The genus Aronia (sometimes treated as a subgenus or section of Sorbus) is distributed in eastern North America from Newfoundland and southern Quebec and Ontario in the north to the Florida Peninsula in the south.

Aronia or chokeberry mostly occurs in wet habitats: wetlands, wet forests, along streams and around lakes, in sandy lowlands. It can be found in dunes, on rocky slopes and steep cliffs, on overgrown or bare rocky outcrops [1]. In North America, chokeberry is considered a low-quality ornamental shrub and thus is seldom cultivated, sometimes even exterminated as a weed by chemical applications [2].

Most American authors [1, 2] distinguish two species of chokeberry: red-fruited A. arbutifolia (L.) Elliott and black-fruited A. melanocarpa (Michx.) Elliott (Fig. 1), emphasizing significant variability in both species, and especially in the latter one.

Fig.1. [Not provided] Herbarium sample of A. melanocarpa with ripe fruit (northern New York State, Adirondack Mountains, Huntington Forest, sphagnum bog, 24 Aug 1976. coll. A. Skvortsov)

Both species of Aronia were cultivated in the European gardens as early as the beginning of the 19th century, their various forms often described as distinct species differing from the ones named above by height, pubescence, fruit color, etc. [4, 5]. Nearly all these "species" (A. depressa Lindl., A. grandifolia Lindl., A. pubens Lindl., and others) are currently treated as garden forms of either A. arbutifolia or A. melanocarpa [3]. The only exception is A. floribunda Spach, which is considered either a garden hybrid between A. arbutifolia and A. melanocarpa [6] or distinct species.

Plants appearing intermediate between A. arbutifolia and A. melanocarpa also exist in the wild. These are as well treated either as hybrids [1] or a third species, A. prunifolia (Marsh.) Rehd. [3]. The wild A. prunifolia has been found identical with the cultivated A. floribunda [3], which made it possible to synonymize these names.

Another viewpoint on Aronia [7] is that it includes only a single, extremely variable species: A. arbutifolia.

At the same time, the black-fruited aronia cultivated in this country and considered to be A. melanocarpa, is characterized by extremely low variability. Due to that, any further selective improvement of the cultivated aronia employing the traditional approach (selection—crosses—selection) has no perspective. The only promising method is chemical mutagenesis [10].

As a participant in the Russian-American Expedition of 1976 across the Eastern US, A.K. Skvortsov observed A. melanocarpa in its natural habitats (rocks, open sand, and bogs in Virginia and New York State) and noticed drastic differences between these and those cultivated in Russia. This triggered a domestic study of cultivated aronia headed in four different directions: 1. variability of cultivated aronia; 2. characters distinguishing cultivated aronia from its wild ancestors; 3. history of aronia introduction in this country; 4. flowering and fruiting biology. The following is a report on the results for the former two research directions.

The goals of the survey included:

• assessment of viability and variability of some aronia populations in the center and at the margins of its range in the USSR;
• a study of fruit morphological variability;
• comparisons of morphology, phenology, and growth dynamics of seedlings originating from different geographic areas, grown in uniform soil and climatic conditions; delineation of variability range for certain characters; description of infraspecific polymorphism.

The work was completed in 1976-1980 at the Flora Department of the Central Botanic Garden, USSR Acad. Sci (GBS AN SSSR) in Moscow.

Fruits were collected in 1977 along a latitudinal profile within the secondary range of aronia in [European] Russia, from its northernmost to southernmost limit: in Arkhangelsk, Petrozavodsk, Vologda, Moscow, and Astrakhan.

In order to obtain comparable material, we collected fruits at the same stage of ripening from plants growing in similar situations and having a similar habit (young shrubs up to 1.5 m tall). 10-15 samples of fruit were collected at each geographic destination, each from a certain mother-plant.

Fruits were also obtained from the US (Washington D.C., Virginia, Massachusetts) and from other regions of the USSR: Riga, Jaunkalsnava (Latvia); Girionys (Lithuania); Volgograd, Saransk [European Russia]; Barnaul [southwestern West Siberia]; Sakhalin I. [Far East].

Each fruit sample was represented by a random trial of 50 fruits, each of which was measured fresh (diameter), then dried and weighted (air mass determined) using a torsion scale. Because the amount of fruits received from other botanic gardens through Index Seminum was not large, we had to reduce those trials to 10 fruits. The mean for each sample was calculated upon running three random trials; then the mean for each geographic destination was computed as well as variability range, mean square deviation, and representative error. Student's t-test was employed for assessing the statistical significance of the differences between sample means.

Cultivated material used for seedling comparison was started from seeds obtained in 1977 through own collecting as well as from other botanic gardens. All seeds were stratified in peat moss at 0-2°C for 4 months. The sample plots were started in the Experimental Nursery of the Central Botanical Garden in the spring of 1978. 10 samples, 30-50 g each, were sown for each of the four geographic destinations where the seed had been collected; additional sample plots were started with seed obtained by mail (a single sample plot for each destination). Every sample had two replications sown separately. Soil and light conditions were uniform for all plots. Care for the plots included weeding, tillage, and a one-time thinning of seedlings. Watering was only allowed during exceptionally long drought periods.

Growth rate and dynamics were assessed during the first two vegetation periods. At the end of each growing season height and stem diameter were measured in all seedlings (ca. 50 plants from each of the four major geographic destinations, in two replications). In mid-summer, seedling dry biomass was assessed (30 plants per geographic destination, in two replications).

