The structure and development of Siberian stone pine (Pinus sibirica Du Tour) annual shoot

In this review, we analyzed our own and literary information on Siberian stone pine (Pinus sibirica Du Tour) male and female annual shoot structure in connection with adaptive significance of various morphological elements. The material was collected in two so called village-side (thinned) Siberian stone pine forests: the first (100-110 years old) was located in the lower part of the forest belt of the North-Eastern Altai, the second (160-170 years old) was located in the southern taiga subzone of the Western Siberian plain (the South-East of Tomsk region). We studied 10 co-dominant trees from each of the forests. We took 5 branches of the upper part of the crown from each tree. The morphological structure of the last 10 annual shoots was registered, i.e. we fixed the number and sequence of different stem unit types. Male shoots produce only one (winter) bud and one (spring) elementary shoot during the growing season. At the beginning of the season, a temporal bud is initiated on female shoots, giving rise to the summer shoot which is formed during the season without any period of bud dormancy. Then a winter bud is formed producing the first (spring) elementary shoot in the beginning of the next season. Spring and summer shoots significantly differed in structure and functions. Cone bearing occurs solely on spring shoots. Elongation and photosynthesis occur basically due to a spring shoot. Branching is the main function of a summer shoot. Only regenerative function (acted by kryptoblasts (dormant buds)) is divided into roughly equal parts between two elementary shoots. The presence of a summer shoot automatically provides a 2-3-fold increase in the lateral long shoot number. It causes very characteristic for the Siberian stone pine and highly expedient differentiation of the branches in two qualitatively different types. Branches mainly consisting of bicyclical annual shoots become powerful (skeletal) whereas all other branches remain weak ("overgrown"). The formation of a summer shoot in the distal part of the annual shoot of the female type causes an abrupt increase in its already pronounced acrotony. In small (male) shoots, the acrotony is hardly expressed: the smallest and 'weakest' axial structures (latent buds) are situated on their distal pole. We analyzed internode length variability on a compound (bicyclical) annual shoot in this work for the first time. The minimal internode length occurred not at annual shoot poles but in the transition zone between spring and summer shoots. Therefore, the elementary shoot formation cycles do not influence internode elongation, which exclusively depends on annual shoot formation cycle. Thus, the formation of axial and lateral structures of an annual shoot occurs in fundamentally different ways. The size and the structure of a mature shoot are mostly determined during the embryonic stage of its development. Basing on these results, before the beginning of shoot elongation, not only the number and relative position of metamers of different types but also all relations and all internal correlations, including those characterizing the growth and formation of elementary branching systems, are predetermined. Funding: The work was carried out due to basic budgetary funds (Federal Agency for Scientific Organizations) with the support of the Russian Foundation for Basic Research (project № 15-04-03924).

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