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Chapter 3. Crop Responses To Chloride

Chapter 3. Crop Responses To Chloride

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108



PAUL E. FIXEN



was topdressed with common salt for the purpose of stiffening the straw. In

his report, Tottingham concluded that NaCl served directly as a fertilizer

and that C1- was the active element.

In 1954, Broyer et al. (1 954) offered sufficiently convincing evidence to

cause the general acceptance of C1- as a plant essential element. However,

for over 20 years it was generally believed that field-grown crops would not

benefit from C1- additions because of the ubiquitous presence of C1- in the

environment.

The potential role of C1- in cropping systems was not seriously considered until the 1970s, when research in the Philippines (von Uexkull, 1972),

Europe (Russell, 1978), and the northwestern United States ( Powelson and

Jackson, 1978) clearly showed that C1- could play an important role in

crop management. These studies stimulated interest by other researchers to

further investigate the role of C1- in crop management.

Yield increases from application of C1- were verified in the field during

the 1980s. In Oregon, yields of soft white winter wheat (Tritium aestivum

L.) fertilized with NH,Cl exceeded those of ( NH,)tSO,-fertilized wheat by

1243 kg grain ha-' averaged across three sites (Chnstensen et al., 1981). In

South Dakota, CaCl, and KC1 applications produced the same hard red

spring wheat yield at equivalent C1- rates, and both produced 240 kg ha-'

more grain than KN03 when averaged across 5 site-years (Fixen et al.,

1986b). Barley grain yield with KCl applied exceeded an equivalent rate of

K as K2S04by 299 kg ha-' at one of five sites in North Dakota (Timm et

al., 1986).

The numerous field responses to C1- likely involve the unique roles C1plays in plants, beyond biochemical functions. Such roles include osmoregulatory functions, plant development, and interaction with other nutrients and diseases. Critical C1- concentrations for these roles appear to be

much higher than for biochemical functions.



11. CHLORIDE IN PLANTS



Chloride has a diverse set of functions in plants. Some involve intracellular processes that are very specific while others involve how plants interact with the environment. Because the biochemical functions of C1- require no more than 100 mg kg-', C1- is classified as a micronutrient.

However, much higher concentrations, in the range of 2000-20,000 mg

kg-', are normally present in plants, indicating that C1-, unlike other

micronutrients, is relatively nontoxic at high concentrations. In fact, many

of the nonbiochemical roles of C1- appear to require concentrations com-



CROP RESPONSES TO CHLORIDE



109



parable to those of macronutrients. Understanding the diverse functions of

CI- in plants is critical to unders~ndingcrop responses to Cl-.



A. BIOCHEMICAL

FUNCTIONS

1. Deficiency Symptoms



Physiological C1- deficiency symptoms in plants grown in nutrient solutions have been well characterized. Symptoms have been described for

several species.

a. Tomato (Lycopersicon esculentum Mill.)

Chloride deficiency in tomato plants starts with a wilting of the leaflet

blade tips, followed progressively by chlorosis, bronzing, and necrosis

(Broyer el al., 1954). Lateral roots branch extensively but are stubby with

club tips (Johnson et al., 1957).

b. Spinach (Spinacia oleracea L.)

After 3-4 weeks in a solution without added CI-, shoot growth of

spinach was reduced 70% while root growth was reduced approximately

50% (Robinson and Downton, 1984). Leaves were smaller, narrower, had

curled edges, and appeared wrinkled. The stem and lower leaf petioles had

prominent accumulation of a n t h ~ y a n i n while

s

the leaf blades had higher

chlorophyll levels. Roots were smaller and had a “herring bone” appearance.



c. Sugar Beets (Beta vulgaris L.)

A “hemng bone” root pattern was also observed on C1-deficient sugar

beets (Terry, 1977). Young leaves developed small yellowish spots with

interveinal areas remaining greener than areas close to veins. Some c u p

ping of the leaf blade at the midrib was noted. Johnson ef al. (1957)

observed premature wilting due to C1- deficiency.

d. Lettuce (Lactuca saliva L.)

The first symptom observed by Johnson et al. (1957) was wilting. Restricted root growth with stubby, club-tipped laterals was prominent.

e. Cabbage (Brassica oleracea L.)

