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Chapter 3. Crop Responses To Chloride
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
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
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
parable to those of macronutrients. Understanding the diverse functions of
CI- in plants is critical to unders~ndingcrop responses to Cl-.
1. Deficiency Symptoms
Physiological C1- deficiency symptoms in plants grown in nutrient solutions have been well characterized. Symptoms have been described for
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
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-
PAUL E. FIXEN
ward on the same plant. As the deficiency progressed, newly formed leaves
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
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
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
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).
PAUL E. FIXEN
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
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).
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
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
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
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
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
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.
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.
PAUL E. FIXEN
Plant Diseases with Reported Suppression Using CI- Fertilized
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
Gray leaf spot
Hollow heart, brown center
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