N-compound accumulation and carbohydrate shortage on N2 fixation in drought-stressed and rewatered cowpea plants

Abstract

In this study nodulated cowpea plants were exposed to a progressive drought-stress followed by rewatering in orderto establish mechanisms related to the nodule sensitivity to N2 fixation. Water stress caused a prominent decreasein the dry matter of leaf and nodule in contrast to the root growth that was stimulated. The drought-stressed plantsshowed a great decline in the ureide concentration of xylem sap whilst the concentration of the nodule leghaemoglobindecreased slightly. The stressed-nodules had significant accumulation of soluble carbohydrates in parallel to intenseincrease in the concentration of ureide, total amino acid and proline. After two days rewatering, the leaf growthshowed a significant enhancement whereas in nodules a poor recovery was observed. In addition, the nodule N2 fixationcapacity did not improve after rehydration whereas the sap ureide experienced only a slight recovery and surprisinglythe content of nodule soluble protein continued to drop. In contrast, a significant recovery in the nodule leghaemoglobincontent was observed. Rewatering greatly induced depletion of both sucrose and total soluble sugarcontents of nodules that were associated to a great decrease in the concentration of ureide, proline and amino acids,especially in leaves. Conversely the poor recovery of cowpea nodules after rehydration was apparently related to a carbohydrateshortage.