INFLUENCE OF ULTRAVIOLET-B ENHANCED SOLAR-RADIATION ON GROWTH AND PHOTOSYNTHESIS OF POTASSIUM DEFICIENT COWPEA SEEDLINGS

UV-B enhanced solar radiation (UV-B E) equivalent to 20 % ozone deplet ion at Madurai (10 degrees N), was imposed on K+ sufficient (K-0.88, 0 .88 mM K2SO4) and K+ deficient (K-0.05, 0.05 mM K2SO4) cowpea (Vigna u nguiculata cv. Pusa-152) seedlings, and changes in growth, leaf photos ynthetic pigments, soluble leaf protein contents and photosynthetic ac tivities were analysed 12 d after start of the stress. Low K supply gi ven either independently or in combination with UV-B E significantly r educed shoot and leaf biomass. The area of first trifoliate leaflets w as greatly declined under combined stresses as compared to that of cot yledonary leaves, A negative correlation between growth and photosynth esis was found in K-0.88 seedlings grown under UV-B E. A wide variatio n between the primary and trifoliate leaves was observed in contents o f soluble leaf proteins under stress. The electron transport activitie s and net CO2 uptake showed a sharp decline in seedlings subjected to combined stresses although the concentration of photosynthetic pigment s remained unchanged. Hence the low K supply increased the UV-B suscep tibility of cowpea seedlings in the field.