FEIGENBAUM, S., Büntehof Agricultural Research Station, Hannover, 3000, Germany
MENGEL, K., Büntehof Agricultural Research Station, Hannover, 3000, Germany

The effect of low light intensity and suboptimal potassium supply on the fixation of molecular nitrogen by root nodules and the nitrogen turnover in the host plant was studied in Medicago sativa using 15N labelled molecular nitrogen. For the application of 15N2 labelled gas a special box was used. Both low light intensity and a low potassium supply resulted in a substantial growth depression. In particular the protein content of tops, roots and nodules was lower in the plants of the low light intensity treatment as compared with the control plants. Decreasing potassium supply had a similar but less‐pronounced effect on protein content. The low protein content was not a consequence of a lack of soluble amino nitrogen or NH3, since these fractions were influenced to a lesser degree by the reduced light intensity and by the low potassium supply. This observation is supported by the data obtained with 15N. N2 fixation and NH3 assimilation were affected by both low light intensity and low K application to the same degree as the overall metabolism, thus showing no particular response to the treatments applied. Copyright © 1979, Wiley Blackwell. All rights reserved