Local Forms of Vigna unguiculata—Response to Osmotic Stress at Vegetative Growth Stage

Cowpea (<i>Vigna unguiculata</i> L. Walp.) is a species with superior tolerance to drought stress compared to other legumes. It is a promising crop with increasing importance in the face of global climate changes. Local forms, well adapted to particular agro-climatic conditions, are useful germplasm resources. Five Bulgarian cowpea landraces, which had displayed differences in osmotic stress tolerance at the germination stage, were subjected to severe stress (15% PEG 6000 in Hoagland nutrient media) during 16 days at the vegetative growth stage (plants with expanded trifoliate leaves). All local forms responded to the imposed stress by biomass and leaf area diminution, a slight increase in leaf water deficit and electrolyte leakage, proline accumulation in roots and leaves, and an increase in root starch and leaf phenol content. Roots presented more pronounced metabolic changes than leaves, including increased total antioxidant activity, phenolic and carbohydrate content, and proline accumulation. Under osmotic stress, tight control of oxidative stress and concerted upregulation of superoxide dismutase, catalase, glutathione transferase, and peroxidase activities in leaves were registered along with changes in certain specific isoforms, while glutathione reductase activity diminished. Antioxidant enzyme activities had different changes in stressed roots, compared to leaves, and among genotypes. The accession most sensitive to osmotic stress at germination presented more symptoms of oxidative stress at the vegetative growth stage.