Handbook of Plant and Crop Physiology

(Steven Felgate) #1

When cotton callus tissues were exposed to salt stress, Banks et al. [27] observed an increase in the
activity of antioxidant enzymes, including ascorbate peroxidase and glutathione reductase. According to
these investigators [27], these responses suggest that the up-regulation of the activity of these enzymes in
response to salt stress is due to de novo transcription of the genes encoding the two enzymes and not to
translation of the existing transcripts or mobilization of existing enzyme pools.
The most recent findings on cotton plants were reported by Feng et al. [28], Murray et al. [29], Gos-
sett et al. [30], and Rajguru et al. [31]. Feng et al. [28] studied the effects of salt stress on VA (vesicular
arbuscular) mycorrhizal formation and of inoculation with VAM (vesicular-arbuscular mycorrhizal)
fungi on saline tolerance of plants, including cotton, maize, soybean, and melon, grown on soils contain-
ing NaCl. They found that at a given NaCl level, cotton, maize, and soybean plants incubated with VAM
had a higher biomass than noninoculated plants. These investigators [28] suggested that the VAM
fungi–plant symbiosis might play an important role in survival of plants grown on saline soils. Also, in-
oculation with VAM fungi could enhance crop production in plants grown on saline soils and reduce the
loss of plant yield caused by salt stress. Gossett et al. [30] reported that the total antioxidant enzyme re-
sponse to NaCl stress in cotton callus tissue is somewhat specific to the combined effects of Naand Cl
ions. Although Rajguru et al. [31] showed that salt treatment reduced ovule fresh weight in several cot-
ton cultivars, superoxide desmolase activity increased in most of the cultivars under the salt stress condi-
tion. Glutathione-S-transferase activity significantly increased in all the cultivars treated with NaCl.
Several studies indicated that decreases in plant growth and crop yields under stress conditions have
been associated with impairment of nutrient and water uptake, abnormal metabolism, and inhibition of
plant protein synthesis [32–77]. In these studies, salt and/or water stress impaired growth and incorpora-
tion of nutrients (i.e., N) into the protein and increased accumulation of inorganic-N in plants. Reduction
of nutrient uptake and utilization by plants was also reported by several investigators in earlier studies
[78–84]. Uptake of N and P by plants was inhibited by high NaCl and Na 2 SO 4 concentrations in the root
medium, and the excess amount of absorbed Nadepressed NH 4 absorption in these studies. Absorption
and metabolism of ammonium (NH 4 ) and nitrate (NO 3 ) in red kidney beans (Phaseolus vulgarisL.) was
significantly reduced under salt or water stress [82–84]. In all of the preceding studies, reduction of root
permeability and the consequent decrease in water and nutrient uptake under high electrolyte concentra-
tions were stated as the cause of this abnormality in water and nutrient absorption and metabolism. Nev-
ertheless, low levels of salts in the presence of N, P, and K stimulated growth and increased yield of cot-
ton,Gossypium hirsutumL. [62,63,85–88]. With further increase in salinity, dry-matter yield decreased,
but it increased with the addition of N at each salinity level. Moreover, plants continued to accumulate N
under saline conditions in spite of the reduction in yield and dry-matter production.
Soil salinity did not inhibit N absorption by bermudagrass (Cynodon dactylonL.), a high-salt-toler-
ant plant [89], and stress had little or no effect on the rate of NO 3 uptake by barley (Hordeum vulgare
L.), another high-salt-tolerant crop, except at the highest osmotic pressure, lowest osmotic potential
(0.54 MPa) of the rhizosphere [43,90]. Also, NaCl in the culture solution did not influence NO 3 uptake
by tomato (Lycopersicon esculentumMill.), a medium-salt-tolerant plant [80].
Abdul-Kadir and Paulsen [91] reported that the soluble protein and free amino acid content of wheat
(Triticum aestivumL.) plants were not consistently affected by MgSO 4 , MgCl 2 , and NaCl. Udovenko et
al. [92,93] found that under salt stress the non–protein-N fraction increased in beans, peas (Lathyrus hir-
sutusL.), barley, and wheat, whereas the protein-N fraction changed irregularly. These investigators
[92,93] concluded that the response of N metabolism to salt stress is similar in plants with varying salt
tolerance. An increased in the soluble-N fractions and free amino acid levels and decrease in protein-N
content of cotton plants under medium (0.8 MPa osmotic potential) and high (1.2 MPa osmotic po-
tential) levels of salinity were reported by Pessarakli and Tucker [63]. However, these investigators found
that the low level of salinity (0.4 MPa osmotic potential) slightly enhanced dry-matter production and
protein content of the plants. On the other hand, this level of salinity (0.4 MPa osmotic potential) and
lower (0.25 MPa osmotic potential) of the culture solution substantially decreased protein content of
red kidney beans [83,84], green beans [57,58,94,95], and alfalfa, Medicago sativaL. [56]. Impaired N
metabolism and decreased protein content of a number of plants under stress conditions have also been
reported by several other investigators [96–102]. Rabe [64,65] and Dubey [38,39] reviewed altered N
metabolism and protein synthesis, respectively, in plants under stressful conditions. These authors re-
ported that N metabolism and protein synthesis in plant species were severely affected under stress.
Water stress induced by Carbowax also caused a marked reduction in protein synthesis by plants
[83,84]. Although these studies were conducted on red kidney beans, a salt-sensitive plant, salt (NaCl)


RESPONSES OF COTTON TO SALT STRESS 683
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