Rates of Hydrogen Ion Efflux by Nodulated Legumes Grown in Flowing Solution Culture with Continuous pH Monitoring and Adjustment

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Abstract
The net efflux of H+ from lucerne (Medicago saliva L.), red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) growing in flowing solution culture and dependent upon symbiotic fixation of atmospheric N, was measured over a 75 d experimental period. Considerable and rapid increases in acidity of the nutrient solution of up to 1.45 pH units were recorded when the pH was riot held constant over a 30 h period. There was little difference in H+ efflux when solution pH was held constant at 4.75, 5.75 or 6.75, but there was an immediate cessation when it was adjusted to 3.75. Differences in the daily net efflux of H+ closely followed the pattern of daily differences in incoming radiation, and there was also evidence of a diurnal pattern of H+ efflux. Although there were initially distinct differences between the species in the calculated rate of net H+ efflux (μg H+ g−1 dry shoot d−), by day 75 these had diminished. In all species, however, the maximum rate of efflux per unit of shoots occurred during the earlier rapid phases of growth. The measured efflux of H+ was well equated with the plant content of excess cations (as measured by ash alkalinity) and, on average, the ratio of acidity produced to N assimilated (expressed as an equivalent) was 0-24.

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