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Review. Flower biology in apricot and its implications for breeding

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Different aspects related to flower biology have a close link to fruit set failures in apricot and other fruit trees. In this work, studies on pollen viability and germinability, stigma receptivity, ovule development and longevity, the different factors affecting the effective pollination period (EPP), are reviewed. The concept of EPP is based on biological parameters that are the successive steps that take place during the reproductive process and it is the frame within which the factors limiting an appropriate fruit set can be studied. The definition of this concept and its detailed study have allowed determination of the different limiting factors and the design of specific treatments to improve it. Knowledge of the incompatibility phenotype for many apricot cultivars has allowed advising about the planting of singlecultivar orchards. The study of the inheritance of this and other traits in apricot and other fruit trees has allowed planning of hybridisations to minimise or eliminate the production of undesirable seedlings, increasing the efficiency of the breeding programme. Studies on the flower biology of apricot have provided valuable information to help select the appropriate parent cultivars for breeding programmes, also this information is transferred to farmers to avoid losses produced by an inadequate cultivar selection. In this review we intend to give an updated overview of the state of the art in the research and the achievements thus far, as well as considering the implications of these studies for fruit breeding in general, with special attention to apricot breeding.
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... Two ovules per ovary are typical of the genus Prunus, as has been confirmed in many fruit species, such as almonds [25,29], apricots [30,31], peaches [32], cherries [12,33], Japanese plums [26,34], and European plums [35,36]. Moreover, the presence of two ovules per ovary was confirmed in all examined cultivars. ...
... A certain degree of megagametophyte development at full bloom may not be enough to ensure a high rate of fruit set. In some cases, high percentages of functional ovules had low fruit set [31]. ...
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Compatibility and synchrony between specialized tissues of the pistil, female gametophytes and male gametophytes, are necessary for successful pollination, fertilization, and fruit set in angiosperms. The aim of the present work was to study the development and viability of embryo sacs, as well as fertilization success, in relation to the fruit set of the cultivars ‘Mallard’, ‘Edda’, ‘Jubileum’, and ‘Reeves’, under specific Norwegian climatic conditions. Emasculated, unpollinated, and open-pollinated flowers were collected at the beginning of flowering, and on the 3rd, 6th, 9th, and 12th days after flowering, from all four plum cultivars over two years (2018/2019). Ovaries were dehydrated, embedded in paraffin wax, sectioned, stained, and observed under a light microscope. Results showed the existence of synchronization between successive phases in the development of the embryo sac and individual phases of flowering. All plum cultivars had higher percentages of viable embryo sacs, fertilized embryo sacs, and fruit set in 2018 than in 2019. These differences may be related to the very low temperatures during the post-full-flowering period in 2019, and to the low adaptation of some studied cultivars to unfavorable conditions. In our study, the cultivar ‘Jubileum’ showed the highest percentage of viable embryo sacs, fertilized embryo sacs, and fruit set compared to other cultivars, i.e., the best low-temperature adaptation.
... Studies on the flower biology of apricot have provided valuable information to help select the appropriate parent cultivars for breeding programmes (Burgos et al., 2004). Different aspects related to the pollen, the stigma and the ovules are considered that influence greatly the possibilities of the flowers to set fruits. ...
... Traditionally the European group of apricot cultivars has been described as self-compatible, in the last decades many widely cultivated apricot cultivars have been described as self-incompatible. In fruit trees, incompatibility complicates horticultural practices, because self-incompatible cultivars require the addition of pollinators and the yield depends on the abundant pollen transfer among trees (Burgos et al. 2004). ...
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In recent years the interest in walnut tree culture increased, therefore it was necessary to introduce new technologies of cultivation of this crop to increase economic efficiency. Distance of 10 m between the rows of walnut tree provide a vegetation cover which can be used in animal feed. Livestock in walnut tree plantations can be an effective method to reduce the payback period of investments. This paper aims to assess agronomic characteristics in four cultivars of walnut tree (Miroslava, Anica, Ovidiu and Velnita) and identify the floristic composition of the vegetal cover of rows of walnut tree in ecological conditions.
... Оскъдното производство на цветни пъпки или високият процент на опадване на пъпките е показател за лоша производителност. Тъй като тези особености зависят от сорта, те могат да бъдат наследени и следователно използването на такива сортове като родители в рамките на селекционна програма не е препоръчително (Burgos et al., 2004). ...
... These characteristics depend on the cultivar and could be inherited. Therefore, the use of such cultivars as parents in a breeding program is not recommended (Burgos et al., 2004). ...