Increased va~abilityamong plants was apparent as was pr~maturewilting of the tips and margins (Johnson et al., 1957). Slowed expansion of the

inner leaf sections caused cupping. Leaves cupped both upward and down-



110



PAUL E. FIXEN



ward on the same plant. As the deficiency progressed, newly formed leaves

became chlorotic.

f. Barley (Hordeum vulgare L.)

The most distinctive symptom was general chlorosis of newly emerging

leaves (Johnson ef al., 1957). The leaves remained wrapped in tubular

form longer than normal, were slower growing, smaller, more fragile than

normal leaves, and eventually became necrotic.

g. Alfalfa (Medicago sativa L.)

Leaflet tips were split along the midrib and irregular chlorotic blotches

appeared in the leaflet center (Johnson ef al., 1957).

h. Corn (Zeu mays L.)

Premature wilting was the only deficiency symptom observed by Johnson et al. (1957).

i. Beans (Phaseolus vulgaris L.)

Increased nastic movement of the leaves was the only symptom observed

by Johnson ef al. (1957). Leaves of Cl--deficient plants were notably more

oriented toward the sun at sunset.

j. Potato (Solanurn tuberosurn L.)

The symptom first observed was a lighter green color, with new growth

tending to have a “pebbled” appearance described as vertical protrusions

on the upper side of leaflets (Gausman et al., 1958a). As the deficiency

intensified, margins of terminal leaflets curled upward and chlorosis developed on terminal leaflet tips and eventually extended approximately onefourth of the distance down the leaflet margins. A “purplish bronzing” of

the older chlorotic areas was the final symptom to develop.



k. Coconut Palm (Cocus nucifera L.)

Trees deficient in C1- were reported to have older leaves with yellowing

and/or orange mottling and dried up leaf tips and edges (von Uexkull and

Sanders, 1986). Also noted were reduced growth rates, fewer nuts set,

reduced nut size, droopy leaves, signs of moisture stress, and stem cracking

and bleeding.

2. Photosynthesis



The direct role of CI- in photosynthesis has been the focus of numerous

studies since Warburg and Luttgens (1944) showed that 0,evolution by



CROP RESPONSES TO CHLORIDE



111



isolated chloroplasts required CI-. Other researchers concluded that CIwas involved in the splitting of water molecules in photosystem I1 (Izawa et

al., 1969) and, more specifically, CI- acts as a cofactor of an NH,OH-sensitive, Mn-containing, 02-evolving enzyme (Kelley and Izawa, 1978).

Using isolated thylakoids, Critchley et al. (1982) suggested that CI- facilitates electron transport by reversible ionic binding to the 0,-evolving

complex or to the thylakoid membrane. A later study by Critchley (1983)

offered evidence that this effect was indeed due to CI- and not to the

accompanying cation.

Terry ( 1977) questioned the methodology involved in investigations of

the role of CI- in photosynthesis. He concluded that the in vitro photosynthesis responses to CI- may not have a physiological basis but instead may

be due to the isolated condition of the chloroplasts. The principal cause of

a 60% reduction in sugar beet growth from CI- deficiency in his study was

lower cell multiplication rates in leaves, not a reduction in in vivo photosynthesis rates.

It was shown by Robinson and Downton (1984) that the C1- content of

chloroplasts is highly regulated. These authors reported a 70% reduction in

spinach growth and leaf CI- concentration due to CI- deficiency, but no

significant change in CI- concentration in the chloroplasts. Thus, photosynthetic rates may not be affected even though CI- is severely deficient.

Robinson and Downton suggest that Terry (1977) measured a reduction in

CI- concentration in isolated chloroplasts due to the unreliability of the

nonaqueous technique used.

The characteristic compartmentalization of CI- and accumulation in

chloroplasts under deficient conditions would result in a very low critical

leaf CI- concentration for photosynthesis. The severely CI--deficient spinach with unaffected photosynthetic rates reported by Robinson and

Downton (1984) had a leaf CI- concentration of 131 mg/kg dry weight.

The agronomic critical level of C1- in crop plants is clearly set by processes

other than photosynthesis.

