ROOT-ZONE TEMPERATURE SENSITIVITY OF NITROGEN-FIXING AND NITRATE-SUPPLIED SOYBEAN [GLYCINE-MAX (L) MERR CV MAPLE ARROW] AND LUPIN (LUPINUS-ALBUS L CV ULTRA) PLANTS

Soybean [Glycine max (L.) Merr.] is a subtropical legume that is now often grown in cool temperate areas. Under these conditions, low root zone temperatures (RZT) may adversely affect N2 fixation and therefore the growth and development of soybean plants. Lupin (Lupinus albus L.), is well adapted to cool temperate areas. Soybean exports N from root nodules as ureides, while lupin exports N as amides. The water solubility of ureides declines sharply with temperature while the solubility of amides is much less affected. Experiments were established to test the effect of suboptimal RZT on N accumulation, N partitioning and dry matter partitioning by N2-fixing and mineral N-supplied soybean and lupin plants. In the first experiment, soybean plants were grown at four constant RZT: 10, 15, 20 and 25-degrees-C with constant 25-degrees-C air temperature. Four nitrogen (N) treatments were applied within each RZT; plants were fertilized with a complete N-free nutrient solution supplemented with either 0, 7.1 or 14.2 mM N, or received 14.2 mM N at the onset of the experiment with this level being gradually decreased to zero over the first 15 days of the experiment. In a second experiment, soybean and lupin were grown at 10, 15, 20 and 25-degrees-C RZT and received either 0 or 14.2 mM N. Nitrogen accumulation and partitioning were more affected by low RZT in N2-fixing than in mineral N-supplied soybean and lupin plants. Dry matter accumulation of N2-fixing soybean plants was also more sensitive to low RZT than was that of mineral N-supplied soybean plants. The slower development of the N2-fixing soybean plants at RZT less than 25-degrees-C was attributed to poor nodule development at temperatures in the 10-degrees-C range and to low nitrogenase activity at temperatures greater than 15-degrees-C. Data from the final harvest indicated that photosynthate shortage may have played a role in the reduced N2 fixation. Lupin had a higher tissue N concentration than soybean at low RZT and a lower root to shoot per cent N ratio, but N2 fixation in lupin seemed to be about as sensitive to low RZT as that of soybean.