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III. Physiological Basis of Salt Tolerance

III. Physiological Basis of Salt Tolerance

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6



H. E. HAYWARD AND C. H. WADLEIGH



shown that the capacity to produce well a t moderate levels of salinity

does not necessarily imply the ability to persist a t higher levels of salt

accumulation. This second criterion is especially useful to the agronomist

in comparing the performance of strains and varieties of a given crop.

Thirdly, the relative performance of a crop a t a given level of soil

salinity as compared to its performance on a comparable nonsaline soil

may be used as a criterion of salt tolerance. This method has certain

advantages over the previously mentioned concepts in that comparisons

between species are more readily evaluated. For example, although preference as to salt tolerance should be given to that variety of alfalfa

having the highest production on saline soil regardless of performance in

the absence of salinity, one could hardly compare salt tolerance in alfalfa

with that in cotton without taking into account the yielding power of

these respective crops when growing on comparable nonsaline soils.

Evaluating salt tolerance on the basis of relative yield will not necessarily result in the same order of classification as power of survival a t

high levels of salinity, but it will provide a more useful basis of appraising agronomic crops to be grown on moderately saline soil. I n variety

and strain testing, tshe data on relative yield should be supplemented by

data on absolute yield; ie., a strain may have a comparably poor relative

yield because of unusual vigor of growth on the nonsaline soil, and yet

yield the best of any of the strains a t the given level of salinity. Everything considered, defining salt tolerance on the basis of relative yield to

that of the nonsaline condition is to be preferred for general agronomic

use.

I n discussing the physiological basis for the various degrees of salt

tolerance which prevail among crop plants, it may be helpful to consider

the characteristics of the natural halophytes. I n a review of this group

of plants, Uphof (1941) discusses the physiological characteristics of

halophytes, but it is apparent that the specific physiology of these plants

is not well known. The early investigators concluded that halophytism

was essentially xerophytism, since both halophytes and xerophytes are

adapted physiologically or anatomically to a scarcity of water. Anatomical studies, such as those of Chermezon (1910), later revealed that

the two groups of plants must be regarded as distinct physiologically.

Halophytes tend to have relatively high values for the osmotic pressure

of the tissue fluids. Fitting (1911) used an indirect method to measure

the osmotic pressure of the cell contents of various species of plants on

the North African Desert. The highest osmotic pressures, 100 atmospheres or above, were found in plants growing on dry or highly saline

soils. Those growing on moist nonsaline soils had osmotic pressures of

10-20 atm. The osmotic pressure of the various species tended to vary



PLANT GROWTH ON SALINE AND ALKALI SOILS



7



with the physiological scarcity of water in the environment in which the

plants were growing. This generalization has been verified by Harris

et al. (1916, 1924), Keller (1920) and others. There may be a wide

variation in the osmotic pressure of the tissue fluids depending on the

environmental stress under which i t is growing. Harris et al. (1924)

found variations in the osmotic pressure of the tissue fluids of leaves of

Atriplex confertifolia from 31.2 to 153 atm. ; in Allenrolfeu occidentalis

from 22.5 to 61.8 atm.; in Sarcobatus vermiculatus from 22.7 to 39.8 atm.;

and in Salicornia utahensis from 36.8 to 51.9 atm.

'Much of the variation in osmotic pressure of the tissue fluids was

found to be associated with variations in chloride content, but not all of

it. Keller (1925) observed that some halophytes may regulate the salt

content of their tissue fluids somewhat independently of the salinity of

the environment-. Salicornia may contain a lower concentration of

sodium chloride than exists in the soil, or i t may accumulate NaCl far

above the concentration of the soil, depending on the degree of soil

salinity. Iljin (1922, 1932) states that only those plants should be considered halophytes whose protoplasm is resistant to relatively high accumulations of sodium ions in the cell sap. Thus, halophytes may be

described as having a t least three attributes which are important to

their survival on saline soil; (a) the capacity to develop rather high

osmotic pressures of the tissue fluids in counteraction to the increased

osmotic pressure of the substrate; (b) the capacity to accumulate considerable quantities of salts in the tissue fluids and to regulate that

accumulation; and (c) a protoplasm which is characteristically resistant

to the deleterious effects of accumulations of sodium salts in the cell sap.

