COMPARATIVE EFFECTS OF NEUTRAL AND ALKALINE SODIUM SALTS ON PHOTOSYNTHETIC PIGMENTS AND REDUCING SUGAR CONTENT IN COWPEA (VIGNA UNGUICULATA (L.) WALP.) CV. KONKAN SAFED

Abstract

Soil salinity and alkalinity are major constraints for legume cultivation, particularly in coastal and semi-arid regions where sodium salts accumulate in the root zone. The present investigation examined the effect of neutral sodium chloride (NaCl; 10, 20 and 40 mM) and alkaline sodium bicarbonate (NaHCO₃; 6, 9 and 13 mM), along with neutral sodium sulphate (Na₂SO₄; 10, 20 and 30 mM), on photosynthetic pigments and reducing sugars in cowpea (Vigna unguiculata (L.) Walp.) cv. Konkan Safed at 21 days after sowing (DAS). Chlorophyll a, chlorophyll b, the chlorophyll a:b ratio and reducing sugars in shoot, root and leaves were estimated. NaCl caused only a mild reduction in chlorophyll content, whereas NaHCO₃ produced a pronounced decline, with chlorophyll a falling from 2.062 g/100 g fresh weight in the control to 0.733 g/100 g fresh weight at 13 mM. Na₂SO₄ increased chlorophyll a at 10 and 20 mM but reduced it sharply at 30 mM. Reducing sugars declined progressively in all plant parts under increasing NaCl stress. Alkaline NaHCO₃ maintained shoot sugars near control levels but reduced root and leaf sugars at the highest concentration. Na₂SO₄ caused an initial reduction in root sugars, followed by partial recovery at higher concentrations. The findings indicate that alkaline salinity is more damaging to the photosynthetic apparatus than neutral salinity, while soluble sugar accumulation may contribute to osmotic adjustment in cowpea under salt stress.