IMPACT OF ELEVATED ATMOSPHERIC CO2 ON NITRATE REDUCTASE TRANSCRIPTIONAND ACTIVITY IN LEAVES AND ROOTS OF PLANTAGO-MAJOR

Vegetative plants of an inbred line, A4, of Plantage major ssp. pleios perma (L.) Pilger were grown at 350 mu l 1(-1) or at elevated (700 mu l l(-1)) CO2 in non-limiting nutrient solution with nitrate. Both the relative growth rate (RGR) and the root to total plant weight ratio (R WR) were increased by elevated CO2. However, the stimulation of both R GR and RWR was transient and did not last longer than 8 days. To inves tigate the physiological mechanisms involved in this stimulation, rela ted changes in C/N metabolism were examined. In the roots soluble suga r concentration increased during the transient period of RGR stimulati on (up to 23%), as did the root respiration rate. Changes in nitrogen metabolism were also restricted to this period and consisted of an inc rease in (1) in vivo and in vitro root nitrate reductase (EC 1.6.6.1) activity, (2) in vitro leaf nitrate reductase activity, (3) leaf and r oot nitrate reductase mRNA and (4) reduced nitrogen concentration in t he roots. The elevated CO2-related signal for the increase in nitrate reductase transcript levels in the roots is discussed in terms of the increased availability of soluble sugars. The results suggest that the short-term enhancement of root carbon and nitrogen metabolism may be responsible for the transient effect of elevated CO, on whole plant RG R.