3. Enzyme Activation



Several enzymes are known to be stimulated by CI-. ATPase, located on

tonoplasts and other sealed vesicles, appears to be stimulated by C1-,

mostly by a direct effect on the enzyme but partly via dissipation of

electrical potential (Churchill and Sze, 1984). This stimulation is preferentially inhibited by nitrate. a-Amylase, the enzyme that hydrolyzes starch to

sugars, requires CI- for activation (Metzler, 1977). Asparagine synthetase is

also stimulated by CI- (Rogness. 1975).



112



PAUL E. FIXEN



B. OSMOREGULATORY

FUNCTIONS

The ability of CI- to move rapidly across cell membranes and its biochemical inertness are two important properties that allow CI- to serve as a

key osmotic solute in plants (Maas, 1986). Chloride serves in this capacity

at relatively low energetic cost to the plant (Sanders, 1984). When C1- is in

short supply, plants may use more energy-costly organic salts for turgor

control. Chloride is located primarily in the cell’s central vacuole as a

component of a simple salt solution involved in cell expansion.

1. Counterion for Cation Transport



Because CI- is very mobile and tolerated at high concentrations, it is

ideally suited to maintain electrical charge balance when cations such as

K+ move across cell membranes.

2. Osmotic Adjustment



The process of osmotic adjustment occurs when solutes such as C1accumulate within a cell, causing the water potential within the cell to

decrease below the external potential. The resulting water potential gradient causes water to enter the cell and the plasmalemma to expand against

the rigid cell wall, resulting in an increase in cell turgidity.

Jensen and Tophoj (1985) conducted an outdoor pot study on barley

that showed that application of KCI increased leaf water content and

improved plant water status during soil water stress. Grain yield was highly

correlated with leaf water content. Tissue concentrations of K+ and CIwere increased similarly by the KCI additions, indicating that both ions

contributed to osmotic adjustment. Potassium chloride application to

wheat in a field study resulted in more negative plant water potentials

(Maurya and Gupta, 1984). Christensen et a/. (1981) showed in a field

study on winter wheat that NH,Cl application resulted in more favorable

plant water status than application of (NH,),SO,. The role of C1- in

reducing the effects of moisture stress under field conditions is still uncertain and will be discussed in more detail in later sections.

3. Stomata1 Operation



Stomata open when water moves into the guard cells, causing them to

become more turgid. The influx of water is caused by an increase in solute

concentration, which in turn causes the intracellular water potential to



CROP RESPONSES TO CHLORIDE



113



become more negative. Usually the major solutes involved are K+, C1-,

and malate (Maas, 1986).

If CI- is in short supply, malate typically is synthesized within the guard

cells from starch produced in chloroplasts and is used as a balancing ion

(Allaway, 1981). However, Assman and Zeigler (1986) have made calculations that indicate there may be no energetic advantage to using C1- as the

balancing ion instead of malate.

The amount of C1- used as a counterion to balance K+ vanes with

species and the level of C1- in the environment. In his review of physiological response to C1-, Maas (1986) illustrated differences among species in

counterion dependency as follows: onion (Allium cepa L.), an absolute

requirement for C1- (Schnabl and Raschke, 1980); broad bean (Viciafaba

L.), mostly organic acids (Outlaw and Lowry, 1977);corn and Commelina,

both CI- and malate (Penny et al., 1976; Ratschke, 1975). Palm trees were

at one time thought to be similar to onion in having an absolute requirement for C1- because their guard cells appeared devoid of chloroplasts (von

Uexkull and Sanders, 1986). von Uexkull and Sanders (1986) suggested

that some of the CI- deficiency symptoms discussed earlier, such as frond

breakage and stem bleeding, were due to improper stomata1 functioning.

However, von Uexkull (1985) reported results showing that many palm

guard cells contain both chloroplasts and starch and that the physiological

role of C1- in palms is still not fully understood.

4. Leaf Movement



Orientation adjustments of leaves are due to turgor changes of motor

cells (Satter and Galston, 1981). Such changes appear to be made by the

same mechanism discussed earlier for guard cells. Researchers have noted

that leaf orientation on wheat in field plots can be influenced by C1treatment (Taylor and Jackson, 1980).



