Impact of low root temperatures in soybean [Glycine max (L) Merr] on nodulation and nitrogen fixation

The inability of soybean [Glycine max. (L.) Merr.] to adapt to cool soil conditions limits its development and yield in short season areas. In particular, nodulation and N-2 fixation by this subtropical crop species is sensitive to cool root zone temperature (RZT). Two controlled environment experiments were conducted to examine the effect of RZT on plant growth, nodulation and N-2 fixation. RZTs were controlled by circulating cooled water around pots on a growth bench. The two experimental designs were: (1) soybean plants grown under a series of RZTs (15, 17, 19, 21, 23 and 25 degrees C); and (2) plants maintained at 19 or 21 degrees C, or transferred to 21 or 19 degrees C at either 1, 7, or 14 days after inoculation. These studies demonstrated that: (1) between 25 and 17 degrees C the onset of N-2 fixation was linearly delayed by 2.5 days for each degree decrease in temperature, while below 17 degrees C each degrees C appears to delay the onset of N-2 fixation by about 7.5 days; (2) by 49 DAI plants at temperatures ranging from 17 to 25 degrees C were fixing some nitrogen, but plants at 15 degrees C were not; and (3) infection and/or early nodule development were very sensitive to low RZT, such that only a few days at 19 degrees C, instead of 21 degrees C can make important differences in the time to onset of N-2 fixation and, subsequent N accumulation and plant growth.