The Influence of Carbohydrate Reserves on the Response of Nodulated White Clover to Defoliation

A growth-chamber study was carried out to determine whether the response of apparent nitrogenase activity (C2 H2 reduction) to complete defoliation is influenced by the availability of carbohydrate reserves Reserve carbohydrate (TNC) concentrations of 6-week-old white clover (Trifoliun repens L) plants were modified by CO2 pretreatments There was no difference in the response of apparent nitrogenase activity to defoliation between plants with different TNC concentrations C2H2 reduction activity declined sharply after defoliation and then recovered similarly in both high- and low-TNC plants Further experiments were conducted to explain the lack of response of apparent nitrogenase activity to TNC levels Bacteroid degradation was ruled out because in vitro nitrogenase activity of crude nodule extracts was still intact 24 h after defoliation Sufficient carbohydrates appeared to be available to the nodules of defoliated plants because adding [14C]glucose to the nutrient solution did not prevent the decline in apparent nitrogenase activity These conclusions were supported by the finding that an increase in pO2 around the nodules of defoliated plants completely restored their C2H2 reduction activity The comparison of the effects of defoliation and darkness suggested that the decrease in apparent nitrogenase activity was not related directly to the interruption of photosynthesis It appears that lack of photosynthates is not the immediate cause of the decline of nitrogen-fixing activity after defoliation