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

Abstract

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 N 2 fixation and therefore the growth and development of soybean plants. Lupin ( Lupinus albus L.), is well adapted to cool temperature 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 N 2-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°C with constant 25°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°C RZT and received either 0 or 14.2 mM N. Nitrogen accumulation and partitioning were more affected by low RZT in N 2-fixing than in mineral N-supplied soybean and lupin plants. Dry matter accumulation of N 2-fixing soybean plants was also more sensitive to low RZT than was that of mineral N-supplied soybean plants. The slower development of the N 2-fixing soybean plants at RZT less than 25°C was attributed to poor nodule development at temperatures in the 10°C range and to low nitrogenase activity at temperatures greater than 15°C. Data from the final harvest indicated that photosynthate shortage may have played a role in the reduced N 2 fixation. Lupin had a higher tissue N concentration than soybean at low RZT and a lower root to shoot per cent N ratio, but N 2 fixation in lupin seemed to be about as sensitive to low RZT as that of soybean.