Evaluating N-free solution culture system for determination of nitrogen fixation in rice

Abstract

Nitrogen fixation by endophytic bacteria has a potential to contribute to the mitigation of global warming by reducing the use of nitrogen fertilizer and thus emission of nitrogen oxide. Screening rice and associated nitrogen fixing endophytic bacteria is limited by a lack of appropriate N-free nutrient solution. This study set out to evaluate N-free nutrient solutions and systems for growing rice, by evaluating nutrient solution with 2 different forms of iron (Fe), Fe-citrate and Fe-EDTA for rice growth and N levels (0 and 70 ppm, N0 and N70). Five rice seedlings were grown in each pot containing 5 L of either nutrient solution with 2 replications. In one set of pots the nutrient solution was not renewed and in another set the nutrient solution was renewed weekly. It was found that chlorophyll content (measured with SPAD502 on the youngest emerged blade) varied with N levels and form of Fe. The SPAD value of N70 was higher than N0 in both iron forms. Nitrogen deficiency in N0 was indicated by chlorosis and necrosis of older leaves and greener younger leaves. In N0 without nutrient solution renewal, the SPAD value was higher with Fe-EDTA than Fe-citrate, with Fe deficiency of Fe-citrate plants indicated by more chlorosis of younger than older leaves. Rice plants were grown successfully in N0 with EDTA-Fe in enclosed root system without renewal for one month with 1 L, and longer with 1.5 and 6 L of nutrient solution. The system will now be used for evaluating nitrogen fixation in rice in association with endophytic bacteria.