C. UPTAKE

AND INTERACTION



WITH OTHER

NUTRIENTS



Plants absorb CI- from the soil solution via at least two mechanisms. For

barley, the first mechanism approaches its maximum uptake rate at C1concentrations near 0.1 -0.2 mM. The second mechanism operates only at

CI- levels of 0.5 mM and above (Elzam and Epstein, 1965). Uptake is

likely metabolically controlled and sensitive to temperature and metabolic

inhibitors. Chloride uptake by excised corn root segments increases with

decreasing pH down to a pH of 5.5, due possibly to the involvement of a



114



PAUL E. FIXEN



protonated camer (Lin, 1981). Illumination increases uptake due to an

increased ATP supply that serves as an energy source for the active uptake

process (MacDonald et al., 1975). Uptake of CI- is competitively inhibited

by Br-, NO,-, and

but not by F or I- (Elzam and Epstein, 1965;

Murarka et al., 1973; Mengel and Kirkby, 1987).

1. Nitrogen Interactions



Nitrogen and C1- interact via several mechanisms including both soil

and plant processes. Rates of some steps in the mineralization of soil

organic matter are affected by C1-. This can influence the form of N

absorbed by crop plants. At the root surface, NO,- and C1- ions are known

to compete with each other in the uptake process.

a. Nitrification Inhibition

Numerous studies have demonstrated that C1- inhibits nitrification in

acid soils (Hahn et al., 1942; Agarwal el al., 197 1 ; Heilman, 1975; Golden

et al., 1979; Christensen and Brett, 1985; Roseberg et al., 1986). Christensen and Brett (1985) reported that in laboratory studies of other investigators, concentrations of 46- 152 mg C1- kg-I soil were necessary for measurable nitrification inhibition. In their own studies of unlimed soil (pH

5 . 9 , where NH,Cl had been applied and inhibition of nitrification occurred, concentrations in the surface 10 cm of soil were 235 and 17 1 mg

C1- kg-I soil, respectively, 1 and 7 weeks after application. Under these

conditions the NH4+ N:NO,- N ratio remained above 3: 1 for 12 days

longer with NH,Cl than with (NH,)2S04. Increasing soil pH to 6.6 with

lime eliminated the nitrification inhibition properties of the C1- treatments.

Most studies evaluating the effects of CI- salts on nitrification were not

designed to separate general osmotic effects from effects specific to C1-.

However, Roseberg et al. ( 1986) determined that at any given soil solution

osmotic potential, CI- salts resulted in a lower rate of nitrification than did

SOZ-salts. They concluded that inhibition of nitrification by CI- salts is

due to a combination of specific C1- ion effects and low soil osmotic

potential. They also suggested that the mechanism of inhibition could

involve either direct or indirect effects of C1- on nitrifying organisms.

b. Form of N Effects

Effect of N form on C1- response by take-all-infected soft white winter

wheat in Oregon was evaluated by Taylor et al. ( 1983) at two planting dates

and soil pH levels of 5.6, 6.0, and 6.2. Yield response to 40 kg C1- ha-’

applied in the fall as KCI was nonsignificant (p = 0.05) when (NH,)2S0,

was the N source, regardless of soil pH and seeding date. When Ca(NO,),



CROP RESPONSES TO CHLORlDE



11s



was the N source, chloride response averaged 1370 kg ha-' for pH levels of

6.0 and 6.2 at the late seeding date. Chloride response was nonsignificant at

other pH and planting date combinations. Split application of a much

higher rate of C1- (86 kg ha-' fall 342 kg ha-' spring) as NH,Cl produced 1550 kg ha-' more grain than (NH,),SO, at the late planting date

and pH of 5.6.

Imgated hard red spring wheat in Montana inoculated with the take-all

fungus at a soil pH of 7.9 showed greater yield response to Cl- fertilization

when ammoniacal N forms were applied than when NO3- was used (Engel

and Mathre, 1988). Grain yield response to 45 kg C1- ha-' as NaCl with

NH,+ was 5 17 - 869 kg ha-' while it was nonsignificant where no N was

applied or when NaNO, was used.



+



c. Competition with NO3Nitrate and C1- compete with each other for uptake in many species

(Harward ef al., 1956; Meyer ef al., 1957; James et al., 1970a; Murarka et

al., 1973; Fuqua et al., 1974; Glass and Siddiqi, 1985; Christensen and

Brett, 1985; Coos ef al., 1987). Increasing the supply of either one tends to

reduce the tissue concentration of the other. Murarka ef al. (1973) showed

in a greenhouse study that even though applications of 100 or 200 mg C1kg-' soil reduced NO3- concentrations in the potato plant, they did not

reduce the amount of protein or dry matter yield. However, the authors

pointed out the potential problems that varying soil C1- levels could have

on use of plant NO,- levels as a diagnostic tool for N management.

