Plant and Soil 92, 427-430 (1986). 9 1986 Martinus Ni]hoffPublishers, Dordrecht. Pn'nted in Netherlands. Ms. 6414 Effects of red and far red lights on nodulation and nitrogen fixation in soybean ( Glycine max L. Merr) P.A. BALATTI and E.R. MONTALDI Instituto de Fisiologia Vegetal, Faeultad de Agronomia U.N.L.P., CaRe 60 y 119, CC 31, 1900 - La Plata, Republica Argentina Received 22 May 1985. Revised August 1985 Key words Light Nitrogen Fixation Nodulation Phytochrome Soybean Summary Rooted leaves and plants were irradiated at the end of the photoperiod with red light for 15 minutes, while another group was also irradiated with red light and immediately afterwards with far- red light. The reduction in Pfr/Ptotal established by far red irradiation promoted nodulation probably by affecting an hormonal balance. Introduction It is well-known that root nodule formation by legumes plants is dependent on the supply of carbohydrates resulting from photosyn- thesis. Hence, the effect of light on nodulation is primarily due to its influence on photosynthesis 4. However, it has been found that light may also affect nodulation by a reaction of low energy 1'3'4. In this photoreaction, nodulation and nitrogen fixation, like many other processes in the plant, are regulated by phytochrome s . The purpose of this study was to know more about the action of red and far red lights on nodulation and nitrogen fixation in the rhizobium- soybean symbiosis. Data presented were obtained from isolated rooted leaves and from whole plants. Rooted leaves provided a plant material in which carbohydrates were not a limiting factor for nitrogen fixation. Materials and methods Leaves of soybean plants cv Bragg grown in a field plot, expanded to 60-80% of their final size were cut at sunset when they had maximum carbohydrate content. Both unifollate and tri- foliate leaves were placed in sterilized vermiculite in 1 liter pots with petioles covered by the substratum. The pots were placed in culture rooms under a cycle of 12 h light at 27~ and 12 h darkness at 18~ Pots were covered with glass plates to diminish transpiration during the rooting process. From the time the leaves were cut till they developed roots they were sprayed daily with a solution of 2.3- 10-6M kinetin so as to provide a source of cytokinins until they were synthetized by the new root system. Leaves were watered with a Hoagland solution diluted to 1/4 concentration as required. In the experiments both leaves and plants were grown in vermiculite previously sterilized at 1.5 atm for 20 minutes. Seeds were surface sterilized with 10% Na hypoclorite for 10 minutes 427