Chlorophyll α fluorescence is a useful and non-invasive tool to screen for the effects of many biotic and abiotic parameters on photosynthesis in plants. The fluorescence of leaves after excitation with light was measured with a portable fluorimeter (Plant Efficiency Analyser, built by Hansatech Instruments King's Lynn Norfolk, PE 30 4 NE, GB) and the subsequent data were analysed using the JIP-test which provides parameters indicating PSII properties. Seeds of Vigna unguiculata were sterilized before germinating and planting in Magenta jars (used in hydroponics culture) with a nutrient solution without nitrate (B&D solution). Some of the plants were inoculated with Rhizobium sp. strain NGR234 (10¹⁹ bacteria/100μl) at roots level. The other plants were treated with different KN0₃ concentrations (0 mM, 0.5 mM, 1 mM, 5 mM, 10 mM and 20 mM) to produce several treatments including that with Rhizobium. During 6 weeks several hundred fluorescence measurements of 2 seconds are made on the leaves of inoculated and control plants, with or without different nitrate (KN0₃) concentrations. The results obtained suggest that this technique can screen nodulation as well as nitrogen deficiency in vivo on the level of the leaves. The results of KN0₃ and the influence of nodulation and KN0₃ on primary photosynthesis will be discussed in respect to the modifications observed in the fast fluorescence rise O-K-J-I-P.