(^15) N uptake rate. The (^15) N concentration of the roots was higher than that of the shoots, particularly under
stress conditions. The^15 N concentration in plants increased with increasing salinity levels. The concen-
tration of^15 N in plants in terms of the ratio of plant total^15 N content to dry matter produced (mg^15 N/kg
dry matter) was significantly higher for moderately stressed than for control plants. This indicates that
plants continued to accumulate^15 N under salt stress conditions in spite of the reduction in dry-matter pro-
duction. Total water absorbed by plants decreased linearly with increasing salinity. This reduction was
even more appreciable than the reduction in^15 N absorption rate. The effect of salinity was more pro-
nounced at the vegetative than at the reproductive stage of growth.
The metabolism of^15 N in salinized cotton plants was adversely affected under medium and high lev-
els of NaCl, at both vegetative and reproductive stages of growth. Significant accumulations of all solu-
ble-^15 N fractions occurred when plants were subjected to medium and high levels of NaCl compared with
the controls. The –0.4 MPa osmotic potential of the culture solution enhanced protein synthesis at the veg-
etative growth stage. Only the 1.2 MPa osmotic potential significantly decreased the protein-^15 N con-
tent of plants as compared with the controls and any other level of NaCl. Protein synthesis was impaired
by a large excess of NaCl in the nutrient solution, which inhibited NH 4 metabolism.
Consequently, under salt stress conditions of sufficient magnitude, plant growth, N absorption and
metabolism, protein synthesis, and water absorption will be altered. This will result in the failure of plants
to fully utilize nutrients and water. Salinity levels in excess of those causing drastic interference with plant
growth, nutrient (i.e., N) absorption and metabolism, and water uptake in salt-sensitive plants such as
beans do not appreciably interfere with these factors in cotton, a relatively high-salt-tolerant plant. This
indicates a link between salt tolerance, growth, nutrient (i.e., N) absorption and metabolism, and water
uptake. Although the contribution of osmotic and specific ion effects cannot be distinguished from this
study, it is likely that both were involved.
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