Effects of Nitrogen Nutrition on Electron Transport Components and Photosynthesis in Spinach

Abstract

Spinach plants (Spinacia oleracea L.) were grown in hydroponic culture in a glasshouse under full sunlight. They were supplied with different concentrations of nitrate nitrogen in solution, ranging from 1 to 12 mM, in order to produce leaves with different nitrogen contents. Oxygen evolution at CO₂ saturation was measured as a function of absorbed irradiance in leaf discs with an oxygen electrode. Electron transport activities, reaction centre densities, cytochrome f and plastoquinone contents, RuP₂ carboxylase and coupling factor activities and soluble protein content were measured in similar material. Although nitrogen and chlorophyll contents per unit leaf area were reduced by 60% by nitrogen deficiency, when expressed on a chlorophyll basis, thylakoid components, electron transport activities and the rate of oxygen evolution at CO₂ saturation were similar between nitrogen treatments. In contrast, the content of soluble protein and RuP₂ carboxylase expressed on a chlorophyll basis was greater the greater the nitrogen content per unit leaf area. Therefore the ratio of RuP₂ carboxylase activity to electron transport activity increased in leaves having greater nitrogen content.