The Relationship Between Photosynthesis and Nitrogen Fixation

Abstract

Surface-sterilized seeds of Trifollum pratense were germinated on moist filter paper in sterile Petri dishes, the seedlings were transferred to tubes of sterile quartz sand and watered with a N free nutrient soln. and the exptl. plants were inoculated with Rhizobium trifolii 200 (an uninoculated lot serving as controls). After 4-6 weeks the exptl. plants were darkened or the tops were removed at ground level. Following various periods of predarkening the tubes of exptl. plants and the illuminated inoculated and uninoculated controls were placed on a gas manifold and gassed with excess N215. After 1-3 days'' incubation at 25[degree]C in darkness for the exptl. plants and in light for the controls, the plants or roots were analyzed for Nl5 content. In other tests for the effect of light on the uptake and incorporation of N independent of fixation, (N15H4)2SO4 was added to tubes of predarkened plants. The NH4+ uptake was detd. by obtaining the N15 content of the plants without further treatment; the NH4+ incorporation was detd. by hydrolyzing the plants, removing the ammonia and amide N by steam distillation, and analyzing the residue for N15. It was found that N fixation could take place in the dark, but at decreasing rates as the period of predarkening was increased. The incorporation of externally applied N15H4+ into protein N was less dependent on light and the uptake of N15H4+ even less light-dependent. Plants decapitated before exposure toN215 were also capable of fixing N. Cultures of the photosynthetic bacterium Rhodospirillum rubrum were grown anaerobically in light at 25[degree]C for 4-6 days on a low-N medium, and then incubated in both the light and darkness at 30[degree]C for 3-5 days on a medium containing negligible amts. of N and in atmospheres containing N215; the bacteria fixed N aerobically in the dark at a rate of about 1% that in the light. It is suggested that the interdependence between photosynthesis and N fixation in both T. pratense and R. rubrum may indicate that an intermediate or product of photosynthesis is necessary for N fixation.