- Total Soluble-N Content of Plants
Soluble N compounds that should be in an ethanol extract of plant tissues include NO 3 , NH 4 , amides,
amino acids, amine, amino sugars, peptide, alkaloids, nucleotide, chlorophyll, and even some fats. Be-
cause only the NH 4 form of N was used in this investigation,^15 N from^15 NH 4 exposure should not be
found in the NO 3 form in the plant tissues in this study.
The total soluble-^15 N concentration of the plant tissues increased with NaCl concentration at 0.8
MPa osmotic potential after 24 hr exposure to^15 NH 4 at the vegetative growth stage, then declined at 1.2
MPa (Table 6). At the reproductive growth stage, total soluble-^15 N increased in a similar manner but did
not decline at the highest salinity level. Thus, the decrease in protein-^15 N at the 1.2 MPa salinity level
did not result from a shortage of soluble-^15 N compounds. The rates of^15 N incorporation into the total sol-
uble-^15 N fraction as indicated by the slope of the regression of the^15 N tissue concentration versus time
(Table 7) followed the same pattern as described before for the 24-hr uptake time. Accumulation of sol-
uble-N compounds in plants under stress conditions has also been reported by several other investigators
for various plant species [43,49,53–61,65,66,69,70,72–76,79,81–86,89–96,101,102,153].
The amounts of soluble-^15 N reflect both concentrations in the tissue and dry-matter production
(Table 1). Significantly less total soluble-^15 N was found after 24 hr with 1.2 MPa salinity at the vege-
tative growth stage. With the 0.8 MPa salinity, a larger amount of total soluble-^15 N was observed than
with other treatments. At the reproductive stage, the amounts of total soluble-^15 N were equal at the high
and low levels of salinity, with the intermediate salinity levels resulting in higher quantities of total solu-
ble-^15 N in the plant parts.
Although the rate of^15 NH 4 absorption was severely curtailed by high salinity at both growth stages
[62], growth was not restricted by decreased total soluble-N concentration. However, impairment of sol-
uble-N utilization at high salinity was reflected in a severe decrease in the protein concentration of plants.
It is not clear whether this lower protein concentration was a cause of reduced growth. Growth was re-
duced at the lower salinity levels without a reduction in the protein concentration.
The ratio of protein-N to non–protein-N (soluble-N) is further evidence for the decrease in the pro-
tein-N content of the NaCl-treated plants. A substantial decrease in the ratio of protein-^15 N to non–pro-
tein-^15 N was observed for the plants subjected to a high level of NaCl (1.2 MPa osmotic potential) com-
pared with the controls at both stages of growth (Table 6). At both stages of growth, the values for the
0.8 MPa osmotic potential of the NaCl-treated plants were significantly lower than the controls.
- Ammonium Plus Amide-N Content of Plants
At both stages of growth, significantly higher concentrations of ammonium plus amide-^15 N accumu-
lated in the shoots of the plants subjected to NaCl stress compared with the controls (Table 6). The con-
centration of ammonium plus amide-^15 N increased with increasing salinity to a maximum at 0.8 MPa
osmotic potential. Because the absorption rate of^15 NH 4 did not change appreciably at these salinity
levels, this increased accumulation of ammonium plus amide-N must have resulted from a reduced rate
of utilization; however, reduced growth is another possible consideration. The concentrations of am-
monium plus amide-N at the 1.2 MPa stress were lower than at the 0.8 MPa osmotic potential.
These values reflect markedly reduced absorption rates at the 1.2 MPa stress [62]. The rate of^15 N
utilization also decreased, allowing a higher^15 N concentration than commensurate with absorption rate.
Slopes for regressions of ammonium plus amide-^15 N and time of uptake for each salinity level (Table
7) indicate a rate of accumulation pattern similar to the concentrations indicated for the 24 hr exposure
time (Table 6).
- Free Amino-N Content of Plants
Free amino acids would be expected to constitute the major portion of the total ethanol soluble-N com-
pounds from plant tissues. In this study, the amino-N and ammonium plus amide-N accounted for 30 to
55% of the total soluble-N. The ninhydrin release method for free amino-N determination was used in this
investigation. This method, however, can result in poor recoveries of a number of amino acids [168]. In
Kennedy’s [168] investigation, recoveries varied from 2 to 60% for 12 amino acids with complete re-
covery of 14 others. In the present study, the apparent low recovery of amino-^15 N from cotton tissues by
the ninhydrin release method is consistent with the results of Kennedy [168] when all aspects of the
methodology are considered. Even with the low recovery, however, the relative effects of NaCl salinity
on amino acid formation and utilization should be valid.
690 PESSARAKLI