Under some conditions, positive interactions between NO,- and C1have been measured. Application of C1- increased N concentration in

leaves of coconut palm in the Philippines (von Uexkull and Sanders,

1986). At soil C1- levels greater than 19 mg kg-' soil, spring wheat C1concentrations in South Dakota increased with increasing soil NO3- levels

(Fixen ef al., 1987). Below 19 mg C1- kg-' soil, they decreased with

increasing soil NO,-. As in other studies, C1- application consistently

reduced plant NO,- concentrations.

2. Phosphorus Interactions



Chloride appears to interact with P in a complex manner. In some cases

P availability has been increased by elevated C1- while in other cases it has

been decreased or not affected.

a. Chloride Effects on P Uptake

Pot experiments with Caribou loam soil using ,*P on potatoes lead

Gausman ef al. (1958b) to suggest that an optimum or critical level of C1existed for maximum P uptake to occur, with uptake decreasing on either



1 I6



PAUL E. FIXEN



side of this level. In their studies the optimum level appeared to be 300 to

450 mg C1- kg-' soil. A later paper from the same research project, but

using potatoes grown in washed sand in the greenhouse, reported increased

32Pactivity at flowering with 100 mg C1- kg-' but reduced uptake above

this concentration. No significant effect was measured at harvest.

Solution culture studies using white clover (Trifoliurn repens L.) in

Australia suggested that an optimum C1- concentration exists for P uptake,

but that the optimum changes with solution P concentration (Rogan,

1977). The reduction in P uptake at C1- levels exceeding the optimum was

attributed to anion competition.

Soft white winter wheat infected with take-all responded to P when

applied with 428 kg C1- ha-' as NH4Cl and KCl but not when applied with

(NH.J2S04and 40 kg C1- ha-' as KCI (Taylor et al., 1983). The authors

suggested that the addition of P overcame the competitive inhibition of P

uptake caused by the high CI- concentration in the root zone.

Others have found no effect of C1- on P uptake. A study of 20-day-old

corn seedlings in solution culture at pH 4 showed no effect of CI- on 32P

uptake during a l-hr period (Carter and Lathwell, 1967). Similar results

were reported for lima beans by Kretschmer ef af. (1953).

b. Phosphorus Effects on C1- Uptake

Field experiments in central Washington on a slit loam soil naturally low

in C1- showed that application of P fertilizer had a pronounced synergistic

effect on uptake of C1- by potatoes from KCl applications (James et al.,

1970b). No effect of C1- on P uptake was measured.

Fine and Carson (1954) found that application of P fertilizer in both

greenhouse and field experiments alleviated salt injury symptoms of oats

and barley growing in saline soil and gave marked yield increases. They

suggested that the function of P may have been to reduce the excessive

quantities of CI- and sulfate accumulating in leaves. It is feasible that at

low soil C1- levels, P application may tend to increase response to CIadditions, whereas at very high soil C1- levels (as encountered in some

saline soils), P application may reduce CI- uptake and the negative effects

of high salts. The literature is not clear on these interactions.

3. Manganese Interactions



Application of chloride-containing salts to acid soils has increased the

Mn concentration of plants. Researchers in Oregon studied CI effects on

Mn uptake by bush beans and sweet corn in poorly drained soils (pH

values from 4.7 to 5.3) that contained Mn concretions (Jackson et al..

1966). Chloride application increased Mn uptake of both crops and re-



CROP RESPONSES TO CHLORIDE



117



sulted in Mn toxicity symptoms on trifoliate leaves. The authors offered

Mn toxicity as a possible explanation for the yield reductions on acid soils

sometimes observed from band application of KCl at planting time.

Additional laboratory studies by the Oregon researchers explored the

mechanism by which CI- increases soil Mn availability (Westerman er af.,

1971). They suggested that CI- enhances the reduction of some Mn oxides

in acid soils and in the process increases extractable Mn.

