Modification of Architecture of Dwarf Subshrub Thymus seravschanicus (Lamiaceae) in the Republic of Tajikistan

Abstract

This work describes a life-form of dwarf subshrub Thymus seravshanicus using the architectural approach, which is one of the modern methods for the in-depth study of the plant structure. The species is widely distributed across various ecotopes in the Republic of Tajikistan (Gissar, Zarafshan, and Khazretishi (Hazrati-Shokh) ranges). For the first time, in the structure of individual plants, this study identifies and characterizes an architectural unit (AU) composed of the main compound skeletal axis (n order), compound skeletal axes of the 1st (n + 1) order, branching shoots, and ephemeral shoots. Depending on the position of the main (n order) compound skeletal axis in space, the AU exhibits diversity, including orthotropic, ascending, and mixed types; features of their development have been described. It is shown that, under different ecological and cenotic habitat conditions, the structure of individuals is formed by the multiple repetition of concurrently three or one AUs. In forb steppe (deciduous “black forest” belt) and forb meadow (subalpine belt) communities, the structure of individuals is formed by the combination and multiple repetition of all AUs. On sites with no free substrate available, the development of individuals starts with the formation of an orthotropic AU and a dense shrub is formed; in areas of unoccupied substrate, the development of individuals begins with the formation of an ascending AU and a clump is formed. In the fescue steppe community (juniper belt), the structure of T. seravshanicus individuals is composed only of an ascending AU; a clone is formed. The polyvariance of shoot development is found to be manifested through (1) the intensive branching common for individuals in the conditions of the black forest belt, and the duration of the continued monopodial growth of shoots is up to 3 years; (2) rosette shoots in the conditions of the subalpine belt, and their monopodial growth continues for up to 6 years; and (3) a lack of occurrence of short metamers in the structure of shoots in the conditions of the juniper belt, and the duration of monopodial growth is up to 2 years. The features that were identified are mechanisms of adaptation of T. seravshanicus to growing conditions and do not lead to changes in the life-form of the dwarf subshrub. These findings expand the understanding of the dwarf subshrub structure and their morphological plasticity and provide a basis for modeling the ways dwarf subshrubs morphologically transform of in different habitats.