Nitrate:Ammonium Nutrition Alleviates Detrimental Effects of Salinity by Avoiding Degradation of Photosynthetic Pigments and Allowing High Photosystem II Efficiency in Sorghum Plants

Abstract

This study was carried out to evaluate if replacement of nitrate (NO3 - ) by ammonium (NH4 + ) in the growth medium prevents the damage in the photosystem II (PSII) efficiency and degradation of photosynthetic pigments caused by saline stress in sorghum plants. Sorghum bicolor plants were cultivated in nutrient solutions containing different NO3 - :NH4 + ratios (percentage of NO3 - in relation to NH4 + of 100:0, 75:25, 50:50, 25:75 and 0:100) at a final concentration of 5.0 mM. Salinity was imposed by adding NaCl at 75 mM in the growth medium; and the analyses were performed after ten days of salt stress. The most striking effects of nitrogen regime were observed in plants supplied with equal amounts of NO3 - : NH4 + (50:50). Under salt stress, 50:50-grown plants displayed an increase in the photosystem II maximum efficiency (Fv/Fm), which was associated with a lower non-photochemical quenching (NPQ). The superior performance of salt-stressed plants grown in 50:50 nutrient solutions was indicated by a higher effective quantum yield of PSII (ΦPSII) and electron transport rate (ETR), as well as increased chlorophyll b and anthocyanins contents. In conclusion, nitrogen nutrition with same amounts of nitrate and ammonium avoids the degradation of photosynthetic pigments and allows higher PSII efficiency in salt-stressed sorghum plants.