Xylogenesis, Photosynthesis and Respiration in Scots Pine Trees Growing in Eastern Siberia (Russia)

Abstract

Wood formation (xylogenesis) in trees depends on photosynthesis and respiration. Temperature and precipitation affect photosynthesis and respiration and accordingly growth processes in a tree. We studied xylem and phloem cell formation, cell wall biomass accumulation, photosynthesis productivity, and trunk respiration in Scots pine trees growing in eastern Siberia (Russia) in the years with contrasting summer-weather conditions. The number of cells in the differentiation zones and the morphological parameters of the cells produced by the cambium were determined on samples taken mainly after 10 days of the growing season from the trunks of 10 trees. The activity of cambium and the accumulation of cell wall biomass at individual stages of tree ring wood formation and their relationship to the photosynthetic productivity of the crown and the cost of stem respiration were assessed. The division of cambial cells into xylem or phloem sides depended on the combination of temperature/precipitation in separate periods of the season and on reactions of photosynthesis and respiration to these factors. Biomass accumulation was bimodal with maxima in June (development of early wood) and predominantly in August (development of thick-walled late tracheids). This was due to the optimal combination of air temperature and moisture, which provided a sufficient influx of assimilates and their low consumption by respiration. It is shown that cambial activity and accumulation of biomass in the cell walls of Scots pine annual wood rings depend on the cumulative effect of temperature and precipitation on photosynthesis and stem respiration throughout the growing season. Fluctuations in external factors changed the balance between the inflow of photoassimilates and their utilization. As a result, photoassimilates were used not only for the synthesis of cell wall biomass, but were also partly converted to reserve substances, in particular, into starch. Our study expands understanding of the internal processes that lead to the formation of wood under the influence of external factors.