Decrease in the numbers of cyanobionts and heterocysts and nitrogen-fixing activity in Azolla leaves upon transfer to low light intensity

The numbers of cyanobionts and heterocysts in successive leaves from the apex along the axis were larger in the Azolla plants grown at a high light intensity than in those grown at a low light intensity. When the plants grown at a high light intensity were transferred to a low light intensity, the numbers of cyanobionts and heterocysts in the mature region gradually decreased. On the other hand, the nitrogenase activity per leaf sharply decreased immediately after the transfer of the plants to a low light intensity and then remained at a low level during the examination. Nitrogenase activity per heterocyst also decreased immediately after the transfer of the plants to a low light intensity. However, the activity gradually increased to the level of the plants grown at a high light intensity. These results suggest that the nitrogenase activity of Azolla changes in two ways. The low nitrogen-fixing activity in each leaf immediately after the transfer and after acclimatization was due to a low nitrogenase activity in each heterocyst and to the small number of heterocysts, respectively.