherries able to find buyer at high prices should also have desired fruit sizes to be unrivalled in export. Therefore, any factors able to influence fruit size should be taken into consideration in culture of those early cherries. Of these factors, pollination and fertilization are the primary ones [⁵5 Dokuzoguz M, Gülcan R. Research on the Improvement of Selected Almonds and Selection of Aegean Rape by Selection. Tubitak, Toag Pub. 1973; 28.]. Therefore, several researches have long been conducted on reproductive biology of various fruit species and types [⁶6 Vitagliano C, Viti R. Effects of some growth substances on pollen germination and tube growth in different stone fruits. Acta Hort. 1989;239:379-82.

7 Bolat I, Pirlak L. (1999a). Effects of some chemical substances on pollen germination and tube growth in apricot. Acta Hort. 1999a;488(1):341-4.

8 Bolat I, Pirlak L. An investigation on pollen viability, germination and tube growth in some stone fruits. J Agri and Forest. 1999b;23:383-8.

9 Tosun F, Yildirim A, Koyuncu F. Studies on the fertilization biology of selected almond genotypes 1. Pollen performances. Fruit Grow. 2007;1:304-8.

10 Acar I, Ak BE, Sarpkaya K. Effects of boron and gibberellic acid on in vitro pollen germination of Pistachio (Pistacia vera L.). Afr. J. Biotechnol. 2010;9(32):5126-30.

