Photoinhibition of Photosynthesis in Leaves of Cowpea (Vigna unguiculata L. Walp. P152) in Response to Zinc and High Irradiance Stresses

Seedlings of cowpea (Vigna unguiculata L. Walp. P152) were fertilized every alternate day for 14 days with half strength Hoagland solution containing 0 ppm (deficient), 5 ppm (sufficient) and 50 ppm (excess) ZnSO .7H O. Photoinhibition of photosynthesis and photosynthetic recovery were studied in detached leaves 42 under zinc (Zn) and high irradiance (HI) stresses. Changes in photosynthetic pigments, soluble proteins, soluble starch, photosystem (PSII) activities and thylakoid membrane proteins were investigated. The degree of photoinhibition was determined by the ratio of variable to maximum chlorophyll (Chl) fluorescence (F /F ). vm Photosynthetic pigments, soluble proteins and soluble starch decreased significantly in Zn-deficient and Zn- excess leaves, the degree of decrease being greater in Zn-excess. Maximum quantum yield of primary photochemistry (F /F ), maximum variable fluorescence (F ) were significantly decreased to a greater extent in vm v Zn-excess leaves than in Zn-deficient ones. The exogenous electron donors, diphenylcarbazide (DPC) and NH OH slightly restored the loss of PSII activity in Zn-deficient leaves when compared to Zn-excess leaves. 2