Effect of Different Air and Root-Zone Temperature on Yield and Nitrogen Fixation of Three Annual Medics

In order to study the effect of air and root-zone temperature on yield, yield components, nodulation and nitrogen fixation of three annual medics, an experiment was conducted in controlled environment (growth chamber) at the Faculty of Agriculture, Tarbiat Modares University in 2006. The experiment was performed as a spilt split plot with the layout of completely randomized design with three replications. Air temperature at three levels including 15/10, 20/15 and 25/20ºC day/night, four levels of root-zone temperatures including 5, 10, 15 and 20ºC and three annual medics (Medicago polymorpha, M. radiata and M. rigidula) were randomized to main plot, sub plot and sub sub plot units, respectively. The results showed that there were significant differences among annual medics for dry matter production, yield components and nitrogen fixation. M. rigidula produced more leaves, stems and root dry matter, leaf and stem to root ratio, leaf number and area and forage yield than other annual medics. Also, three annual medics at 25/20ºC day/night air temperature (the highest one) produced more nodulation dry matter (8.85 mg/pot) and nitrogen fixation (7.7 mg/g dry matter) than other temperatures. Plants at the former temperature produced 8 and 2 times more nodulation and nitrogen fixation than 15/10ºC day/night air temperature (the lowest one), respectively. Low root-zone temperature up to 5ºC had severely negative effect on yield and nitrogen fixation in the three studied annual medics. Interaction among annual medics, air and root-zone temperatures showed that M. rigidula was better than other annual medics for yield, nodulation and nitrogen fixation at 25ºC air temperature and 15ºC root-zone temperature . The result showed that M. rigidula had normal growth and development compared with other annual medics at low root-zone temperatures. Thus, M. rigidula may be a better annual medic for cultivation in cold and moderate regions. Therefore, in the zones where soil temperature is lower than 5ºC during the season, cultivation of annual medics is not successful, but in the zones where soil temperature is greater than 10ºC, annual medics have normal growth and produce average yield due to better nitrogen fixation.