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

Abstract

Wood formation (xylogenesis) in trees depends on two main factors providing growth processes with assimilates and energy, photosynthesis and respiration. Temperature and precipitation affect photosynthesis and respiration and, consequently, growth processes in wood. The aim of our study was to characterise the relationship of growth processes (cambium activity and biomass deposition) in Pinus sylvestris L. (Scotch pine) trunks with crown photosynthetic activity and trunk respiration in years with contrasting summer-weather conditions. Formation of xylem and phloem cells, accumulation of cell wall biomass, photosynthetic productivity and trunk respiration were studied in P. sylvestris trees growing in Eastern Siberia (Russia). We estimated the number of cells in differentiation zones and morphological parameters of cells produced by cambium, determined cambium activity, accumulation of biomass in tracheid walls and their relationship with crown photosynthetic productivity and stem respiration costs at certain stages of annual ring wood formation. It turned out that cambium cell division towards xylem or phloem depends on the combination of temperature and precipitation in some periods of the season, as well as on the reaction of photosynthesis and respiration to these factors. Biomass accumulation had a bimodal character with maxima in June (early wood development) and predominantly in August (development of thick-walled late tracheids). This corresponded to an optimum combination of air temperature and humidity, providing sufficient assimilate influx and low respiration consumption. We also showed that cambial activity and biomass accumulation in the cell walls of annual wood rings depend on the cumulative effect of temperature and precipitation on photosynthesis and stem respiration during the growing season. Fluctuations in external factors changes the balance between the inflow of photoassimilates and their utilization. As a result, photoassimilates are utilised not only for synthesis of cell wall biomass, but also partially converted into reserve substances, particularly starch. Our study expands the understanding of plant development processes that lead to wood formation under the influence of external factors.