Physical Environment and Symbiotic Nitrogen Fixation. IV. Faotors Affecting the Early Stages of Nodulation

Abstract

The influence of root temperature on the initial nodulation of Trifolium subterraneum L. was examined, and observations were made on the effect of light period and shoot temperature on this character. The maximum constant root temperature at which nodules would form was 33[degree]C, and the minimum was in the vicinity of 7[degree]C. The most rapid initial nodulation (2-3 days after inoculation) was observed at 30[degree]C, and plants growing at this temperature had the highest rate of nodule appearance. Below 22[degree]C, there was a marked increase in the "time to first visible nodule" and a general decline in the rate at which they appeared. Differences were observed in the time to first visible nodule, and in the rate of nodule appearance, between different cultivars of T. subterraneum. There was an indication of a temperature X cultivar interaction for these characters. With the 3 strains of Rhizobium trifolii UBed, no differences in their ability to form nodules were observed, although it was known that their subsequent symbiotic behavior differed under certain root temperature conditions. Transfer experiments suggested that the infection of root hairs was stimulated by 30[degree]C root temperature, and that infection continued at a rapid rate up to 33[degree]C, but was severely retarded above this temperature. Meriste-matic development of the nodules, but not of the roots, was severely retarded above 30[degree]C. Although infection was retarded by lower root temperatures (7-17[degree]C), it appeared that these temperatures had a greater effect on nodule meristem initiation than on infection. Furthermore, there was an indication that low temperature retardation of nodulation was not due solely to direct effects on root hair infection or nodule initiation. Leghaemoglobin formation in the nodules was also retarded by lower root temperatures. Shoot temperatures in the range 18-25[degree]C had little effect on nodule formation. With a root temperature of 12[degree]C, a daily shoot temperature regime of 15/10[degree]C delayed initial nodule formation, but did not affect the rate of nodule appearance, when compared with a regime of 18/14[degree]C. With 4- and 8-hr. daily light periods, initial nodulation was retarded; for plants receiving 12, 16, or 20 hr. light/day, the time to first visible nodule values were similar.