values at the vegetative and the reproductive stages of growth, respectively. At the higher salinity level
(1.2 MPa), the reduction in N uptake was proportionately greater at the vegetative stage than at the re-
productive stage of growth. Reduced N uptake at the vegetative stage was largely due to the reduction in
dry-matter production, except at the high salinity level, where the N concentration was reduced appre-
ciably (Tables 1 and 5). At the reproductive stage, N concentration was essentially the same at all salin-
ity levels, indicating that plants had adjusted somewhat to salinity and its effect on N uptake. The dry
weights at the high salinity level were still less than 50% of the control levels, as were values for the to-
tal N uptake. Although generally these observations are similar to the previous discussion of^15 N data,
some small deviations are apparent. Concentration data for^15 N (Table 4) indicate that short-term^15 N con-
centrations increased in the shoots at the 0.8 MPa salinity level.
C. Nitrogen Metabolism and Assimilation in Cotton Plants
- Protein-N Content of Plants
After a 24-hr exposure to^15 NH 4 , the protein-^15 N content of plants treated with a high level of NaCl (1.2
MPa osmotic potential) was significantly less than in either the control, low, or medium NaCl treatments
(Table 6). This is in agreement with the observations of Lin et al. [8] that with increasing NaCl concen-
688 PESSARAKLI
TABLE 5 Total N Uptake of Cotton Plants During Vegetative and Reproductive Stages of Growth as
Influenced by NaCl Salinity
Treatment, osmotic potential Total N/pot (two plants) (mg)
Growth stage (MPa) Shoots Roots Total
Vegetative Control 185.1 20.5 205.6
0.4 114.8 23.0 137.8
0.8 89.8 17.8 107.6
1.2 43.1 7.2 50.3
LSD (.05)a 15.9 5.3 17.7
Reproductive Control 371.2 63.6 434.8
0.4 333.8 66.7 400.6
0.8 245.9 62.0 307.9
1.2 156.5 41.6 198.1
LSD (.05)a 96.1 15.0 105.6
aThe least significant difference among the means at the .05 probability level.
Source: Ref. 62.
TABLE 6 Concentration of^15 N Fractions and Protein-^15 N to Nonprotein-^15 N Ratio of Cotton Shoots
Influenced by NaCl Stress for Two Stages of Growth After 24 hr of Uptake
Ammonium Free Protein-^15 N to
Osmotic potential Protein-N Total soluble-N plus amide-N amino acid-N nonprotein-^15 N
(MPa) (mg) (mg) (mg) (mg) ratio
Vegetative
control 489.6 318.6 6.97 129.0 1.54
0.4 560.9 463.9 9.37 189.1 1.21
0.8 544.3 741.3 21.96 287.1 0.73
1.2 230.4 523.2 13.67 273.1 0.44
LSD (.05)a 68.3 120.4 2.10 51.9 0.24
Reproductive
control 229.9 145.5 4.87 60.6 1.58
0.4 232.8 204.8 5.31 65.9 1.14
0.8 217.3 363.6 19.71 96.4 0.60
1.2 98.8 393.6 9.64 108.1 0.25
LSD (.05)a 46.6 76.2 1.83 11.6 0.47
aThe least significant difference among the means at the .05 probability level.
Source: Ref. 63.