After a 24-hr exposure to^15 NH 4 , a higher concentration of amino-^15 N was found in the NaCl-
stressed plants, as compared with the controls, at both growth stages (Table 6). This increased concen-
tration was sufficient to equal or exceed the reduction in dry weight, except at the highest NaCl level
(1.2 MPa osmotic potential), when dry weight was reduced most drastically. Yet, at the higher NaCl
salinity level, the amino-^15 N concentration either remained constant or increased slightly as protein con-
centration values declined to less than half of the values for all other treatments. Slopes of the regression
lines of the amino acid and exposure time (Table 7) reflect rate of amino acid accumulation. Thus, the in-
corporation of amino acids into protein was impaired by a high level of NaCl. This level of salinity that
was required for interference with protein formation in cotton, a relatively high-salt-tolerant plant, was
much higher than reported for green beans [57,58,94,95], red kidney beans [83,84,98], soybeans, Glycine
maxL. [102], peas [97], alfalfa [56], corn [68,165], rice [40,49,79,101], and wheat [91], which all have
lower degrees of salt tolerance than cotton.
D. Total Water Uptake by Plants
At both stages of growth (except for the 0.4 MPa osmotic potential during the reproductive stage), salt-
stressed plants absorbed significantly less water than the controls (Table 8). Plants at 0.4 MPa stress did
not exhibit a statistically significant difference in water uptake during the reproductive stage of growth
compared with the controls. Reduction in water absorption by plants due to salinity stress has been re-
ported by many investigators [15,22,24,55–62,74,82,84,86,94,95,129,170–172]. These investigators gen-
erally agreed that root permeability of plants (expressed as hydraulic conductivity of the root system) was
decreased significantly under salt stress. This is an explanation for the reduction in water absorption rate
and may contribute to a similar reduction in nutrient uptake and consequently reduction in crop yield un-
der salinity conditions.
III. SUMMARY AND CONCLUSIONS
Cotton plants grown in normal (control) and NaCl-treated Hoagland solutions were studied at two stages
of growth (vegetative and reproductive). Plant growth in terms of dry-matter production was measured.
Nitrogen absorption (total-N and^15 N) and water uptake were determined. Plant parts (shoots and roots)
were analyzed separately for N content and distribution of^15 N in ammonium plus amide-N, free amino-
N, total soluble-N, and protein-N after the plants were provided^15 NH 4 NO 3 in nutrient solutions for 6, 12,
and 24 hr.
Dry-matter production of the cotton plants was significantly reduced by decreasing the osmotic po-
tential (increasing salinity) of the nutrient solution. The low and medium levels of salinity did not have a
significant effect on the^15 N absorption rate, but the high salt levels caused a substantial reduction in the
RESPONSES OF COTTON TO SALT STRESS 691TABLE 8 Influence of NaCl Salinity on Water Absorption by Cotton Plants During the 24- hr^15 N Uptake
Period for the Vegetative and Reproductive Stages of Growth
Water uptake/pot (two plants) (mL)
Treatment, osmotic potential Uptake time, hr
Growth stage (MPa) 6 12 24
Vegetative Control 125.1 160.0 202.5
0.4 87.5 122.5 172.5
0.8 57.5 120.5 135.0
1.2 40.0 70.0 87.5
LSD (.05)a 46.0 23.4 28.2
Reproductive Control 165.0 275.0 490.0
0.4 147.5 245.0 430.0
0.8 130.0 167.5 215.0
1.2 75.0 107.5 145.0
LSD (.05)a 78.7 102.8 205.5
aThe least significant difference among the means at the .05 probability level.
Source: Ref. 62.