Prospects for crossing species in the genus Crataegus L. (Rosaceae)

Hawthorn is a unique plant used from a long time ago for food and as a medicinal raw material. In Russian regions, there are hawthorn forms adapted to local conditions that produce a good harvest. New forms are obtained through hybridization, so it is important to reveal species that are most suitable for interspecific hybridization to improve the characteristics and adaptability of plants. In this work, for the first time, the effectiveness of crossing between 13 most successful Crataegus species in the conditions of the Peter the Great Botanical Garden (St. Petersburg), morphology, fertility and viability of the pollen derived from crossing, were studied. C. maximowiczii and C. caucasica were identified as universal pollinators. Our goal was to identify the most successful crosses between representatives of the genus Crataegus in the conditions of St. Petersburg, in which the maximum number of full-fledged fruits and seeds is formed, as well as to assess the morphological formation of pollen and its ability to germinate on an artificial nutrient medium. The objects of the study were plants of the species Crataegus Tourn. ex L. from the open ground collection of the Peter the Great Botanical Garden, Komarov Botanical Institute RAS (St. Petersburg): North American species C. horrida Medik., C. prunifolia (Lam.) Pers., C. macracantha Lodd., C. submollis Sarg; Far Eastern species C. maximowiczii C.K. Schneid., C. chlorosarca Maxim., C. chlorosarca Maxim. var. atrocarpa (E. Wolf) Cinovskis, C. pinnatifida Bunge; European-Asian species C. monogyna Jacq., C. lasiocarpa Lange, C. chlorocarpa Lenne et C. Koch., C. media Becht. Rosa Flore Plena; Central Asian species C. almaatensis Pojark., C. caucasica C. Koch. The morphology of pollen grains was studied (a Tescan Vega 2 scanning electron microscope, Tescan, Czech Republic) in low vacuum mode, at a chamber pressure of 15 Pa, accelerating voltage 15 kV. Pollen fertility was assessed by staining with acetocarmine. The viability of pollen was determined by germination on an artificial nutrient medium, including agar-agar (0,8 g), sucrose (15 g) and boric acid (0.003 g), dissolved in 100 ml of distilled water. Forced xenogamy type pollination was used to determine the ability to hybridize and to establish optimal interspecific crosses. The flowers were castrated at the white bud stage. After castration and pollination of the flowers, the shoot was covered with a gauze bag. At least 10 shoots were allocated for each crossing combination. The number of flowers and fruits assessed the effectiveness of crossing. Along with the normal shape of pollen grains, disturbances in their morphological structure were revealed in each species. The largest number of deformed pollen was in C. horrida (~ 30 %), the smallest in C. almaatensis (4.5 %). The pollen of different hawthorn species was well stained with acetocarmine, which indicates its morphological formation (from 63.3 % in C. lasiocarpa to 92.6 % in C. almaatensis . Despite this, some pollen grains (51.7-90.6 %) did not germinate on an artificial nutrient medium. Pollen germinating with long or medium-length pollen tubes (most suitable for hybridization) was most abundant in C. prunifolia (28.3 %), slightly less in C. horrida (10-13 %), C. chlorosarca Maxim. var. atrocarpa , C. macracantha (12 %), and C. submollis (~11 %). All studied plants were cross-pollinated. Fruit and seed set during xenogamy ranged from 63.2 % ( C. submollis ) to 96.2 % ( C. maximowiczii ). Species were able to interbreed with each other. In some cases, during reciprocal crosses, fruit set occurred at different degrees when the same species in one case serves as the mother plant and in the other as the paternal plant (for example, in the cross of C. prunifolia × C. maximowiczii ). However, in most crosses with different combinations of maternal and paternal forms, the difference in fruit set was insignificant. The most successful combinations of interspecific crosses have been identified among the following species: C. maximowiczii × C. submollis , C. maximowiczii × C. horrida , C. maximowiczii × C. prunifolia , C. submollis × C. chlorosarca , C. horrida × C. submollis , C. caucasica × C. monogyna , C. pinnatifida × C. caucasica , C. chlorosarca × C. macracantha , C. chlorosarca × C. caucasica , C. chlorosarca × C. almaatensis , C. chlorosarca × C. pinnatifida , C. macracantha × C . chlorosarca , C. macracantha × C. caucasica . It was found that the most universal pollinators which produced the maximum number of sets were C. maximowiczii and C. caucasica .