Application of the above criteria to an evaluation of the relative salt

tolerance of economic crops is not sharply defined, and the varying

physiological responses of different crop plants to saline soils prevent any

generalization. Brown and Cooil a t the U.S. Regional Salinity Laboratory found in 1947 that the osmotic pressures of the tissue fluids of alfalfa

tops were 12.3, 14.5, 17.9, and 19.9 atm. when grown on artificially

salinized soils in which the average osmotic pressures of the soil solutions

were 0.9, 4.2, 6.6, and 8.2 atm. respectively. Thus, even though there

was but little variation in the net osmotic gradient between soil and

plant tops, there were marked reductions in yield. If the yield on the

control plot that had 0.9 atm. osmotic pressure in the soil solution be

taken as 100 per cent, the yields on the other plots were 62.5, 32.4, and

21.5 per cent respectively. That is, the marked reduction in yield did.

not reflect the relative constancy in osmotic gradient. The increase in

osmotic pressure of the tissue fluids of the tops of these alfalfa plants

could be largely accounted for by the increase in chloride salts in the



8



H. E. HAYWARD AND C. H. WADLEIGH



tissue fluids. Alfalfa is regarded as one of thc more salt tolerant crops,

and the theory could he advanced that its salt tolerance is related to thc

intake of salt and the resiiltant increase in osmotic pressure of the tissue

fluids as the salinity of the soil is increased. Such a theory could not

be applied to certain other forage crops.

Ayers and Kolisch * determined the osmotic pressure of the expressed

sap of seven different leguminous forage plants grown on soil irrigated

with water containing 0, 2500, 5000, and 7500 p.p.m. of added salts.

Observations on red clover, Trifolium pratense, harvested in July showed

osmotic pressures of the expressed sap of 11.5, 20.6, and 23.7 atm. respectively, for the first t.hree treatments. The most saline irrigation

water, 7500 p.p.m., killed the plants. By August, the plants irrigated

with water containing 5000 p.p.m. of salt were killed, and by September

only one or two plants survived that were irrigated with water containing

2500 p.p.m. added salts. All control plants survived but they did not

thrive during the hottest part of the summer. Thus, red clover showed

very poor salt tolerance, yet the increase in the osmotic pressure of the

tissue fluids for a given increase in salinity of the substrate was greater

than that observed for alfalfa. This suggests that capacity to adjust

internal osmotic pressure with respect to the substrate may be a poor

criterion of salt tolerance. It is pertinent to note that for comparable

levels of salinization, the expressed sap of red clover contained nearly

three times as much chloride as that of alfalfa. It,appears that red clover

plants were capable of effecting internal osmotic adjustments to compensate for the external increase in salinity, but the protoplasm of these

plants was not sufficiently resistant to the deleterious effects of the ions

so accumulated.

I n this connection, the observations of Ayers and Kolisch * on two

species of trefoil are of interest. The osmotic pressure of the expressed

sap of the herbage of birdsfoot trefoil, Lotus corniculatus var. TENNUIFOLIUS, which is a very salt tolerant legume (Ayers, 1948) was 12.0, 16.6,

17.3, and 19.1 atm. respectively for the same qualities of irrigation water

used on red clover. Comparable values for big trefoil, Lotus uliginosus,

were 10.6, 16.9, 18.4, and 21.9 atm. osmotic pressure. There was a greater

internal adjustment in osmotic pressure over a range of soil salinization

in big trefoil than in birdsfoot t.refoil, yet the big trefoil showed relatively

poor salt tolerance. At a given level of salinity, however, the expressed

sap of the herbage of big trefoil contained nearly twice as much chloride

as did the birdsfoot trefoil.

*This, and subsequent references in which the author’s name is followed by an

asterisk, relate to unpublished data obtained at the US. Regional Salinity Laboratory.



PLANT GROWTH ON SALINE AND ALKALI SOlLS



9



Additional evidence available on other economic crops (see below)

indicates t.hat the salt tolerance of a given species depends upon three

attributes: ( a ) the capacity to increase the osmotic pressure of the tissue

fluids to compensate for increases in osmotic pressure of the substrate;

(b) the capacit,y to regulate the intake of ions so as to bring about the

increase in osmotic pressure and yet avoid an excess accumulation of

ions, and (c) the inherent ability of the protoplasm to resist deleterious

effects of accumulated ions. These are the same three attributes that

were stipulated as essential for halophytism. It is apparent that the

main deficiencies of economic crops which lack salt tolerance are the

inabi1it.y to regulate adequately the intake of salt and the specific sensitivity of their protoplasm to accumulations of salt within the tissues.