11 Çetinbas M, Çukadar K, Butar S. Determination of pollen performances of selected some apricot genotypes. Fruit Sci. 2016; 3(2):20-3.-¹²12 Sarisu A, Yildirim AN. Investigations on fertilization biology of some early cherry cultivars. Süleyman Demirel University J Nat and App Sci. 2018;22(Special Issue):407-13.]. For an efficient fertilization and productivity, fertilization characteristics of the cultivars should definitely be analysed in cherry orchards and the most appropriate cultivar pattern should be set up accordingly [¹³13 Sarisu HC, Karamürsel ÖF, Öztürk FP, Demirtas I, Koçal H, Gür I, Cengiz Ö, Sevik I. Introducing different cherry cultivars to inner and crossover areas. J Agri Sci. 2019;25:11-20.]. In cherry flowers, anther upper surface is higher than stigma surface. Growth of filaments take place before blooming. Pistil development is completed until balloon stage [¹⁴14 Zhang C, Whiting M. The occurrence of protruding pistil in sweet cherry (Prunus avium L.) and its consequence on fertilization. Sci Hortic. 2012;140:149-56.]. It was indicated in some previous studies that relative position of anthers and stigmas influenced the behavior of pollinators in apricot, peach and nectarine [¹⁵15 Ruiz D, Egea J. Analysis of the variability and correlations of floral biology factors affecting fruit set in apricot in a Mediterranean climate. Sci Hortic. 2008;115:154-63.]. Temperature accelerate blooming, but such a case is not directly proportional to pistil development and usually results in reduced fruit set [¹⁶16 Rodrigo J, Herrero M. Effects of pre-blossom temperatures on flower development and fruit set in apricot. Sci Hortic. 2002;92:125-35.,¹⁷17 Beppu K, Okamoto S, Sugiyama A, Kataoka I. Effects of temperature on flower development and fruit set on 'Satohnishiki' sweet cherry. J Jap Soc Hort Sci. 1997;65(4):707-12.]. Flower pistil is composed of stigma, style and ovary. Pollination is a process started with the advent of pollen into stigma and water absorption. Then pollen moves to ovary through style [¹⁸18 Kessler SA, Shimosato-Asano H, Keinath NF, Wuest SE, Ingram G, Panstruga R, Grossniklaus U. Conserved molecular components for pollen tube reception and fungal invasion. Sci. 2010;330:968-71.]. According to Nyéki and coauthors morphological structures and sizes of flower organs are genetically controlled [¹⁹19 Nyéki J, Brozik S, Ifju Z. Meggy es cseresznyefajtak klonjai viragnyilasanak indukalt periodicitasa. Both. Közl. 1974; 63:165-76.]. Morphological characteristics also have functional importance. For instance, large ovary positively influences fruit set in sour cherry, but long style negative influence [²⁰20 Bubán T. Flower Development and Formation of Sexual Organs. In: Nyéki J, Soltész M editors. Floral Biology of Temperate Zone Fruit Trees and Small Fruits, Akadémiai Kiadó, Budapest; 1996. p. 3-54.]. According to Nyéki (1980) the relations between the lengths of male and female organs influence fruit set in stone-fruits and fruit set greatly decreased in quite short female organ [²¹21 Nyéki J. Floral biology and fertilization of fruit Varieties. Mezogazdasági Kiadó, Budapest, 1980;73.]. In some cases, pollen tube develops slowly and egg losses vigour during this slow phase of growth [²²22 Çetinbas M, Çukadar K, Sarisu HC, Butar S. The effects of different chemical applications on pollen performances of selected apricot genotypes. YYU J Agr Sci. 2018;28(1):49-54.]. For fertilization to be realized, initially pollination should be realized, then germinated pollen over the stigma should generate a style to reach ovary and ovule [²³23 Thompson M. Flowering, Pollination and Fruit Set. In: Webster AD, Looney NE editors. Cherries: Crop Physiology, Production and Uses, Cab International; 1996. p. 223-41.]. For sure, inherent nutrients and hormones are used while pollination and fertilization. IAA plays a significant role in pollen-pistil interaction and has the greatest level at stigma and dissipate into vascular tissue following the germination of pollen [²⁴24 Chen D, Zhao J. Free IAA in stigmas and styles during pollen germination and pollen tube growth of Nicotiana tabacum. Physiol. Plant. 2008; 134: 202-15.]. However, these hormones and nutrients are not sufficient all the time and mostly insufficient [²²22 Çetinbas M, Çukadar K, Sarisu HC, Butar S. The effects of different chemical applications on pollen performances of selected apricot genotypes. YYU J Agr Sci. 2018;28(1):49-54.,²⁵25 Güçlü F, Koyuncu F, Yildirim A, Celepaksoy F. Investigations on fertilization biology of some selected almond genotypes II. Effects of some chemical treatments on pollen performances. Suleyman Demirel University J Agri Fac. 2011;(1):28-33.]. For pollen to be germinated, there should be sucrose, boron, calcium, potassium, magnesium and gibberellic acid and indole acetic acid-like growth regulators in the growing conditions. Therefore, several researchers worked with some minerals and plant growth regulators under both in vivo and in vitro conditions [⁶6 Vitagliano C, Viti R. Effects of some growth substances on pollen germination and tube growth in different stone fruits. Acta Hort. 1989;239:379-82.,⁷7 Bolat I, Pirlak L. (1999a). Effects of some chemical substances on pollen germination and tube growth in apricot. Acta Hort. 1999a;488(1):341-4.,²²22 Çetinbas M, Çukadar K, Sarisu HC, Butar S. The effects of different chemical applications on pollen performances of selected apricot genotypes. YYU J Agr Sci. 2018;28(1):49-54.,²⁵25 Güçlü F, Koyuncu F, Yildirim A, Celepaksoy F. Investigations on fertilization biology of some selected almond genotypes II. Effects of some chemical treatments on pollen performances. Suleyman Demirel University J Agri Fac. 2011;(1):28-33.

26 Asif MI, Al-Tahir OA, Farah AF. The effects of some chemicals and growth substances on pollen germination and tube growth of date palm. Hort Sci. 1983;18(4):479-80.-²⁷27 Wang Q, Lu L, Wu Li Y, Lin J. Boron influences pollen germination and pollen tube growth in Picea meyeri. Tree Phys. 2003;23:345-51.].

This study was carried out in order to contribute to the interpretation of productivity levels of early sweet cherry cultivars in practical cultivation and laboratory studies and fruit sets with open pollination were conducted to determine fertilization biology of ‘Early Lory’, ‘Prime Giant’ and ‘Cristalina’ commonly grown in sub-tropic regions of Turkey, western and southern coastline consists with warm winters and quite hot summers, for early production. Early ripening of these varieties and superior quality characteristics compared to local varieties increase their market values, therefore they are preferred in sub-tropic regions. For this purpose, pistil morphology and biological characteristics, germination capacity of pollen and effects of some plant growth regulators and minerals on germination of pollens were investigated.

