Root-zone temperature and soybean [Glycine max (L) Merr] vesicular-arbuscular mycorrhizae: Development and interactions with the nitrogen fixing symbiosis

Suboptimal root-zone temperature (RZT) has been shown to decrease soybean [Glycine max (L.) Merr.] nodulation and nitrogen fixation. Hoc-ever, there are few studies on the effects of suboptimal RZTs on vesicular-arbuscular (VA) mycorrhizal colonization of their host plants, and no investigation of those effects in soybean. In addition, there have been no investigations of the tripartite symbiotic association (VA mycorrhizal fungi, Bradyrhizobium japonicum and soybean) over a range of RZTs. A cont rolled-environment experiment was conducted to examine the effect of RZTs on VA mycorrhizal colonization, nutrient uptake, nodulation and nitrogen fixation, and plant growth and development. The experiment was organized as a randomized complete block split-plot design; the main-plot units consisted of four RZTs, 15, 18.2, 21.6 and 25 degrees C; the four harvest stages and inoculation, or not,with mycorrhizal fungi formed the sub-plot units. The results of this study indicated that (1) the optimal RZT for mycorrhizal infection of Glomus versiforme was 21-22<degrees.C - above and below this range, mycorrhizal colonization was inhibited; (2) mycorrhizal colonization increased until flowering (the third harvest), but decreased thereafter at all tested RZTs; (3) mycorrhizal colonization had a negative effect on nodule establishment (nodule number) at lower RZTs, but a positive one at higher RZTs; and (4) the lower nodule number at lower RZTs was more than compensated for by increased mass per nodule so that mycorrhizal infection stimulated N-2 fixation at lower RZTs.