Peculiarities of accumulation of glucofructans in Allium L. (Amaryllidaceae) species

Fructose-containing carbohydrates (glucofructans) are now intensively studied, which is associated with the appearance of new data on their protective role in the plant organism and their pharmacological properties. These compounds are present in a number of ornamental plants, cereals, and vegetables, including onions. Various types of pharmacological action of onions, which can be associated with the presence of fructans, are revealed. The aim of this work was to study the peculiarities of accumulating glucofructans in 8 Allium L. species, introduced in the Botanic Garden of the Ural Branch of the Russian Academy of Sciences (Yekaterinburg). We studied species of onions belonging to the following morphological groups: bulbous - A. aflatunense B.Fedtsch. and A. caeruleum Pall.; bulbous-rhizomatous with a weakly expressed rhizome and large bulbs (2-4 cm and more) - A. altaicum Pall., A. obliguum L., and A. strictum Schrad.; bulbous-rhizomatous with a well-pronounced rhizome and small bulbs (up to 1.5-2 cm) - A. ledebourianum Schult. & Schult. f., A. nutans L., and A. ramosum L. The material for analysis was collected in 2016-2017, in conditions of culture in the open ground, from plants that were in a generative state. For each specific specimen, an average sample of 5-10 individuals was taken. Freshly harvested bulbs and rhizomes were ground to particles of 7-10 millimeters in size and were kept in a drying oven at a temperature of 100°C for 30 minutes, for inactivation of enzymes; then, the raw material was dried at a temperature of 60°C until air-dry state. Immediately prior to the analysis, the raw material was further milled and a fraction was selected with particle sizes of 0.5-1 mm. During the analysis the low-molecular and high-molecular fractions of glucofructans were isolated. The low molecular fraction was obtained by three-fold extraction of the dry plant material with 95% ethanol at 80°C, evaporated to dryness, and the precipitate dissolved in distilled water. From the plant residue, a high molecular fraction was extracted with water (three times for 60 minutes, in a boiling water bath), hydrolysis with concentrated hydrochloric acid was carried out for 8 minutes at a temperature of 100°C. The fructose content was determined spectrophotometrically using a reaction with resorcinol. The concentration of low- and high-molecular glucofructans was calculated as a percentage of air-dry raw materials. For each sample of raw materials, the ratio of concentrations of high molecular and low molecular glucofructans (GF_H/GF_L) was calculated. Differences in the content of glucofructans (low-molecular and high-molecular) between samples of species belonging to different life forms were estimated using the method of one-way analysis of variance and using the nonparametric Kruskel-Wallis criterion. In the course of work, we revealed the relationship between the content of glucofructans and the features of the morphology of the underground organs. The highest content of high molecular glucofructans was found in A. aflatunense and A. caeruleum, belonging to the group of bulbous species: from 22.22 to 41.40% (See Table 1). A high concentration of GF_H was also found in raw material of A. obliquum that belongs to the group of onion-rhizome plants with well-developed bulbs. These species have a high ratio of concentrations of high molecular and low molecular glucofructans (GF_H/GF_L). Somewhat lower than on bulbous species, but still high enough (16.87-27.71%) was the content of high molecular glucofructans in the raw materials of two other species from the group of onion-rhizome plants with well-developed bulbs - A. altaicum and A. strictum, as well as A. ramosum that belongs to the group of onion-rhizome plants with weakly expressed bulbs. In A. strictum and A. ramosum, the GF_H/GF_L ratio is high - 3.94-8.15; in A. altaicum it is slightly lower: 1.26 in bulbs and 1.62 in the rhizomes. The lowest content of glucofructans and the value of GF_H/GF_L were in species from the group of onion-rhizome plants with small bulbs - A. ledebourianum and especially A. nutans, in which high molecular glucofructans are almost completely absent. Univariate analysis of variance revealed statistically significant differences between species of Alliun of bulbous life form (in which the ability to accumulate high molecular glucofructans is the highest) and species of bulbous-rhizome form with a powerful rhizome and weakly expressed bulbs, with a reduced ability to synthesis of these compounds (p = 0.01004, by the criterion of Unequal N HSD). There were no reliable differences between species of different life forms by the content of low molecular weight glucofructans, and also by the ratio GF_H/GF_L. Differences in the accumulation of glucofructans in different organs of the studied species are revealed. In A. altaicum and A. obliquum, belonging to the group of bulbous-rhizome species with well-developed bulbs, the content of high-molecular glucofructans in bulbs was almost 1.5 times higher than in rhizomes, and the content of low-molecular glucofructans was two or more times higher (See Table 1). But in A. ramosum, which has weakly expressed bulbs, the picture is inverse: the content of high-molecular glucofructans in bulbs is about 1.3 times less than in rhizomes; the differences in the content of low molecular glucofructans in bulbs and rhizomes are insignificant. The accumulation of high molecular glucofructans significantly changed during the growing season (See Tables 2 and 3). During the period of spring regrowth, their content is very low; the maximum accumulation was noted during flowering and fruiting, and by the end of the growing season it decreased. The content of oligofructans in different species reached a maximum in various phenophases, which is related to the specific features of seasonal development of the studied species.

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