CO2 Exchange and Mesophyll Structure in Second-Year Needles of Abies sibirica Ledeb.

Seasonal changes in the net photosynthesis and dark respiration in second-year needles of Siberian fir were investigated. The functional state of needles was shown to determine the structural features of mesophyll cells. Current-year shoot growth is one of the major factors affecting the structure and functional activity of second-year needles during the growing period. The greatest changes occur in the number of mitochondria in mesophyll cells and the respiration rate of second-year needles. The rate of photosynthesis in second-year needles was shown to depend on the number of thylakoids in mesophyll chloroplasts.