The Sunflower (Helianthus) genus belongs to the Asteraceae family (Asteraceae) and includes more than 100 species of annual and perennial herbaceous plants. Cultivated sunflower is the most famous representative of the Sunflower genus. The south-western part of North America is considered the birthplace of sunflower, where wild species are growing now. The sunflower is rich in a variety of organic and mineral substances. The content of these substances depends on the organ of the plant. Wild sunflower species, both annual and perennial, have long been involved in various breeding programs, being sources of abiotic and biotic resistance genes. The presence of the pool of useful genes in the wild species is obviously explained by the severity of their natural habitat, which has concentrated various complexes of adaptive traits in them. Wild species of sunflower can be not only sources of valuable genes for cultivated sunflower, but also have their own interest because of the high content of substances useful to humans in their organs. It is known about the high content of such biologically active substance as inulin in Helianthus tuberosus L. This wild sunflower species is considered one of the main sources of inulin among higher plants. However, data on the content of this reserve substance in the underground organs of other species of perennial sunflower are not known in the scientific literature. The aim of the work was to study the morphological and biochemical characteristics of underground organs of 6 species of perennial sunflower, including the content of such a reserve substance as inulin. As the study material the species of perennial sunflower Helianthus maximiliani, H. mollis, H. nuttallii, H. rigidus, H. salicifolius, H. tuberosus, growing on the experimental plot of the department of landscape industry and genetics of the Biological faculty of Zaporozhye National University, were used. Underground organs of these species were collected at the end of October 2017. To determine the content of organic matter, ashing was carried. The content of inulin was determined by the difference between the sum of monosaccharides and free monosaccharides determined according to Bertrand with modifications. Repetition in both experiments is threefold. We investigated the morphological features of the underground part of the source material. In the plants of H. salicifolius, the underground part was represented by a woody rhizome. There was a thick strong main root. The root system had a large number of secondary roots. Part of the lateral roots was initially located almost parallel to the soil surface, and at a distance from the main root, it deepened and went perpendicular to the soil surface, giving branches from the following orders. H. mollis plants had a branched rhizome with a small number of secondary roots. Almost all lateral roots were located parallel to the soil surface. In H. rigidus plants, the underground part was represented by a branched rhizome with thick roots. There was a large number of lateral roots. Part of the lateral roots spread in the soil layer with a bend inward, branching and forming a dense grid of small roots. H. nuttallii plants had a branchy lignified rhizome with a small number of lateral roots. There was a thick and strong main root. Almost all lateral roots were perpendicular to the soil surface. H. maximiliani plants had a woody rhizome with a thick and strong main root. The root system had a large number of secondary roots. Part of the lateral roots spread in the soil layer, forming a dense grid of small roots. H. tuberosus plants were characterized by a branched rhizome. The root system is powerful, the roots penetrate deep into the ground. The main part of the underground stems (stolons) is located at a depth of 20-25 cm, where numerous tubers with bulging buds are formed. It was established that the underground part of Helianthus salicifolius, H. maximiliani and H. tuberosus had the greatest mass. The underground part of H. mollis and H. nuttallii was 3-4 times smaller in mass compared to the above-mentioned species. It was established that the species differed significantly in the content of organic substances in the underground organs. The largest proportion of organic matter was found in the underground organs of H. tuberosus species plants (77.50%). H. nuttallii and H. rigidus had 10–15% less organic matter in their underground cells than H. tuberosus. The smallest amount of these substances was contained in H. mollis, H. maximiliani and H. salicifolius. In their underground organs, organic matter was almost a third less than that of H. tuberosus. The biochemical studies performed showed that the largest number of monosaccharides was found in the underground organs of H. tuberosus and H. nuttallii, and a little less in H. rigidus. H. salicifolius, H. maximiliani and H. mollis had almost half of them. Each species was also characterized by a different proportion of free sugars. Most of them were in H. rigidus. H. mollis, H. nuttallii and H. salicifolius had a high proportion of free monosaccharides. And the smallest number of free monosaccharides was found in H. tuberosus and H. maximiliani, which was 6-7 times lower than H. rigidus. Based on the difference between all monosaccharides detected and free monosaccharides, the largest amount of inulin (about 9%) was found, as expected, in H. tuberosus. Close in terms of the amount of inulin to H. tuberosus was the wild species H. nuttallii (about 8%). H. rigidus had significantly less (no more than 6%) of this reserve substance. Wild species of H. salicifolius, H. maximiliani and H. mollis in their underground organs contained only 3-4% of inulin.
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