Nodule structure and functioning in Cicer arietinum as affected by nitrate

Chickpea (Cicer arietinum L.) cv. C-235 inoculated with Rhizobium sp. (Cicer) strain cv4Az was raised in sand culture under natural conditions with nitrogen-free nutrient solution. 45-d-old plants were treated with 20 and 50 mM KNO3 and sampling made 2 and 6 d after treatment. KNO3 application induced premature nodule senescence. Light microscopic investigations showed that KNO3 treatments resulted in structural degradation of the central bacteroidal tissue. The mass of green nodules increased by 35 % under these treatments. This was accompanied by a rapid decline in leghemoglobin (Lb) content of the nodules being 51 - 67 % lower than in control. The total soluble nodule proteins showed relatively minor changes under KNO3 treatments thus suggesting preferential degradation of Lb. These changes were associated with a rapid decline in N2-fixing activity. However, the decline in total soluble sugars was relatively minor as compared to acetylene reducing activity, thus indicating that sugar deprivation is not the cause of decreased nitrogen fixation ability. Glutathione reductase and ascorbate peroxidase activity showed a 10 - 20 % decrease in comparison with the control. Accumulation of H2O2 and structural degradation of the nodular tissue are considered to be the factors leading to nodule senescence under nitrate treatments.