In the pouch experiment, the RZT was controlled, by water bath systems, at 25, 20 and 15[degrees]C, respectively (Zhang et al.
The levels of RZT and inoculations were combined factorially and were allocated to the blocks in a split-plot fashion.
Compared with 25[degrees]C, the optimal RZT for soybean growth and nodulation, 15[degrees]C RZT strongly inhibited plant nodulation and growth, while 20[degrees]C RZT had little inhibitory effect (Table 1).
In the greenhouse experiment, the performance of NEB17 was not altered by the changes in RZT, indicating that it may also be effective under longer season conditions, where spring soil temperature is warmer.
Abbreviations: NEB, non-Bradyrhizobium endophytic bacteria; PGPB, plant growth promoting bacteria; RZT, root zone temperature.
In the establishment of the N fixing symbiosis, all stages (root hair curling, infection thread formation and penetration, nodule development and function) are inhibited by suboptimal RZTs (Lindemann and Ham, 1979; Lynch and Smith, 1993).
In the present experiment, the seasonal soil temperature data from 1998 and 1999 showed that the average daily RZTs at a depth of 5 cm were below 25[degrees]C, especially during early stages of growth (Fig.
Zhang and Smith (1994) demonstrated that low RZTs delay all of the steps in the infection of soybean roots by B.