MATERIAL AND METHODS

Plant material

Experiments were conducted in 2017 and 2018. ‘Early Lory’, ‘Cristalina’ and ‘Prime Giant’ early sweet cherry cultivars grafted on Gisela 6 rootstocks in the trial orchard established in 2012 of Fruit Research Institute (Egirdir, Isparta-Turkey) were used as the plant material of the experiments. ‘Cristalina’ comes to the first harvest maturity among these cultivars with a few days. It has large, dark red fruits with high marketing quality. Early Lory is early, medium-large, forming a homogeneous fruit on the tree, dark red, sweet and quality. Prime Giant has the largest fruits and is an early variety that matures a few days later than other varieties.

Pistil morphology

Pistils were removed from the receptacles and their fresh and dry weights were weighed with a precise scale (±0.001 g) and the difference between fresh and dry weight was considered as moisture content (%). Pistil and style lengths (mm), ovary and stigma diameters (mm) were measured with a digital caliper (±0.01 mm) [¹⁶16 Rodrigo J, Herrero M. Effects of pre-blossom temperatures on flower development and fruit set in apricot. Sci Hortic. 2002;92:125-35.].

Ovules

A day after anthesis, 10 pistil samples were taken from each cultivar in both years. Samples were fixated into FAA (90cc 70% ethyl alcohol + 5cc glacial acetic acid + 5cc formaldehyde) solution. In histological works, ovaries were subjected to preparations, they were cross-sectioned, their development was assessed under microscope and their images were taken. In preparations, microwave radiation-supported paraffin technique was used [²⁸28 Askin MA, Ozeker E, Dolgun O. Possibility of using microwave radiation in preparation techniques. Fruit. 1999;1:912-6.]. Following the preparation, samples were cut with a rotary microtom at 10 micrometer thickness and they were opened at 50 °C water bath. Then cross-sections were put over the glass slides and dried in an oven at 65 °C.

Development of embryo sac and ovule were determined in accordance with the method specified by Cerovic and Micic [²⁹29 Cerovic R, Micic N. Functionality of embryo sacs as related to their viability and fertilization success in sour cherry. Sci Hortic. 1999;79:227-35.]. The embryo sacs reached to 4 and 8 nuclei stage were accepted as fully developed. Safranine staining technique was used to monitor the tissues. Development status of primary and secondary ovules a day after anthesis was investigated and imaged under microscopy.

Fruit set ratio of the cultivars

Fruit set ratio of open pollinating flowers were determined at harvest period as indicated by Rodrigo and Herrero [¹⁶16 Rodrigo J, Herrero M. Effects of pre-blossom temperatures on flower development and fruit set in apricot. Sci Hortic. 2002;92:125-35.] and Alburquerque and coauthors [³⁰30 Alburquerque N, Burgos L, Sedgley M, Egea J. Contributing to the knowledge of the fertilization process in four apricot cultivars. Sci Hortic. 2004;102(4):387-96.] in 4 replicates with 50 flowers in each replicate. In replicates, flowers were counted at balloon stage and labelled. After fruitlet drops unfertilized, fruit counts were followed until harvest maturity. Since the early sweet cherry varieties were studied, the harvest maturity fruit counts were the fruit set values. Percentage fruit set ratios were calculated with the obtained values.

Effects of chemical treatments on pollen germination

To obtain pollen germination of the cultivars, 100 flowers at balloon stage were collected from each cultivar and different sections of the trees and they were immediately brought to laboratory. Anthers of the flowers were removed over white papers, then anthers of the cultivars were placed into petri dishes and kept under 75 W lamp for a night to burst. Pollens in petri dishes were transferred to small bottles and bottles were shaken to remove pollens from anthers. These bottles were kept in desiccators in a fridge until use in analyses. Pollen germination medium was prepared by using 100 mL distilled water, 1 g agar and 10 g sucrose and this medium was used as control treatment. Chemical treatments were implemented through supplementing control treatment with 10 ppm GA₃, 50 ppm KNO₃, 5 ppm IAA and 25 ppm H₃BO₃ separately. Growth media were poured in petri dishes hot and pollens were seeded into media before the freeze of media. About 12 hours after seeding, counts were performed under Nikon-Eclipse 80i microscope at 10X magnification and mean germination ratio of pollen grains of each flower was determined. Experiments were conducted in randomized block design with 4 replications.

Statistically evaluation

The samples were collected in randomized block design with four replicates of each kind and one tree per repetition from orchard. Resultant data were subjected to variance analysis with SAS-JUMP statistical software. Effects of the cultivars and chemicals were assessed and significant differences were tested with LSD multiple comparison test at P<0.05 level.

