The physiology of spore-negative and spore-positive nodules of Myrica gale

The physiology of spore-negative and spore-positive root nodules was investigated in Myrica gale L. grown in water culture in a growth chamber. Spore (—) nodules were induced with Frankia cultures and spore (+) nodules with crushed nodules. Gas exchange was measured in a flow-through system. The time course of acetylene reduction following addition of acetylene was essentially the same in both spore (—) and spore (+) nodules with a stable maximum between 2 and 4 minutes followed by a steep decline to a minimum (37% of the maximum) between 9 and 30 minutes depending on the plant. The minimum was followed by a partial recovery. Nodule CO₂ evolution showed a similar pattern but the minimum rate (83% of the maximum) was not nearly as low. Plants nodulated with one spore (—) and one spore (+) strain were compared at 6, 8 and 10 weeks after inoculation. At 6 weeks the spore (—) plants had 52% greater specific nitrogenase activity and 46% more biomass than the spore (+) plants. At 8 and 10 weeks, however, the differences between plants with spore (—) and spore (+) nodules became smaller. Plants nodulated with 4 spore (—) and 5 spore (+) strains were compared at 8 weeks after inoculation. Collectively the spore (—) plants exhibited a 32% greater specific nitrogenase activity, a 15% lower energy cost of nitrogenase activity (CO₂/C₂H₄), and invested 31 % less biomass in nodules than the spore (+) plants. The spore (—) plants also produced 16% more biomass indicating that spore (—) strains are generally more desirable than spore (+) strains. However, two spore (+) strains were as effective as the spore (—) strains.