SHORT-TERM EFFECTS OF HIGH LIGHT INTENSITIES ON SOYBEAN NODULE ACTIVITY AND PHOTOSYNTHESIS

There is little information available on the effects of highlight inte nsity (HLI) on nitrogenase activity in legume nodules. Inhibitory as w ell as stimulatory effects have been described. The hypothesis that an increase in carbohydrate production is involved in these effects was tested by comparing the effects of high light, high CO2, or low O-2 ex posure of the shoot. The HLI treatment consisted of tripling the light intensity to 1200 mu E m(-2) s(-1), compared with the growth intensit y of 400 mu E m(-2) s(-1). Acetylene reduction activity (ARA) measurin g nitrogenase activity was studied in relation to shoot CO:! exchange. HLI stimulated ARA. The stimulation was progressive and reached 17% a fter 10 hr of treatment. Photosynthesis (P) was initially doubled, but photoinhibition appeared after about 8 hr of HLI. Under HLI, P became limited by N fixation. Other treatments increasing photosynthesis wer e compared with HLI: elevating the ambient CO2 concentration around th e shoot to 900 ppm, or lowering the O-2 concentration to 2%, increased photosynthesis, respectively, by 55% and 70% without effect on ARA. I t is concluded that ARA was not regulated by the availability of carbo n assimilates and that specific factors promoting or inhibiting ARA ar e produced by leaves under HLI.