tration, the protein content in cotton seedlings decreased. The depressing effects of salt on the protein con-
tent of cotton plants at high levels of NaCl could be attributed to decreased amino-N incorporation into
protein as reported for red kidney beans [83,84], rice [40,79,101], and other plants [38,39,64–66,
92,93,165,166]. The decrease in polyribosome levels, as reported for corn, Zea maysL. [165], and for bar-
ley and pea shoots [166], is probably another reason for the decrease in protein synthesis in cotton shoots.
The low level of NaCl (0.4 MPa osmotic potential of the nutrient solution) significantly increased
the protein content of cotton shoots at the vegetative stage of growth. This is in agreement with the find-
ings of Renu and Goswami [2] on uptake and accumulation of labeled^14 C photosynthates in cotyledonary
leaf of cotton treated with gibberellic acid under salt stress. According to these investigators [2], the low
levels of salinity stimulated^14 CO 2 uptake and accumulation of carbohydrates in the cotyledonary test,
whereas high salinity decreased it. The report of Zhu and Zhang [15] showing an increased concentration
of proteins when maize, sunflower, cotton, and castor bean plants were either soil dried, salt treated, or
flooded is also in support of the present study. However, the same level (0.4 MPa) of salt stress sub-
stantially decreased protein synthesis in a number of other plants with lower degrees of salt tolerance
[56–58,83,84,94,95,98]. Impaired N metabolism with the consequence of reduced protein content of sev-
eral other plant species with various degrees of salt tolerance under stress conditions has been reported by
several investigators [39,40,43,49,50,65,66,68,72,77,79,89,91–93,96,97,99–102,165,167].
Water stress is also known to impair N metabolism and reduce protein synthesis in plants
[39,65,66,68,71,83,84,98,99,166]. In addition to the osmotic effect of salt, the specific ion effects of Na
and/or Clhave certainly contributed appreciably to the inhibition of^15 N incorporation into protein.
However, this study was not designed to distinguish specifically between osmotic and ionic effects.
The rates of^15 N incorporation into protein as measured by the concentration of^15 N in the protein frac-
tion at 6, 12, and 24 hr of exposure to^15 NH 4 appear to have been influenced only by the high level of NaCl
at both growth stages (Table 7). The rate of incorporation was reduced at the 1.2 MPa salt level by fac-
tors of 2.5 and 2.8 at the vegetative and reproductive stages of growth, respectively. The rate of^15 N in-
corporation into protein at the vegetative stage was approximately 2.5 times greater than the rate at the re-
productive stage, based on the^15 N concentration. The total^15 N in the protein fraction was greater at the
reproductive than at the vegetative stage of growth, apparently because of the much greater amount of shoot
dry weight (Table 1). Furthermore, the total protein-^15 N decreased with increased salt level at both stages
of growth. This reflects the combined effect of salt on shoot growth and^15 N incorporation into protein.
RESPONSES OF COTTON TO SALT STRESS 689TABLE 7 Slope (b) and Intercept (a) of the Regression Lines for Concentration of^15 N Fraction in Cotton
Shoots Influenced by NaCl Salinity Versus 6, 12, and 24 hr Exposure Time for Two Stages of Growtha
Growth stage
Treatment, osmotic potential Vegetative Reproductive
15N Fraction (MPa) baba
Protein Control 24.22 99.08 10.38 24.65
0.4 26.44 81.19 10.43 23.18
0.8 24.71 61.10 9.36 11.58
1.2 9.89 6.80 3.61 11.56
Soluble Control 13.20 13.70 5.14 17.53
0.4 21.28 58.50 0.06 1.79
0.8 30.64 19.25 16.36 28.95
1.2 20.88 30.25 16.17 2.36
Ammonium Control 0.32 0.53 0.19 0.06
plus amide 0.4 0.40 0.16 0.23 0.17
0.8 0.93 0.49 0.80 0.58
1.2 0.51 1.71 0.30 2.52
Free amino Control 6.03 22.79 2.65 1.05
0.4 9.63 48.44 2.73 1.13
0.8 13.88 58.63 4.41 9.63
1.2 14.24 76.61 5.12 15.17
aCorrelation coefficient, r, values lie between 0.92 and 1.00.
Source: Ref. 63.