CO2 GAS-EXCHANGE AND GROWTH IN RHAPONTICUM CARTHAMOIDES UNDER THE CONDITIONS OF MIDDLE TAIGA SUBZONE OF NORTHEASTERN EUROPE .1. DEPENDENCE OF PHOTOSYNTHESIS AND RESPIRATION ON ENVIRONMENTAL-FACTORS

Light and temperature dependences of CO2 gas exchange and responses of photosynthesis and respiration to changes in irradiance, day length, and nitrogen supply were studied in the leaves of a rare medicinal pla nt Rhaponticum carthamoides (Willd.) Iljin The temperature dependence of the net photosynthesis (P-n) in the leaves had a wide optimum zone (8-28 degrees C). Photosynthetic rate P-n was saturated at photosynthe tically active radiation (PAR) intensities above 300 W/m(2), attaining a level of 16-18 mg CO2/(dm(2) h). The light compensation point of ph otosynthesis varied in the range 9-13 W/m(2) PAR. The intensity of ada ptive radiation was 40- 60 W/m(2). After prolonged shading, plants mai ntained P-n rates, measured at both low and high light intensities, at the levels close to those of the control plants, due to an increase i n chlorophyll concentration and decrease in chlorophyll alb ratio, res pectively. The increase of the initial slope of the light curve of pho tosynthesis indicated that the shaded plants used light energy more ef ficiently in photosynthesis. The decrease in PAR intensity and reducti on of assimilate formation led to a lower respiration rate in plant or gans. An increased nitrogen supply resulted in enhanced respiration of plant organs but did not affect P,rate in the leaves. From the gas-ex change evidence, Rhaponticum carthamoides can be classified as a light -requiring plant, with well-pronounced shade endurance and flexible me tabolism. The data are discussed with respect to plant adaptation to t he environment.