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Evaluation of the cross- compatibility of different cultivar parental combinations is important for improving the plant breeding process. The proper choice of cross-compatible cultivars for the controlled hybridization is a prerequisite for obtaining more hybrid seeds and plants. In the present study an evaluation of the compatibility of 29 parental combinations, with 12 different apricot cultivars and 6 F1 hybrids (Modesto x Harcot) was done. For the period 2011-2019 in a collection orchard at the Fruit Growing Institute -- Plovdiv, a total number of 10 817 flowers were hand-pollinated and afterwards 750 hybrid seeds were obtained. The percentage of fruit set varied from 0 to 27.42{\%}, depending on the climatic factors of the year and the presence of the same S-alleles, determining cross- incompatibility in the used parental cultivars. The viability of some of the obtained seeds and plants was assessed. The presence of the same S-allele in both parents does not lead to a decrease in the number of obtained seeds, their germination and the percentage of obtained hybrid plants.
... (Milosević et al., 2010) . (Burgos et al., 2004) ‫نکوناام‬ . (Nekonam et al., 2010) (McLaren et al., 1996;Fikret Balta et al., 2007;Ruiza and Egea, 2008) . ...
... Several factors are considered common causes of flower bud drops viz. water stress, lack of chilling and high temperatures (Burgos et al., 2004). ...
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... In contrast to pear, in stone fruits, viable embryo sacs can disappear up to sixth day after BOF [49], persist up to twelfth day after BOF [31,50], or the development of the embryo sacs can be slowed down so that a high percentage of ovules without differentiated embryo sacs can be found [51]. This means that a certain degree of megagametophyte development at full bloom may not be enough to ensure a good crop, since high percentages of functional ovules can have a low fruit set [52]. So, high yields require much more gamete production resources in ovules compared to pollen. ...
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Since the European pear (Pyrus communis L.) is a self-incompatible fruit species, synchrony and compatibility between female parts of the mother plant and male gametes from the pollen donor must be fulfilled. Besides pollination and fertilization, normal embryo and zygote development is one of the prerequisites for the satisfactory yields in pears. The main goal of this experiment was to investigate the functionality of embryo sacs and the embryo's early stages of growth in relation to the fruit set of diploid ('Celina') and the triploid ('Ingeborg') pear cultivars under specific Norwegian climatic conditions. For this purpose, flowers were collected at the beginning of flowering, and on the third, sixth, ninth, and twelfth days after the beginning of this phenophase for two consecutive years. Ovaries were dehydrated, embedded in paraffin wax, sectioned, stained, and observed under the light microscope. In the analyzed cultivars, results showed different tendencies in embryo sac development and degradation processes, in both experimental years, which is probably due to the genetic background of the examined cultivars. Also, fertilization success and fruit set were higher in the second year of study due to the higher average temperature during the flowering period. Diploid cultivar 'Celina' showed much better adaptation to high temperatures in relation to triploid cultivar 'Ingeborg'.
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In fruit trees, gametophytic self-incompatibility is one of the major problem because of preventing self-fertilization controlled by a single locus with some allelic variants. Among the fruits, apricots also show self-incompatibility especially originated from Middle-Asian and Iranian-Caucasian. In present study, it was determined self-compatible/incompatible of some apricot cultivars which cultivated in Turkey. 17 Turkish and 42 foreign apricot cultivars were used in this study. Analyses were carried out using three primer pairs (SRc-R-SRc-F, EM-PC2consFD / EM-PC3consRD and AprFBC8). A total 5 S-RNase alleles (S2, S3, S6, S9 and S11) were determined in the 11 new Turkish cultivated apricots and a total 8 (S3, S7, S8, S9, S11, S12, S13 and SC) were determined in old Turkish apricot cultivars. A total 11 S-RNase alleles (S2, S3, S4, S6, S7, S8, S9, S11, S12, S19 and SC) were determined in foreign 42 apricot cultivars. It was determined self (in)compatibility alleles of 11 new Turkish and 38 foreign apricot cultivars first time with this study. These results can be used for plantation new orchards and breeding programs. Also, pollinator cultivars should be considered for plantation new orchards for these self-incompatible cultivars because of some Turkish and foreign apricot are mostly self-incompatible.
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We observed trees of the Japanese apricot, Prunus mume ‘Nanko’ (Rosaceae), bearing two types of flowers: 34% had blue fluorescent pollen under UV irradiation, and 66% had non-fluorescent pollen. The fluorescent pollen grains were abnormally crushed, sterile, and devoid of intine and pollenkitt. The development of microspores within anthers was investigated: in the abnormally developed anthers, tapetal cells were vacuolated at the unicellular microspore stage, and fluorescent pollen was produced. Compounds responsible for the blue fluorescence of pollen were identified as chlorogenic acid and 1-O-feruloyl-β-D-glucose. The anthers with fluorescent pollen contained 6.7-fold higher and 3.8-fold lower amounts of chlorogenic acid and N¹,N⁵,N¹⁰-tri-p-coumaroylspermidine, respectively, compared to those with non-fluorescent pollen. The tapetal vacuolization, highly accumulated chlorogenic acid, and deficiency of N¹,N⁵,N¹⁰-tri-p-coumaroylspermidine imply that low-temperature stress during the early unicellular microspore stage caused a failure in microsporogenesis. Furthermore, potential effects of the visual difference on the bee behavior were also discussed through the colorimetry. The sterility, likely induced by low-temperature stress, and the preference of honeybees for fluorescence may reduce the pollination efficiency of P. mume.