RESULTS and DISCUSSION

Fruit set ratio of the cultivars

When the fruit set ratios of sweet cherry cultivars for two years were compared to each other, it was observed that ‘Cristalina’ and ‘Prime Giant’ had greater fruit set ratios than the ‘Early Lory’. Statistical assessments on 2017 data revealed significant differences between fruit set ratios of the cultivars (P<0.05). Considering an average 20-25% fruit set ratio was acceptable for optimum productivity in sweet cherry cultivars, it was observed that ‘Cristalina’ and ‘Prime Giant’ reached to sufficient productivity levels in both years, but ‘Early Lory’ had slightly low yield level in 2017 (Table 1). Sweet cherry has a great place in agricultural export of Turkey and establishment of orchards with early cultivars is quite a significant issue for sub-tropic regions. In orchard establishment with early cultivars, fertilization biology, incompatibility and flowering dates of the cultivars should definitely be taken into consideration. With regard to fruit set ratios of the present cultivars, ‘Cristalina’ and ‘Prime Giant’ had greater fruit set ratios than ‘Early Lory’. Pistils play the primary role in fertilization of the cultivars. Flower productivity is represented by fruit set ratios [³¹31 Williams RR. Factors Affecting Pollination in Fruit Trees In: Luckwill LC, Cutting CV editors. Physiology of Tree Crops, Acad. Press., London and New York; 1970. p. 193-200.]. It is not expected that all fruits over a tree will turn into fruit. For instance, a fruit set ratio of 5% is sufficient in commercial apple culture [³²32 Modlibowska I. Pollen tube growth and embryo-sac development in apples and pears. J Pomol Horticult Sci. 1945; 21:57-89.]. Differences in fruit sets are generally attributed to development of ovules and nutritional status of the flowers. Besides, nutrient levels of flowers and leaves are also closely related to productivity [³³33 Ystaas J. The influence of cherry rootstocks on the content of major nutrients of three sweet cherry cultivars. Acta Hort. 1990;274:517-9.

34 Hanson EJ, Proebsting EL. Cherry nutrient requirements and water relations. In: Webster AD, Looney NE editors. Cherries: Production and Uses CAB International; 1996. p. 243-57.

35 Betran JA, Val J, Millan LM, Monge E, Montanes L, Moreno MA. Influence of rootstock on the mineral concentrations of flowers and leaves from sweet cherry. Acta Hort. 1997;448:163-7.

36 Ystaas J, Froyne O, Ystaas J. The influence of eleven cherry rootstocks on the mineral leaf content of major nutrients in "Stella" and "Ulster" sweet cherries. Acta Hort. 1998;468(1):367-72.-³⁷37 Sitarek M, Grzyb ZS, Olszewski T, Ystaas J. The mineral elements concentration in leaves of two sweet cherry cultivars grafted on different rootstocks. Acta Hort. 1998;468(1):373-6.]. Several physiological factors also influence fruit set ratios [³⁸38 Furukawa Y, Bukovac MJ. Embryo sac development in sour cherry during the pollination period as related to fruit set. Hort Sci. 1989;24(6):1005-8.].

Pistil morphology

Measurements on ovaries of pistils of the cultivar flowers revealed that there were significant differences in pistil fresh weights in 2017 (P˂0.05) and the greatest fresh weight was obtained from ‘Prime Giant’ (13.69 mg). ‘Prime Giant’ had greater moisture content than the other two cultivars in both years of the experiments (Table 2). ‘Cristalina’ had the shortest pistil length in both years. Similar findings were also observed in style lengths and the differences in style lengths were found to be significant (P˂0.05).

