Inhibition by NO−3 of acetylene reduction in bean (Phaseolus vulgaris L. cv. Contender) and soybean (Glycine max L. cv. Amsoy 71) was measured in parallel with nodule carbohydrate and nitrate metabolism. In bean the onset of inhibition of C2H2 reduction (6 h) coincided with decreased import of assimilates and a lowering of carbohydrate pools (sucrose, glucose and starch). Nitrate reductase (EC 1.6.6.1) activity was induced in all plant organs after 3 h but no nitrite was detected in the nodules. In soybean, nodule carbohydrate concentrations and import of assimilates into the nodules increased markedly between 6 to 24 h after supply of nitrate when the nitrogenase (EC 1.7.99.2) was progressively inhibited. High nitrate reductase activity was observed in the nodules and nitrites accumulated because of insufficient nitrite reductase activity. The nitrate‐induced inhibition of nitrogenase was compared with the inhibition observed with low oxygen around the roots (1% O2) or with direct assimilate deprivation (girdling or decapitation). Soybean and bean appeared equally sensitive to these treatments as regards to acetylene reduction. The results are discussed in relation to the current hypotheses explaining nitrate‐induced inhibition of dinitrogen fixation: assimilate deprivation or nitrite poisoning. Present data are in favour of the first for bean and of the second for soybean.

Foliar application of pendimethalin herbicide (5 ppm) to three weeks old seedlings of Zea mays L. and GossypiumhirsutumL. resulted in a decreasing trend of chlorophylls a an b in both seedlings compared to controls. Chlorophyll b excelled chlorophyll a in maize, whereas in cotton a reverse manner was observed. The chlorophyll ratio a/b in cotton surpassed the chlorophyll ratio in maize.

Addition of nitrate significantly increased nitrogenous fractions (NO-3, T.N.,T., S.N)*, fresh and dry weights of roots and shoots in both plant species. Carbohydrates (T.S.,T.,S.S)*, acetylene reduction activity (A.R.A)* and nodule fresh weights were significantly decreased as compared with untreated nodulated plants. The depressive effect of nitrate on acetylene reduction activity is discussed in relation to hypotheses explaining photosynthate demand for nitrate assimilation in the nodules (assimilate deprivation), or nitrite poisoning. Present data are in favour of the assimilate deprivation hypothesis.

Nitrate uptake was examined in the roots of 15-day old nitrogen depleted broad bean (Vicia faba L.) seedlings during a 10 h. exposure to 7 mM KNO3. The seedlings were subjected to partial defoliation, thermal treatment at stem bases and girdling. The loss of nitrate from the medium during incubation of the seedlings indicated that about 30% of the nitrates were absorbed during the first 5 h. It was evident that the energy expanded for nitrate uptake and reduction came from the tops, since partial defoliation, thermal treatment and girdling restricted nitrate uptake and reduction. The data are interpreted as indicating a close interrelationship between nitrate uptake and reduction and carbohydrate metabolism, particularly in relation to the export of photosynthate from the tops to the roots.

Two field experiments were conducted to test the efficiency of selected rhizobia in charcoal and filter-mud as suitable carriers. Rhizobium isolates procured from naturally initiated nodules were compared with imported strains for nodulation in two forage plants, Medicago sativa and Clitoria ternateaa at two different sites in the Sudan. The local isolates proved to be as efficient as the imported strains. In the field experiment, there was a marked positive effect in response to inoculation with the carrier based inoculants. Fresh and dry weight, total nitrogen content and number of nodules per plant were significantly higher in modulated plants as compared with the control (non-inoculated and non-inoculated plants which received urea. The range of increase was between 16 and 94%.