Permalink : http://hdl.handle.net/2115/47117

KEYWORDS : photosynthesis;transpiration;moisture of needles;stoma;water deficiency

The paper discusses variations in the photosynthesis and water regime for Pinus sylvestris L. under extreme conditions of the forest-steppe zone of the Irkutsk Region. The daily cycle of Pinus sylvestris L. is characterized by variations in net-photosynthesis, which reaches a maximum in the early hours, with a simultaneous decrease of gas-exchange intensity and transpiration. This is evidence of the substantial role the stomata plays in the regulation of these processes. The air temperature (r = -0.65±0.05, n = 12) is the principal negative factor for carbon dioxide gas exchange. The intensity of transpiration is determined by the level of solar radiation coming into the biocenosis. Insufficient soil moisture, and absolute air humidity (r = 0.70±0.04, n = 12) play a very important role in the processes of gas exchange and transpiration, together with such factors as cool cloudy weather. Therefore, with the solar radiation absorbed by the needles and the elevated air temperature, we observed not only an increase in air humidity in the sub-stomata cavities, but also an increase in the concentration of carbon dioxide in the mezophyll of the needles. Variations in the temperature of the needles provoke an increase of transpiration and the decrease in the intensity of gas exchange. In this case, gas exchange is negatively impacted by the re-assimilation of carbon dioxide discharged in the process of respiration. This dependence is seen from the graphics characterizing the dependences of gas exchange and transpiration upon variations of solar radiation and temperatures. Hence, under extreme conditions, the stomatal mechanism acquires a dominant importance and performs an adaptive function.