Polyvariance of Thymus mugodzharicus (Lamiaceae) ontogeny of the cushion-like life form

The study of morphological and biological features of the cushion plant development makes it possible to explain their adaptation to living in specific habitat conditions. The cushion biomorph of Thymus L. genus. is rarely formed and has not been practically described in the literature. This research is devoted to studying the cushion-like life form of Thymus mugodzharicus. The species is distributed generally in the territory of Kazakhstan, it also occurs in the Southern Urals and in the Northern part of Russia. Its habitats are confined to petrophytic steppe communities located along mountain slopes. The aim of the research was to study the polyvariance of T. mugodzharicus ontogeny of the cushion-like life biomorph to detect the mechanisms of its adaptation to growth conditions. We collected the material in Eastern Kazakhstan, in the spurs of the southern slope of the Narym Range (48°51'12,8"N, 083°51'22,0''E, height 998 m above sea level), in shrubby forb feather grass steppe. The species of T. mugodzharicus was located on the stone-free substrate in the crevices of stones and in microdepressions of the relief. When studying the the cushion-like life form of T. mugodzharicus we relied on IG Serebriakov's classification of life forms (1962). T. mugodzharicus shoots and their systems were described on the basis of the original data on the structure of Thymus shoots obtained by EB Kolegova and VA Cheryomushkina (2012); terminology of IG Serebriakov (1959), MT Mazurenko, AP Khokhryakov (1977), EL Nukhimovskii (1997), and YuA Bobrov (2009). We studied the ontogeny of individuals according to the concept of discrete description proposed by TA Rabotnov (1950). The calendar age of individuals of different ontogenetic states was determined by annual rings. The following biometric characteristics were taken into account in individuals in different ontogenetic states: the number and length of vegetative and generative shoots and plagiotropic ramposite (sympodial) bough axes, and the diameter of the plant. We found out that the main causes that inhibit the growth of shoots and, as a consequence, the formation of the cushion-like life form of T. mugodzharicus are ecological habitat conditions: dry winter, parching effect of the wind, hot summer and water collection in the crevices of stones and microdepressions of the relief. We established that the basis of the T. mugodzharicus cushion consists of two variants of long-term composite bough axes (according to MT Mazurenko and AP Khokhryakov 1977) (CBA): orthotropic and plagiotropic. They are distinguished by the direction of growth, the number of shoots and the duration of growth. Due to weak branching of the shoots and the lengthening of the annual growths of the CBA, the cushion structure is crumbly. T. mugodzharicus cushion is characterized by the absence of peaty mass inside the shrub and buds in the shoot basal part, crumbly arrangement of shoots, apical growth suppression, equal annual shoot growth, smooth surface and regular acrotonic branching, as well as single number of additional roots. The nature of cushion shoot growth and death corresponds to the development of shrub shoots. At the initial stages, the species ontogeny is characterized by a rapid change in ontogenetic states. Thus, the duration of the regenerative period is not more than two years, the species primary shoot forms already in the second year, and generative shoots appear in the third. In the generative period, the species undergoes a change in the axes growth and CBA differentiation. Depending on the conditions of growth (in the crevices of stones or in the microdepressions of the relief), the polyvariance of the cushion development in the old generative ontogenetic state and its different forms have been revealed. The article contains 3 Figures, 1 Table and 21 References.

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