CO2 EXCHANGE AND GROWTH OF RHAPONTICUM CARTHAMOIDES UNDER THE CONDITIONS OF THE MIDDLE TAIGA SUBZONE OF NORTHEASTERN EUROPE .2. PHOTOSYNTHESIS RESPIRATION RATIO AS AN INDEX OF THE PRODUCTIVITY AND ADAPTIVE RESPONSES OF PLANTS/

The ratio of respiration (R) to gross photosynthesis (P-g) in Rhaponti cum carthamoides plants of various ages was studied by measuring CO2 e xchange. The effects of irradiance, day length, and nitrogen supply on the components of the dry matter balance and R/P-g ratio were examine d in two-year-old plants. P value was calculated by adding respiratory losses to the amount of substrate assimilated per day. When growth ra tes of one-to four-year-old plants were at their highest, respiratory losses comprised about 45% of P-g. Plant adaptive responses to environ mental factors were aimed at the maintenance of a constant R/P-g ratio , In moderately shaded plants, this was attained by retaining the prod uctivity of net photosynthesis and decreasing respiration rates, where as, in severely shaded plants or at a shortened day length, this was a ttained by decreasing plant weight. For plant productivity, the optimu m nitrogen content in plant biomass was 25-30 mg/g. At higher nitrogen contents, the R/P-g ratio increased, and the net assimilation rate an d relative growth rate decreased. It is concluded that Rhaponticum car thamoides plants can adapt to light conditions to maintain maximum eff iciency of growth; however, they are not adapted to high levels of nit rogen supply. It is shown that daily photosynthetic production can be calculated from the net photosynthetic rate at the adaptive radiation intensity.