Growth of Siberian Fir Needles and Their Structural Features in the Northeast of European Russia

Siberian fir (Abies sibirica Ledeb.), the subject of this research, is the least studied forest-forming species in the northeast of European Russia. The purpose of the work is to study changes in the structure of fir needles in the process of its growth. The paper is based on a growth model and examines the relationship between the linear dimensions of Siberian fir needles and changes in the anatomical structure and ultrastructure of its assimilation apparatus during the period of its growth outside the bud. A logistic model was successfully tested as a model of changes in linear dimensions of the fir needles (length, width, and thickness) and proved to be a convenient tool for analyzing the processes of needle development at all structural levels. The model makes it possible to identify the phases of needle development and confirm the synchronism in changes in the linear dimensions of needles with the development of assimilation cells. It has been shown that, when the needles of Siberian fir emerge from the bud, they are already qualitatively capable of photosynthesis, as is evidenced by the presence of developed thylakoids and grana in chloroplasts, as well as photosynthetic pigments. The assimilation apparatus reaches its highest qualitative and quantitative development after the end of the last phase of needle growth in length, which gives us grounds to conclude that, under optimal environmental conditions, already developed needles reach the highest functional activity during this period.