The studied parameters were measured separately for each sample plot. Because visual observations suggested very little variability among sample plots from a certain geographic destination, the mean value for each character (height, stem diameter, and biomass) was calculated for a geographic destination. For statistical analysis, mean square deviation and representation error were calculated. Student's t-test was used for assessment of differences in respective characters for each pair of geographic destinations.

Morphology of vegetative organs was studied in live plants and herbarium samples representing aronia in its natural range and in cultivation. Herbarium specimens from cultivated plants were obtained from Arkhangelsk, Petrozavodsk, Moscow, Volgograd, and Astrakhan. Collections were studied in the following depositories: Moscow State University; Central Botanical Garden, USSR Acad. Sci.; V.L. Komarov Botanical Institute, USSR Acad. Sci.; N.G. Kholodny Institute of Botany, Ukrainian Acad. Sci.; Tartu University (Estonia); Nikita Botanic Garden. The authors are especially grateful to Dr. Raimond Tsinovskis (Riga), who kindly shared his large herbarium collection.

Size and mass of fruits collected in different regions of the USSR vary insignificantly: a single fruit weighs 115-125 mg (Table 1). This might be attributed not just to the biological attributes of the species, but also to uniformity in cultivation situations, which may level differences in soil and climatic conditions.

Morphological characters of aronia vegetative organs have proved to be constant in different regions of the USSR. Even the leaf shape and size show only insignificant variability (Table 2).

Unfortunately, none of the seeds collected or received via mail from North America germinated. Seeds from across this country germinated all at once, in 10 days upon being sowed.

During their first year, the total biomass of the above-ground parts was measured for plants of various provenances (Table 3). While it was found that the biomass of a single plant can vary within 40-60 mg, mean values for plants of different provenances were practically the same.

In the second-year plants, the biomass of stems and leaves was measured separately. The leaf biomass was observed to be somewhat larger than that of stems. Due to some variability in biomass among different cultivated plots, the Student's Criterion indicated unreliable differences between mean biomass values of different geographic samples.

Comparisons of annual increments among samples of various provenance (Table 3) did not reveal any significant differences.

Variability of the measured parameters turned out to be low (variability coefficient was 10%). These data demonstrate high genotypic stability of characters in aronia across all of its secondary area.

Differences of cultivated aronia from its wild American ancestors

The measurements of fruit diameter and dry mass demonstrated reliable differences between the plants from primary and secondary aronia range. Mass of fruits from plants collected in a number of geographic destinations within the USSR is 2-3 times as large as that of fruits originating from North American plants (see Table 1). Plants grown in the West and Central European botanic gardens also have fruits about 1.5-2 times as light as those of our cultivated aronia. V.I. Protsenko [9] arrived at similar results.

In addition, the following morphological and qualitative differences were observed. In the North American wild-collected plants, fruits are oval or slightly pyriform (pear-shaped), shiny. In the cultivated aronia, they are globular, mostly somewhat depressed (at least at the apex), and always opaque. Fruits from cultivated aronia are more juicy than those wild-collected, hence they shrink much on drying, thus partially alleviating drastic size differences between wild-collected and cultivated specimens observed in fresh material.

Browsing through herbarium collections suggested differences in size and quantity of flowers: North American plants have fewer and smaller flowers than specimens of cultivated aronia. Reliable differences in the shape and size of leaf blades were also revealed (see Table 2). We observed the significant infraspecific variability in North American plants previously described by American researchers. North American herbarium samples were greatly variable in leaf size and shape: from small, round leaves to large, lanceolate ones with attenuate apex.

While North American aronia is only moderately hardy (Zone 4, according to [3]), aronia cultivated in the USSR is extremely hardy, so that it is possible to assign it to Zone 2.

Taking all these facts into account, one cannot identify plants cultivated in the USSR with any of the forms occurring within aronia's natural area. The cultivated aronia is reliably different from wild-collected in a number of quantitative and qualitative characters.

Since the cultivated aronia exhibits distinct differences from its wild ancestors, remains constant in its characteristics, and has acquired a very wide range across Northern Eurasia, it appears to be quite reasonable to describe it as a new species (Fig. 2).

Fig. 2. [Not provided] Herbarium sample of cultivated aronia, A. mitschurinii (USSR, Moscow, Moscow University Botanical Garden. Cultivated. 6 Aug 1973. coll. A. Skvortsov)

Aronia mitschurinii Skvortsov et Maitulina sp. nova.— Frutex sat robustus, ad 3 m (vulgo 1.25-2 m) altus, ramulis, foliis pedicellisque plus minusve cinereo-tomentosis, foliis late ovalibus, corymbo multifloro, baccis sphaericis vel apice ± applanatis, atrofuscis opacis in vivo 9-12 mm in diametro.

Habitat: in hortis numerosis-sintis in URSS culta, specimina silvestria abhuc ignota.

Typus: Mosqua, Hortus botanicus principalis, culta, 10.9.1980 leg. J. Maitulina, MHA.

A specie affini (et verosimiliter parentali), A. melanocarpa (Michx.) Elliott habitu robustiore, pubescentia fere omnium partium, corymbo conspicue majore necnon fructibus multo majoribus opacis manifeste discernitur.

The Central Botanical Garden USSR Academy of Sciences

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Translation I. Kadis
30 December 2011