Grain legumes are important crops, but they are salt sensitive. This research dissected the responses of four (sub)tropical grain legumes to ionic components (Na
+ and/or Cl
−) of salt stress. Soybean, mungbean, cowpea, and common bean were subjected to NaCl, Na
+ salts (without Cl
−), Cl
− salts (without Na
+), and a “high cation” negative control for 57 days. Growth, leaf gas exchange, and tissue ion concentrations were assessed at different growing stages. For soybean, NaCl and Na
+ salts impaired seed dry mass (30% of control), more so than Cl
− salts (60% of control). All treatments impaired mungbean growth, with NaCl and Cl
− salt treatments affecting seed dry mass the most (2% of control). For cowpea, NaCl had the greatest adverse impact on seed dry mass (20% of control), while Na
+ salts and Cl
− salts had similar intermediate effects (~45% of control). For common bean, NaCl had the greatest adverse effect on seed dry mass (4% of control), while Na
+ salts and Cl
− salts impaired seed dry mass to a lesser extent (~45% of control). NaCl and Na
+ salts (without Cl
−) affected the photosynthesis (
Pn) of soybean more than Cl
− salts (without Na
+) (50% of control), while the reverse was true for mungbean. Na
+ salts (without Cl
−), Cl
− salts (without Na
+), and NaCl had similar adverse effects on
Pn of cowpea and common bean (~70% of control). In conclusion, salt sensitivity is predominantly determined by Na
+ toxicity in soybean, Cl
− toxicity in mungbean, and both Na
+ and Cl
− toxicity in cowpea and common bean.
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