IV. PHYSIOLOGICAL

BASISOF ALKALITOLERANCE

Very little is known concerning the physiological basis for the tolerance of plants to alkali soils. There appears to be considerable variat*ion

among halophytes as to their tolerance to alkali as contrasted with salinity. Hilgard (1906) points out that Allenrolfea occidentalis and Salicornia subterminalis are two of the most salt tolerant halophtes, but their

tolerance to “black alkali” (alkali) is relatively poor. On the other

hand, Sarcobatus vermiculatus and Sporobolus airoides are also highly

salt tolerant, and have a remarkably high tolerance of “black alkali.”

I n evaluating tolerance of plants to alkali soils distinction must be

made as to whether the soil is (a) high in exchangeable sodium but having a moderate pH, (b) high in exchangeable sodium, but with a pH

of 8.5 or above, and (c) high in exchangeable sodium but with a considerable accumulation of titrat,able carbonate. The latter condition represents the status in “black alkali” soils as described by Hilgard (1906).

Although concrete evidence is very meager, it may be inferred that

tolerance of a species to high percentages of adsorbed sodium is modified

by the pH of the soil and the accumulation of soluble carbonate.

Breazeale (1927) concluded from his studies, however, that sodium carbonate occurs in “black alkali” soils in insufficient concentration to be

toxic. Thus, the infertility of most of these soils must be sought in their

poor permeability to water and to other nutritional disturbances.

Ratner (1935, 1944) presents evidence that plant growth is inhibited

on high-sodium soils owing to availability of calcium. Hence, tolerance

to soil alkali may involve the capacity by the plant to secure an adequate

supply of calcium under conditions of relatively low availability. Bower

and Wadleigh (1948) studied the influence of various levels of exchangeable sodium upon growt,h and cationic accumulation by dwarf red kidney

beans, garden beets and Rhodes and Dallis grasses under controlled cul-



10



H. E. HAYWARD AND C. 13. WADLEIGH



tural conditions in the greenhouse. The culture media consisted of a

mixture of sand and synthetic cation- and anion-exchange resins (“Amberlites”) containing the desired amounts of various cations and anions

in adsorbed form. Adsorbed K, H2P04,NOs and SOr were supplied in

constant amounts to all cultures, the potassium making up 10 per cent

of the cation exchange capacity. Six levels of exchangeable sodium, wiz.,

0, 15, 30, 45, 60, and 75 per cent of the cation exchange capacity, constituted the treatments. The remainder of the cation exchange capacity

was satisfied by calcium and magnesium, the Ca:Mg ratio being 3 : l .

The p H value of all cultures was approximately 6.5.

The tolerance of the different species to the presence of exchangeable

sodium in the substrate varied greatly. Beans were found to be especially sodium-sensitive. Growth of this species was markedly decreased a t

exchangeable-sodium-percentages as low as 15 and almost completely

inhibited a t the three highest levels of sodium employed. I n sharp contrast with the data for beans, Rhodes grass and garden beets were found

to be very sodium-tolerant. Significant reductions in the growth of these

species occurred only a t the highest level of sodium. The growth of

Dallis grass was not significantly lowered a t exchangeable-sodium-percentages of 30 or less but a t the higher sodium levels practically no

growth was obtained.

The Ca, Mg, K, and Na contents of the roots and tops of each species

were determined after harvest. Accumulation of Ca, Mg, and K by the

plants as a whole tended to decrease and that of sodium to increase

progressively as higher proportions of exchangeable sodium were supplied.

The magnitude of the decreases in Ca, Mg, and K accumulation and the

extent of sodium accumulations varied greatly among the species studied

and between the roots and top parts of the plant. These observations

suggest the possibility that the species that are more tolerant to high

levels of exchangeable sodium are the ones which normally take in considerable amounts of sodium, whereas the more sensitive species are the

ones which normally tend to exclude sodium.



V. How SALINEAND ALKALISOILSAFFECTPLANT

GROWTH

Saline soils may affect plant. growth in two distinct ways: (a) the

increased osmotic pressure of the soil solution effects an accompanying

decrease in the physiological availability of water to the plant; and (b)

t.he concentrated soil solution may be conducive to the accumulation of

toxic quantities of various ions within the plant. Alkali soils may possess

three attributes, any one of which may seriously inhibit or entirely prevent plant growth: (a) the relatively high percentage of adsorbed alkali

cations on the exchange complex of these soils may effectively depress



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