Potassium chloride increased Mn release from 14 soils collected from six

countries located in temperate and subtropical regions (Krishnamurti and

Huang, 1987). Release from the calcareous vertisol was the lowest even

though its total Mn content was the highest. The researchers indicated that

redox, complexation, and exchange reactions appeared to be involved with

the observed Mn release.

Lindsay ( 199 1 ) calculated the solution species of Mn in equilibrium with

manganite and pyrolusite at a pe pH of 16.6 when CI- and SO:- are at

0.001 A4 and CO, is at 10-3.52atm. Under these conditions Mn2+ is the

dominant solution form. However, the activity of MnCP increases 10-fold

with every 10-fold increase in CI- activity. Therefore, when the CI- activity

increases to approximately 0.25 M the activities of MnCI+ and Mn2+ are

equal. At 0.025 M Cl-, MnZ+ activity would be 10 times the MnCP

activity.

If a volumetric soil water content of 0.25 is assumed, 0.025 M C1- is

equivalent to I68 mg kg-I soil CI- or 302 kg CI- ha-' distributed evenly in

the top 15 cm of soil. If the same 302 kg ha-' were mixed with only 10%of

the top 15 cm of soil, the MnCP activity in the fertilized zone could be

equal to the Mn2+activity and essentially double the quantity of total Mn

in solution. Therefore, it would seem that fertilization with CI- could

significantly increase Mn availability via direct complexation. However,

this effect would be temporary due to the mobility of CI- in soil.



+



D. DISEASE

INTERACTIONS

In recent years, the most studied effects of C1- on crop plants have been

those that relate to crop diseases. Chloride application has suppressed or

reduced the effects of numerous diseases on a variety of crop species. A

partial list of such occurrences is given in Table I.

Although CI- interactions with crop diseases are well documented, the

mechanisms involved in these effects are not well defined. Generally,

proposed mechanisms fall into two categories, either suppression of the

pathogen or an increase in host tolerance. In the following discussion a

third category will be included for those situations where no disease-related

effect on the pathogen or the host was detected.



118



PAUL E. FIXEN

Table 1



Plant Diseases with Reported Suppression Using CI- Fertilized

Crop



Diseases



Winter wheat

Spring wheat

Barley

Durum wheat

corn

Pearl millet

Coconut palm

Potatoes

Celery

Rice



Take-all root rot, tanspot, stripe rust, Septoria, leaf rust

Common root rot, tanspot, leaf rust, Septoria

Common root rot, Fusarium root rot, spot blotch

Common root rot

Stalk rot

Downy mildew

Gray leaf spot

Hollow heart, brown center

Fusarium yellows

Stem rot, sheath blight



Adapted from Fixen ef a/. ( 1987).



1. Suppression of Crop Pathogens



a. Take-all Root Rot

Numerous studies across diverse soils have demonstrated suppression of

the wheat take-all fungus (Gaeumunnomyces gruminis var. tritici) by ammoniacal fertilizers. In a review paper, Powelson et af. (1985) cited 12

references that document such suppression, with the first published in

1941. The mechanism most often suggested is that plant uptake of NH4+

instead of NO,- decreases the pH of the rhizospere. Christensen and Brett

(1 985) used data from the literature to determine that an NH4 N :NO, N

ratio of 3: 1 or greater is required for rhizosphere acidification to occur.

The reduced pH offers a competitive advantage to acid-tolerant microorganisms, such as the fluorescent Pseudomonas, and decreases the growth of

G. gruminis hyphae along the root (Cook, 1981).

The nitrification inhibition properties of C1- discussed earlier may be

partially responsible for the suppression of take-all by C1- (Christensen and

Brett, 1985). The studies leading the Oregon researchers to this hypothesis

showed that C1- application slowed nitrification, reduced take-all severity,

and increased winter wheat grain yield especially on moderately acid soils.

Take-all incidence (percentage of plants infected) was unaffected by C1- in

these studies (Christensen el af., 1987).

Powelson et uf. (1985) have suggested that water stress could reduce the

efficacy of NH4+ and C1- fertilizers for take-all control. They referred to

reviews by Cook ( 1981), Rovira and Wildermuth ( 198l), and Schroth and

Hancock ( 1982), pointing out that root-colonizing epiphytic bacteria do



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