Nodule structure and functioning in Cicer arietinum as affected by nitrate

Chickpea (Cicer arietinum L.) cv. C-235 inoculated with Rhizobium sp. (Cice r) strain cv4Az was raised in sand culture under natural conditions with ni trogen-free nutrient solution. 45-d-old plants were treated with 20 and 50 mM KNO3 and samplings made 2 and 6 d after treatment. KNO3 application indu ced premature nodule senescence. Light microscopic investigations showed th at KNO3 treatments resulted in structural degradation of the central bacter oidal tissue. The mass of green nodules increased by 35 % under these treat ments. This was accompanied by a rapid decline in leghemoglobin (Lb) conten t of the nodules being 51 - 67 % lower than in control. The total soluble n odule proteins showed relatively minor changes under KNO3 treatments thus s uggesting preferential degradation of Lb. These changes were associated wit h a rapid decline in N-2-fixing activity. However, the decline in total sol uble sugars was relatively minor as compared to acetylene reducing activity , thus indicating that sugar deprivation is not the cause of decreased nitr ogen fixation ability. Glutathione reductase and ascorbate peroxidase activ ity showed a 10 - 20 % decrease in comparison with the control. Accumulatio n of H2O2 and structural degradation of the nodular tissue are considered t o be the factors leading to nodule senescence under nitrate treatments.