Nitrate and Carbohydrate Effects on Nodulation and Nitrogen Fixation (Acetylene Reduction) Activity of Lentil (Lens...

Lentils (Lens esculenta Moench, cv. Tekoas) grown in a nutrient solution containing 15 millimolar nitrate had 84% fewer nodules than lentils grown in nitrate-free nutrient solution. Nodules from the nitrate-grown plants weighed 71% less than nodules from the nitrate-free plants. Nitrate-grown plants also fixed much less nitrogen (measured by acetylene reduction) than the nitrate-free plants. When lentils were grown in a solution containing 15 millimolar nitrate and 75 millimolar fructose, glucose, or sucrose, however, the nitrogen fixation activity of their nodules was similar to that of nodules from nitrate-free plants. Leaves of lentils grown in the nitrate-sugar solutions had only about 7% as much nitrate reductase activity and accumulated only 10% as much nitrate as leaves from lentils grown in the nitrate solution alone. Roots of lentils grown in the nitrate-sugar solutions had similar nitrate reductase activity but accumulated only 17 to 25% as much nitrate as roots from lentils grown in the nitrate solution. The results indicate that the added sugars alleviated the inhibitory effects of nitrate on symbiotic nitrogen fixation not only by increasing the carbohydrate supply so lentils could support both nitrogen fixation and nitrate reduction but also by inhibiting the accumulation of nitrate and, hence, lowering nitrate reductase activity in the leaves.