Abstract
Nitrate absorbed by soybean (Glycine max L. Merr.) roots from the soil can promote plant growth, while nitrate transported to nodules inhibits nodulation and nodule nitrogen fixation activity. The aim of this study was to provide new insights into the inhibition of nodule nitrogen (N) fixation by characterizing the transport and distribution of nitrate in soybean plants. In this research, pot culture experiments were conducted using a dual root system of soybeans. In the first experiment, the distribution of 15N derived from nitrate was observed. In the second experiment, nitrate was supplied–withdrawal–resupplied to one side of dual-root system for nine consecutive days, and the other side was supplied with N-free solution. Nitrate contents in leaves, stems, petioles, the basal root of pealed skin and woody part at the grafting site were measured. Nitrate transport and distribution in soybean were analyzed combining the results of two experiments. The results showed that nitrate supplied to the N-supply side of the dual-root system was transported to the shoots immediately through the basal root pealed skin (the main transport route was via the phloem) and woody part (transport was chiefly related to the xylem). There was a transient storage of nitrate in the stems. After the distribution of nitrate, a proportion of the nitrate absorbed by the roots on the N-supply side was translocated to the roots and nodules on the N-free side with a combination of the basal root pealed skin and woody part. In conclusion, the basal root pealed skin and woody part are the main transport routes for nitrate up and down in soybean plants. Nitrate absorbed by roots can be transported to the shoots and then retranslocated to the roots again. The transport flux of nitrate to the N-free side was regulated by transient storage of nitrate in the stems.
Highlights
Nitrogen (N) is an important macronutrient required for crop growth (Nacry et al, 2013; Obrien et al, 2016), and its utilization and transport are directly related to crop growth and yield
Nitrate absorbed by soybean roots is transported to the shoots through the xylem by transpiration, where it is reduced to nitrite, and reduced to ammonium
The nitrate contents of the basal root pealed skin and woody part were significantly higher on the N+ side than on the N– side in all three phases. These results indicated that the supply of 100 mg L−1 NO3−-N to one side of the dual-root system of soybeans markedly increased the nitrate contents in the basal root pealed skin and woody part on both the N+ and N– sides
Summary
Nitrogen (N) is an important macronutrient required for crop growth (Nacry et al, 2013; Obrien et al, 2016), and its utilization and transport are directly related to crop growth and yield. Transport and Distribution of Nitrate supplement to N, is absorbed by crops, which can accelerate crop growth (Martinoia et al, 1981; Forde, 2002; De Angeli et al, 2006; Wang et al, 2012; Vidal et al, 2020), and nitrate transport and assimilation in soybean directly affect plant growth (Yamashita et al, 2019). Nitrate absorbed by soybean roots is transported to the shoots through the xylem by transpiration, where it is reduced to nitrite, and reduced to ammonium. Ohyama (1984) fed 15NO3− to soybean roots and found that despite the high capacity of the roots to transform nitrate, a small proportion of 15NO3− was directly translocated to the shoot. A short-term supply of 6.4 mM nitrate promoted N metabolism in the roots but had a pronounced inhibitory effect on nitrate metabolism in nodules (Streeter, 1985)
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