The effect of short term withdrawal of NaCl stress on nodulation of white clover

Abstract

The number of nodules formed by white clover (Trifolium repens L.) released from NaCl stress for 3 days (137 mol m-3) at different periods was examined. The NaCl stress-free periods were, 0 to 3 days prior to rhizobial inoculation, 0 to 3, 3 to 6, and 6 to 9 days after rhizobial inoculation. Plants not subjected to NaCl stress at 0 to 3 days after inoculation had 28.7 nodules per plant (74% of control), while plants continuously stressed had 5.2 nodules (13% of control). A NaCl stress-free period immediately after inoculation was the best among the stressed treatments, indicating that the early stage of nodulation was more sensitive than the later stages. Microscopic observation showed that imposing NaCl stress during the first 3 days after inoculation suppressed root hair curling to 9.1% of control, while the numbers of rhizobia attached to roots counted by dilution plates were not affected. Thus, there were no significant effects of NaCl stress on rhizobia. The sensitivity of the early stage of infection to NaCl stress was attributed to the inhibition of root hair curling.