Nitrogen-fixing nonlegumes in Egypt. I. Nodulation and N2(C2H2) fixation by Casuarina equisetifolia

This study examines nodulation and some aspects of N2 fixation by Casuarina equisetifolia growing in Egypt. Plants examined were 2 to 10 years of age, and all were nodulated. Nodules varied from young meristematic lobes on lateral roots to clusters occasionally reaching 10 cm in diameter; they had the characteristic features of the Myrica/Casuarina type. Nodules became brown, woody and lost their N2-fixing activities in the late summer months. Acetylene reduction was used to assay nodule activity. Maximum rates of N2(C2H2) fixation were observed at 35 degrees C. Acetylene reduction by excised nodules was linear for 6 hours, then slowly declined and finally ceased after 10 hours. Variations in N2(C2H2) fixation rates due to season and time of day were also examined. Active fixation was recorded throughout different seasons of the year, except in the late summer months. Diurnal fluctuations showed that N2(C2H2) fixation was higher at night than during the day or afternoon. The results are discussed and compared to the findings of other investigators.