stress resulted in an appreciably greater reduction in^15 N incorporation in the protein fraction than water
stress created by the Carbowax treatment. These results [83,84] indicated inhibition of N utilization
caused by an ionic effect in addition to the osmotic effect of either NaCl or Carbowax.
Although investigations on the effects of salt and/or water stress on nutrient (i.e., N) absorption, uti-
lization, metabolism, and protein synthesis mostly indicated a reduction in the absorption rate of N and
decrease in the protein content of plants, a few controversial results make generalization difficult. These
and other inconsistent results that demonstrated either an increase or no effect on the nutrient (i.e., N) ab-
sorption and metabolism can probably be explained as resulting from a dilution effect. This was suggested
by Frota and Tucker [82,83], Saad [84], Pessarakli and Tucker [60–63], and Pessarakli [55,86,94,95], in
which plant growth was affected more than the nutrient uptake and metabolism by salt stress and as a re-
sult the relative concentration of N was higher for the stressed plants.
Although the mechanisms by which salinity stress or drought adversely affect plant growth are still
controversial, it is generally agreed that impairment of N absorption and metabolism is a critical factor.
For a detailed review of the adverse effects of stress on plants and crops, readers are referred to the most
comprehensive source, the new edition [103] and the original edition [104] of the Handbook of Plant and
Crop Stress.
If it could be determined at what particular stage of growth high salinity most negatively affects plant
growth and metabolism, the mechanisms by which these adverse effects occur might be identified and the
detrimental effects prevented. In this regard, in addition to this work, several investigators have already
attempted to study the effects of stress at different stages of plant growth [62,63,86,88,105–126].
The purpose of this investigation was to determine the physiological effects of salt stress on growth
in terms of dry-matter production, nitrogen (^15 NH 4 ) absorption and metabolism, protein synthesis, and
water uptake by cotton plants at two stages of growth.
II. RESULTS AND DISCUSSION
A. Dry-Matter Production of Cotton Plants
At both the vegetative and the reproductive stages of growth, salt stress (particularly at medium and
higher NaCl levels) drastically reduced dry-matter production (Table 1). The results of Khan et al. [5]
showing a substantial decrease in cotton yield at different stages of growth under salinity stress confirm
this finding. The report of Qadir and Shams [12] indicating that the imposed salinity stress had a delete-
rious effect on the germination and vegetative growth of cotton plants is also in agreement with the pre-
sent work. According to these investigators [12], leaf area, stem thickness, and shoot and root weights de-
creased with increasing substrate salinity level. Kurth et al. [127] also observed adverse effects of both
NaCl and CaCl 2 salinity on cotton growth in terms of cell enlargement and cell production. Several other
684 PESSARAKLI
TABLE 1 Dry Matter Production of Cotton Plants Subjected to NaCl Salinity During Vegetative and
Reproductive Stages of Growth
Treatment, osmotic potential Plant dry weight / pot (two plants) (g)
Growth stage (MPa) Shoots Roots Total
Vegetative Control 5.42 0.93 6.35
0.4 3.79 0.97 4.76
0.8 2.71 0.77 3.48
1.2 1.71 0.39 2.10
LSD (.05)a 1.43 0.20 1.58
Reproductive Control 20.13 3.90 24.03
0.4 16.72 3.98 20.70
0.8 12.11 3.52 15.63
1.2 7.60 2.42 10.02
LSD (.05)a 4.22 0.83 4.60
aThe least significant difference among the means at the .05 probability level.
Source: Ref. 62.