Examination of self-compatibility in promising plum (Prunus domestica L.) genotypes developed at the Fruit Research Institute, Čačak
Introduction
Plum (Prunus domestica L.) is the most important fruit crop in Serbia. With average annual production of 507,987 t (2010–2014), Serbia ranks among the world’s top three plum-producing countries (FAOSTAT, 2016). The total volume is produced from about 48 million trees (Republic Bureau of Statistics, 2016), giving an average yield of 10.58 kg per tree, which is much lower than the average yields achieved in the world’s largest producing countries. The underlying causes of this include the spread of Plum pox virus (Ranković et al., 1995), extensive cultural practices, instability of the domestic and global markets and use of outdated cultivars (Milošević and Milošević, 2011). Other factors critical to fruit set and yield optimisation in orchards are the success of pollination and fertilisation (Koskela et al., 2010), which depend on degree of self-fertility (Nikolić and Milatović, 2010), certain pollen performance traits (Surányi, 2006, Sharafi et al., 2013), pollen tube growth and ovule longevity (Nikolić et al., 2012).
The crop reliability of fruit species depends on genetically controlled mechanisms and external factors (Makovics-Zsohár and Halász, 2016). Like other species in the Rosaceae family, the European plum exhibits gametophytic self-incompatibility. Gametophytic incompatibility is a widely distributed reproductive barrier defined as the ability of the pistil to reject genetically related pollen (Sutherland et al., 2009) thus preventing inbreeding, while favouring allogamy. Gametophytic incompatibility is genetically controlled by two S-locus genes, one controlling the style (S-RNases), and the other governing the pollen (S-specific F-box protein) (Sutherland et al., 2008). The presence of identical S alleles in the pollen and the style results in the arrest of pollen tube growth in the style in both self-incompatible and cross-incompatible cultivars. European plum (Prunus domestica L.) is a hexaploid (2n = 6x = 48) allopolyploid species which combines genomes from diploid (2n = 2x = 16) cherry plum (Prunus cerasifera Ehrh.) and tetraploid (2n = 4x = 32) blackthorn (Prunus spinosa L.) (Zohary and Hopf, 2000), and thus has three component genomes (D1D2C), each with one gene control and probably with multiple alleles (Botu et al., 2002). This means that there is wide variation in the self-fertility of genotypes of European plum and some are completely self-incompatible (Nyéki and Szabó, 1996, Szabó, 2003, Neumüller, 2010). Partially self-fertile and self-sterile cultivars require compatible polleniser with overlapping bloom times to produce high, stable yields (Sutherland et al., 2004).
The self-fertility of some plum genotypes has been determined from fruit set under conditions of floral isolation and artificial pollination in the field. However, the results of this method show substantial weather-dependency and observation of pollen tube growth in the pistil using fluorescence microscopy has proved a more reliable method (Viti et al., 1997 Milatović et al., 2013). Nowadays molecular methods are also used to evaluate the self-fertility of European plum cultivars, but the hexaploid genome makes allele identification a complex, difficult task (Halász et al., 2014) and there is no clear information about the genetic diversity of self-incompatibility alleles (Kota-Dombrovska and Lācis, 2013 Makovics-Zsohár and Halász, 2016).
The Fruit Research Institute, Čačak has a seventy-year tradition of plum breeding. So far it has produced 15 recognised new plum cultivars and a large number of promising genotypes are currently under intense evaluation. Self-fertility, abundance and regularly cropping are some of the most important goals of breeding (Lukić et al., 2016). The objective of this study was to determine the self-compatibility of promising plum genotypes developed at the Fruit Research Institute, Čačak by observing pollen tube growth in the pistil using fluorescence microscopy and determining fruit set. The results would indicate whether promising genotypes could be grown in single-cultivar plantings or would need appropriate polleniser. The findings were also expected to be useful in further breeding work.
Section snippets
Plant material and experimental design
Six promising plum genotypes developed at the Fruit Research Institute, Čačak using planned hybridisation i.e. 38/62/70 (‘Hall’ × ‘California Blue’), IV/63/81 (‘Large Sugar Prune’ × ‘Scoldus’), 32/21/87 (‘Stanley’ × ‘Scoldus’), 34/41/87 (‘Valjevka’ × ‘Čačanska Lepotica’), 22/17/87 (‘Čačanska Najbolja’ × ‘Zelta Boutilcovidna’) and the newly released cultivar ‘Nada’ (‘Stanley’ × ‘Scoldus’) grafted on Myrobalan seedlings (Prunus cerasifera Ehrh.) were evaluated in 2009, 2010 and 2011.
Research was
Pollen germination in vitro
There were differences between the tested genotypes with respect to pollen germination in vitro (Fig. 1). Average in vitro pollen germination was lowest in the cultivar ‘Nada’ (25.64%) and highest in hybrids 32/21/87 (38.92%) and 38/62/70 (38.63%). In vitro pollen germination differed between experimental years. The lowest average value was recorded in 2009 and the highest in 2010. Not all genotypes showed the same temporal pattern and the ANOVA revealed that the pollen germination was subject
Discussion
Pollen functionality as reflected in in vitro pollen germination and pollen tube growth rate is an important contributor to successful fertilisation and fruit set (Hedhly et al., 2005). Given that 25% pollen germination is the threshold between poor and good pollen germination in plums (Wertheim, 1996), the results suggest that all genotypes except the cultivar ‘Nada’ exhibited good pollen germination. The pollen germination values for the plum genotypes studied in this research are consistent
Conclusion
The results of this research on pollen germination in vitro showed that all genotypes, except the cultivar ‘Nada’, exhibited good pollen germination, thus ensuring successful fertilisation and fruit set. The low percentage of pollen germination in the cultivar ‘Nada’ means that it should not be used as a polleniser in commercial orchards or in future breeding programmes.
In vitro pollen germination, quantitative in vivo pollen tube growth parameters and number of fruit set were affected by the
Acknowledgements
This work was conducted as part of Research Project TR–31064: Development and preservation of the genetic potential of temperate zone fruits which was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia.
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