S ummary The fixation of 15 N 2 and its transport were studied as a function of leaf development along the main stem axes of Azolla. Main stem axes dissected from Azolla caroliniana Willd. plants, and sequential groups of leaves along the axes, had 25 to 35 % less nitrogenase activity than the undissected controls when assayed immediately. However, they exhibited activities equivalent to those of the intact controls when allowed to recover for 12 h or more prior to assaying. Nitrogenase activity was determined as a function of leaf age by measuring 15 N 2 fixation and C 2 H 2 reduction in parallel samples. The reduction of both substrates was very low in the apical region, increased rapidly with leaf maturation and plateaued prior to declining as leaves began to senesce. Whereas the nitrogen content and dry matter decreased with increasing leaf age, the C:N ratio increased. These findings are consistent with the demonstration that nitrogen fixed by the endophytic Anabaena in mature leaves is transported toward the stem tip where undifferentiated but actively dividing filaments of the endophyte are associated with the apex and leaf primordia.
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