Humidity and light affect the growth, development and nitrogenase activity of stem nodules of Sesbania rostrata Brem.

The growth and nitrogenase activity of stem nodules of Sesbania rostrata Brem were strongly influenced by the humidity surrounding the nodule. Nodules developed under high humidity showed the fastest rates of growth and the highest specific nitrogenase activity (determined by acetylene reduction). Development of stem nodules in both the light and dark was similar and showed a similar lowering of growth and activity with decreasing humidity. Stem nodules developed in the dark were pale and unlike the dark green nodules that developed in the light, they did not have chloroplasts in the nodule cortex. Adventitious root development required darkness and relative humidities approaching 100%. In stem nodules that were grown at high humidity and subsequently exposed to lower humidities, there was only a small loss of nitrogenase activity, and high rates of acetylene reduction were maintained for 6 d after treatments were begun. The area of nodule attachment to the stem or root was measured in nodules of many sizes and found to be consistently greater in stem nodules. This may have implications for the comparative water balance of nodules on these two organs.