Comparative response of nodulated and nitrogen-supplied cowpea (Vigna unguiculata (L.) Walp. var. Tuy) plants to infection by Cowpea Mosaic Virus

The effect of infection by the Cowpea Mosaic Virus (CpMV) on several parameters relevant to symbiotic nitrogen fixation was determined in cowpea (Vigna unguiculata (L.) Walp. var. Tuy) plants nodulated with two strains of Rhizobium cowpea: IVIC-124 and IVIC-38. Plants were virus-infected at the seedling stage before Rhizobium inoculation. The effect of CpMV infection on plant growth was analysed in nodulated and nitrogen-supplied plants at 18, 25 and 35 d after germination. At all developmental stages of nodulated plants CpMV infection caused a reduction of leaf chlorophyll content, leaf area, dry weight of shoots and roots, total nodule weight and nodule number. Most of the nodules from 18- and 25-d-old CpMV-infected plants did not exhibit leghaemoglobin pigmentation. CpMV infection delayed the onset of nitrogenase activity in nodules of the rhizobial strain IVIC-124 and the enzyme activity measured on a per plant basis was reduced in both strains at the first and second harvests. Significant nitrogenase activity was detected in 35-d-old infected plants. Some of the nodules of the rhizobial strain IVIC-124 and most of the nodules from plants nodulated with the strain IVIC-38 developed leghaemoglobin; however, the nodule-specific nitrogenase activity, estimated on a milligram nodule dry weight basis, was always higher in virus-infected plants, particularly in 18-d-old CpMV-infected plants harbouring the IVIC-124 strain. CpMV-infected nodules had a larger peribacteroidal space, a reduced number of peribacteroid units, a greater number of bacteroids per unit, a lower number of vesicles and 88% lower total reducing sugar content. Starch accumulation was detected in infected leaves of nodulated plants during the first harvest, while high levels of leaf reducing sugars and protein were present at the second harvest. In healthy nodulated plants the rhizobial strain IVIC-124 was shown to be more efficient than IVIC-38 in promoting plant growth. However, the results indicate that nodulation by rhizobial strain IVIC-124 and growth of plants harbouring this strain were affected to a greater extent by virus infection. The effect of CpMV infection on leaf chlorophyll content, leaf area, carbohydrate level, leaf proteins and growth of nitrogen-supplied plants, as well as the symptoms induced in the leaves, were less conspicuous than in nodulated plants.