The differences in ovary diameters of the cultivars were not found to be significant. ‘Prime Giant’ had greater stigma diameter than the others (Table 4). Additionally; ‘Cristalina’ had smoother stigma surface, ‘Early Lory’ had greater lobe formation over the mid-section of the stigma. ‘Prime Giant’ had wider stigma and the stigma had sectional appearance (Figure 1). With regard to pistil moisture contents, ‘Prime Giant’ had greater moisture content than the others in both years of the experiments. Considering the pistil lengths, ‘Cristalina’ had lower values in both years (Table 3). Similar results were observed in style lengths and the differences in style lengths of the cultivars were found to be significant (Table 3). With regard to stigma diameters, ‘Prime Giant’ had wider stigmas than the others (Table 4). Stigma is located on top of style, style is located right beneath it and ovary is located at the bottom. According to Nyéki and coauthors morphological structures and sizes of these components are genetically controlled [¹⁹19 Nyéki J, Brozik S, Ifju Z. Meggy es cseresznyefajtak klonjai viragnyilasanak indukalt periodicitasa. Both. Közl. 1974; 63:165-76.]. Morphological characteristics also have functional significance. For instance, larger ovary positively and longer style negatively influence fruit set in sour cherry [²⁰20 Bubán T. Flower Development and Formation of Sexual Organs. In: Nyéki J, Soltész M editors. Floral Biology of Temperate Zone Fruit Trees and Small Fruits, Akadémiai Kiadó, Budapest; 1996. p. 3-54.]. Nyéki indicated that the relations between male and female organ lengths influenced fruit set of stone-fruits and fruit set quite decreased in quite short pistils [²¹21 Nyéki J. Floral biology and fertilization of fruit Varieties. Mezogazdasági Kiadó, Budapest, 1980;73.].

Ovules

Microscopic assessments were conducted on ovary samples of the cultivars taken a day after anthesis and development of ovules were evaluated. A recess or a problem was not observed in development of embryo sacs of both the secondary and the primary ovules of the cultivars a day after anthesis. It was observed that embryo sacs completed their differentiation and achieved 4-8 nuclei stage. In ‘Cristalina’, normal development of ovules was observed a day after anthesis (Figure 2). In ‘Early Lory’, high deformation was observed in primary ovules (Figure 3). Deformation in nucellus tissue of the primary ovules results in loss of fertilization ability. Such a case then influences overall fruit set. An abnormal development was not observed in primary ovules of ‘Prime Giant’ (Figure 4).

In present study, normal development was observed in ovary development of ‘Cristalina’ a day after anthesis. High deformation was observed in primary ovules of ‘Early Lory’. Deformation of nucellus tissue of primary ovules resulted in loss of fertilization capability. Such a case may then influence overall fruit set. Despite shorter pistils of ‘Cristalina’, normal ovule development resulted in greater productivity. An abnormal development was not observed in ovules of ‘Prime Giant’. There are two ovules in ovary of stone-fruits. However, only one of these ovules has a capability of fertilization. The one with a fertilization capability or the functional one is called as primary ovule and undeveloped one is called as secondary ovule [³⁹39 Eaton GW. A study of the in Prunus avium and its relation to fruit setting. Can J Plant Sci. 1959;39:466-76.]. According to Williams; in “Strong Flower” stigma receptivity, development of embryo sac and cell division of unfertilized ovules take longer time [⁴⁰40 Williams RR.The effect of summer nitrogen applications on the quality of apple blossom. J Hort Sci. 1965;40:31-41.]. Life of apple ovule at 11 °C could be 11-12 days [⁴¹41 Child RD. Pollination Studies in Fruit Trees. Ann. Rep. Long Ashton Res Stn. 1967;115-20.]. Pistil functionality of sour cherry may take 2-3 days [⁴²42 Nyéki J. Meggyfatják virágzása és termékenyülése. Agrartud Közl. 1976;35:377-86.], however, about 2-25% of ovules degenerated until anthesis [⁴³43 Bartz M, Stösser R. Quantitative Auswertung der pollenschläuche im griffel von sauerkirschen (Prunus cerasus L.) in beziehung zum fruchtansatz. Gartenbauwiss. 1989;54:132-7.]. In sour cherry, ovules are generally inactivated about 3-6 days after anthesis [¹⁸18 Kessler SA, Shimosato-Asano H, Keinath NF, Wuest SE, Ingram G, Panstruga R, Grossniklaus U. Conserved molecular components for pollen tube reception and fungal invasion. Sci. 2010;330:968-71.]. Such a duration is 1-5 days [⁴⁴44 Postweiler K, Stösser R, Anvari SF. The effect of different temperatures on the viability of ovules in cherries. Sci Hortic. 1985;25:235-9.] or 13 days [⁴⁵45 Guerrero-Prieto VM, Vasilakakis MD, Lombard PB. Factors controlling fruit set of "Napoleon" sweet cherry in Western Oregon. Hort Sci. 1985;20:913-4.] in sweet cherry based on cultivar and temperature. Differences in cultivars may influence the vigor of ovule. Such a case is especially effective on functionality and vigor of embryo sac right after anthesis [⁴⁶46 Stösser R, Anvari SF. On the senescence of ovules in cherries. Sci Hortic. 1982;16(1):29-38.].

