Effect of salt stress on nitrogen nutrition and yield quality of nodulated soybeans

Abstract

A pot experiment was carried out with soybean plants using vermiculite as a growth medium to study the effect of salt stress on: a) the uptake and distribution of biologically fixed nitrogen and applied fertilizer nitrogen using ¹⁵N as a tracer and b) the contents of protein, free amino acids, sugars, and starch in pods as criteria of yield quality.

The results obtained are summarized as follows:

1.	Salt-affected plants derived lower amounts of atmospheric nitrogen (NDFA), and also absorbed lower amounts of fertilizer nitrogen (NDFF) per plant as compared to control plants.
2.	The percentage of nitrogen derived from fertilizer (NDFF %) in the salt-affected plants was higher than in the control plants, suggesting that the salt-affected plants are less dependent on fixed nitrogen due to their lower nodulation ability.
3.	Nodules as compared to other plant organs were much less dependent on fertilizer nitrogen. This was more evident in the control plants compared to the salt-affected ones.
4.	Regardless of the effect of salinity, plant roots accumulated a higher percentage of NDFF than NDFA, and the accumulation of NDFF decreased as the plant developed to maturity.
5.	Pod accumulation of both NDFF and NDFA which gradually increased as the plant developed to maturity, was lower in the salt-affected plants particularly at the filling stage.
6.	The contents of protein, total free amino acids, sucrose, and starch, as criteria of yield quality, were all decreased in the pods of the salt-affected plants. However, the concentrations of protein and sucrose were not as severely affected compared with the other criteria tested. The depressive effect of salinity on the pod protein content seemed to be due to the decrease in the efficiency of pod protein production percent rather than to the decrease in the incorporation of amino acids into protein.

Key Words:

• nitrogen nutrition
• salt stress
• yield quality
• soybean.