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Armenia is considered as one of the centers of apricot origin. The reproductive development and adequate pollination of cultivars present the most important stage for the apricot productivity. Morphometric and cytoembryological study of reproductive biology of Armenian apricot cultivars and wild seedlings revealed differences in formation of flower structure. Formation of short pistils had negative effect on effective pollination and fruit and seed production. Almost all flowers of investigated samples contained only one developed ovule with cells of embryo sac in the pistil. For the majority of cultivars low level of ovule abnormalities was registered. The highest level of pollen sterility was detected for cultivars with low level of ovule abnormalities as compensatory mechanism for cultivar productivity. Our research form the conditions for the creation of preliminary dataset of Armenian apricot gene pool, making possible the identification of the causes of low yield and the selection of the cultivars with low levels of male and female sterility. Morphometric and cytoembryological profiles of Armenian apricot cultivars can provide important information for the success of breeding programs.
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Initial fruit set in 6 ‘Italian’ prune orchards for 2 years was markedly influenced by post-bloom temperature. In the warm year initial sets ranged from 36 to 64%; in the cool year initial sets were 1 to 13%. Visible frost damage occurred in only 1 orchard and was not responsible for the general failure in the cool year. Embryo sacs were studied in pistils from the same orchards in the 2 years. The cause of erratic fruit setting in ‘Italian’ prune is attributed to its genetically determined sensitivity to cool weather in the post-bloom period. Cool temperature delays pollen tube growth, fertilization, and early embryo and endosperm development so long that the ovule begins to degenerate. Ovule breakdown begins in the nucellus at the chalazal end, so that even if fertilization is finally accomplished, fruit set is not stimulated.
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This report identifies S-RNases of sweet cherry (Prunus avium L.) and presents information about cDNA sequences encoding the S-RNases, which leads to the development of a molecular typing system for S-alleles in this fruit tree species. Stylar proteins of sweet cherry were surveyed by two dimensional polyaclylamide gel electrophoresis (2D-PAGE) to identify S-proteins associated with gametophytic self-incompatibility. Glycoprotein spots linked to S-alleles were found in a group of proteins which had Mr and pI similar to those of other rosaceous S-RNases. These glycoproteins were present at highest concentration in the upper segment of the mature style and shared immunological characteristics and N-terminal sequences with those of S-RNases of other plant species, cDNAs encoding these glycoproteins were cloned based on the N-terminal sequences. Genomic DNA and RNA blot analyses and deduced amino acid sequences indicated that the cDNAs encode S-RNases; thus the S-proteins identified by 2D-PAGE are S-RNases. Although S1 to S6-alleles of sweet cherry cultivars could be distinguished from each other with the genomic DNA blot analysis, a much simpler method of PCR-based typing system was developed for the six S-alleles based on the DNA sequence data obtained from the cDNAs encoding S-RNases.
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Based on the cDNA sequences encoding sweet cherry self-incompatibility associated ribonucleases (S-RNases), a PCR-based S-allele typing system for sweet cherry cultivars has been recently developed. Using this technique, we determined S-genotypes of the three newly released Japanese cvs Kouka-Nishiki, Beni-Sayaka and Beni-Shuho and one British cv Merton Glory that was classified as a Universal Donor, which is able to be used as a pollen donor for all cultivars in pollen incompatibility groups I to XIII. Furthermore, we also determined the partial sequences of the S-RNase genes of 'Rainier' (S1S4)' and 'Sato-Nishiki (S3S6)', which leads to the development of a more reliable S-allele identification method of PCR-RFLP for sweet cherry cultivars. Total DNA isolated from leaves of the four cultivars along with those from ten cultivars with known S-genotypes were PCR amplified with two sets of primers that were designed from DNA sequences encoding the signal peptide (Pru-T2) and two conserved domains (Pru-C2 and Pru-C4R) of sweet cherry S-RNases. By comparing the size of PCR products on agarose gel, the S-genotypes of 'Kouka-Nishiki', 'Beni-Sayaka', 'Beni-Shuho' and 'Merton Glory' were suggested to be S1S3, S1S6, S4S6, and S4S6, respectively. Two of these three S-genotypes (S1S6 and S4S6) were found for the first time. DNA sequencing of PCR products from S-alleles of 'Rainier' and 'Sato-Nishiki' revealed that Ban II, Nru I, Apa LI and Ava I sites, respectively, were unique in the S1-, S3-, S4- and S6 sequences flanked by Pru-T2 and Pru-C4R primers. RFLP analysis of the PCR products using these enzymes confirmed that S1-, S3-, S4- and S6-alleles of the four cultivars contained the respective restriction enzyme recognition sites.