Effects of chemical treatments on pollen germination

Cultivars were subjected to germination tests in 1% agar and 10% sucrose media for 12 hours and germination ratios varied between 13.81-31.81% in the first year and between 13.58-22.45% in the second year. Plant growth regulator and chemicals supplemented to growth media influenced germination ratios differently (Table 5). The greatest germination ratio in all three cultivars and in both years was obtained from H₃BO₃ treatments. According to variance analysis, only the treatments were found to be significant in the first year and both treatments and cultivar x treatment interactions were found to be significant in the second year (P<0.05) (Table 5). In the first year, the greatest germination ratio (39.58%) was obtained from H₃BO₃ treatment and the lowest from the control treatment (21.32%). The other treatments were not significantly different from the control treatment. In the second year of the experiments, boric acid treatments yielded greater germination ratios than the other treatments and the control group. With boric acid treatments a germination ratio of 42.74% was achieved in ‘Cristalina’, 35.98% in ‘Early Lory’ and 42.63% in ‘Prime Giant’. As compared to the control groups, about 27% greater germination was observed in ‘Cristalina’, 16% in ‘Early Lory’ and 29% in ‘Prime Giant’ (Table 5).

With regard to germination capability of pollen grains, ‘Early Lory’ had greater germination ratios than the other cultivars. Improvement of germination capacity of pollen is a significant issue for fertilization. In this sense, among the plant growth regulators and chemicals, H₃BO₃ treatments were found to be prominent and the greatest germination ratios were obtained from H₃BO₃ treatments of all cultivars. Thusly, Bolat and Pirlak reported that boric acid treatments had positive effects on germination of pollen in some apricot cultivars [⁷7 Bolat I, Pirlak L. (1999a). Effects of some chemical substances on pollen germination and tube growth in apricot. Acta Hort. 1999a;488(1):341-4.]. It was also indicated in several studies that boric acid treatments yielded quite well outcomes for germination of pollen [²²22 Çetinbas M, Çukadar K, Sarisu HC, Butar S. The effects of different chemical applications on pollen performances of selected apricot genotypes. YYU J Agr Sci. 2018;28(1):49-54.,²⁵25 Güçlü F, Koyuncu F, Yildirim A, Celepaksoy F. Investigations on fertilization biology of some selected almond genotypes II. Effects of some chemical treatments on pollen performances. Suleyman Demirel University J Agri Fac. 2011;(1):28-33.,²⁷27 Wang Q, Lu L, Wu Li Y, Lin J. Boron influences pollen germination and pollen tube growth in Picea meyeri. Tree Phys. 2003;23:345-51.,⁴⁷47 Sanzol J, Herrero M. The effective pollination period in fruit trees. Sci Hortic. 2001;90:1-17.].

CONCLUSION

It was observed in this study that ‘Cristalina’ and ‘Prime Giant’ had greater fruit set ratios than ‘Early Lory’. ‘Cristalina’ had lower values for pistil and still lengths. ‘Cristalina’ and ‘Prime Giant’ had normal ovule development a day after anthesis. High deformation was observed in primary ovules of ‘Early Lory’. Besides environmental conditions, biological characteristics such as style length, stigma diameter and ovule development influenced fruit sets of the cultivars. Significant differences were not observed in germination ratios of the cultivars, but boric acid treatments improved pollen germination ratios of all cultivars. In this sense, among the minerals, boric acid treatments could be recommended to improve fruit set ratios in practical fruit culture. Present investigations on flower biology of ‘Cristalina’, ‘Early Lory’ and ‘Prime Giant’ may constitute bases for further researches to be conducted on early sweet cherry cultivars. ‘Cristalina’ and ‘Prime Giant’ regarding their floral characteristics were advised as suitable for the subtropical regions of Turkey.

Acknowledgments

I would like to thank the Ministry of Agriculture and Forestry General Directorate of Agricultural Research and Policies, which supported the study financially.

REFERENCES

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