Plant and Soil 8 9 , 2 5 3 - 2 7 1 (1985). 9 1985 Martinus NijhoffPublishers, Dordrecht. Printed in the Netherlands.

Crop production and management u n d e r

saline

Ms. BR 17

conditions*

A. ME1RI and Z. P L A U T

Inst. o f Soils and Water, ARO, Bet-Dagan 50250, Israel

Key words Climate CO 2 concentration Drip Fertilization Salt tolerance Salinity Sprinkler Stand

Irrigation interval Leaching

Summary This review evaluates m a n a g e m e n t practices that m a y minimize yield reduction under saline conditions according to three strategies: (I) control of root-zone salinity; (II) reduced damage to the crop; (III) reduced damage to individual plants. Plant response to salinity is described by an u n c h a n g e d yield up to a threshold soil salinity (a), then a linear reduction in relative yield (b), to a m a x i m u m soil salinity that corresponds to zero yield (Y0). Strategies I and II do not take into consideration any change in the parameters of the response curve, while strategy Ili is aimed at modifying them. Control of root zone salinity is obtained by irrigation and leaching. F r o m the review of existing data it is concluded that the effective soil salinity parameter should be taken as the m e a n electrical conductivity of the saturated paste extract or o f the soil solution over time and space. Several irrigation and leaching practices are discussed. It is shown that intermittent leaching is more advantageous than leaching at each irrigation. Specific cultivation and irrigation practices that result in soil salinity reduction adjacent to y o u n g seedlings and the use of water of low salinity at specifically sensitive growth stages m a y be highly beneficial. Recent data do not show that reduced irrigation intervals improve crop response m o r e u n d e r saline than u n d e r nonsaline irrigation. Alternate use of water of different salt concentrations results in mixing in the soil and the crop responds to the m e a n water salinity. Reduced damage at the field level when soil or irrigation water salinity is too high to maintain full yield o f single plants requires a larger crop stand. For row crops reduced inter-row spacing is more effective than reduced intra-row spacing. Reduced damage at the plant level while the salinity tolerance of the plants remains constant shows up in the response curve parameters as larger threshold and slope and constant salinity at zero yield. This is the effect o f a reduced atmospheric water d e m a n d that results in reduced stress in the plant under given salinity. Management can also change the salt tolerance o f the crop. This will show up as higher salinity at zero yield, as well as changes in threshold and slope. Such changes in the response curve were f o u n d at different growth stages, under different atmospheric CO2, under different fertilization, and when sprinkler irrigation was compared with drip irrigation.

Introduction Sound agricultural management consists of a number o f practices aimed at optimizing crop production. These practices may need to be modified and new ones introduced when brackish water is employed. A comprehensive list which relates yield to total soluble salts in the root medium was compiled for many crops by Maas and H o f f m a n 29. These data are, however, not suitable for specific salt damage resulting from canopy wetting with saline water under sprinkler irrigation, or * Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. l l l l - E 1984 series. 253

254

MEIR1 AND PLAUT

for specific toxic effects. Most of these data were obtained from experiments in which salinity was imposed after seedling establishment and do not necessarily apply to germination and the early seedling stages. The plots were rapidly salinized to obtain salinity that was essentially uniform with depth by irrigation with water of different salinity levels at a high leaching fraction (LF). Therefore, after the rapid initial salinization, temporal and spatial variations in salt content in the soil profile were small. Because of frequent irrigations, fluctuations in soil solution salinity (ECsw) and soil matric potential ( ~ m ) w e r e also minimized. Optimum fertilization for nonsaline conditions was used, assuming no interaction between the effects of fertilization and salinity. To assemble data from experiments reporting different absolute yields and conducted in various soil types, both plant response and soil salinity were normalized, the former by using relative rather than absolute yields, and the latter by using the mean electrical conductivity of the saturated paste extract for the soil of the root zone (ECe). Thus, the possible effects of variations in salt content in the profile, solute chemistry, exchange reactions in the soil, and soil moisture management were not considered. The steady-state conditions and the normalization of the results enabled standardization of the reported data. However, this approach ignores the influence of different environmental conditions on the crop response. Knowledge and control of these are our management tools. This review examines management practices that may minimize damage under saline conditions. These practices were classified according to three strategies: (I) control of root zone salinity below harmful levels by irrigation (method, amount, interval), leaching (amount and timing), and special cultivation practices (pre-planting leaching, seedbed shape, and placement of seeds); (If) accepting damage at the single-plant level and changing cultivation practice (inter- and intrarow spacing of plants) to reduce salinity damage at the field level; and (lII) changing management factors to reduce salinity damage at the plant level (irrigation method, climatic conditions, atmospheric CO2 concentration, fertilizer application). Most of the work on management under saline conditions was devoted to the control of soil salinity. Information on the other aspects is scarce, was gained recently, and is much less understood. Selection and breeding for salt tolerance are not included in the present review. Control of root-zone salinity below harmful levels

The control of root-zone salinity is mostly the outcome of suitable irrigation and leaching management. The salt content of a given soil

CROP M A N A G E M E N T AND SALINITY

255

volume at time T (ST) is the sum of the initial salt content (So) and the salt introduced (CiVi) minus the salt removed (CaVd): Sw

:-

(1)

S 0 at- C i W i - - C d W d

where C -- concentration, V = volume, i = irrigation, d = drainage water. The change in concentrations and volumes between irrigation and drainage result mainly from evapotranspiration, which can, in practice, be considered a distillation process that leaves the salts behind. The relative volume of water that carries salts out of the root zone is defined as the leaching fraction (LF) 4s. The minimum LF that will keep the soil salinity below a required level is the leaching requirement (LR) 4s. The best available reference for this salinity level for various crops is the salt tolerance table presented by Maas and H o f f m a n 29. This table assumes a response curve consisting o f a plateau at m a x i m u m yield below a threshold salinity a and a linear descending line with slope b relating decreasing yield to increasing salinity, until zero yield is reached at a salinity o f Y 0 (Fig. 1). The soil salinity corresponding to any relative yield can be obtained from equation (2): Y/Y,~,

-- 1 -- b(ECe -- a)

(2)

where Y = yield, Ym= = yield of nonsaline control, a = threshold, b -- slope. 100

0 .,J

uJ

50 1.1.1

I.J I.gl

ZERO LD

ECe

,-~

Fig. 1. Schematic response curve to salinity (after Maas and Hoffman29).

256

MEIRI AND P L A U T

The mean salinity (ECe) o f the saturated paste extract from the root zone was used as the effective salinity, i.e. that salinity parameter which is best correlated with the crop response. In 1973 Bernstein and Francois 3 challenged the use o f ECe and recommended using the weighted mean salinity of the absorbed water E'C~ as the effective salinity. They argued that the effective salinity is that against which water uptake actually occurs and that most of the water is absorbed from the soil volumes o f lower salinity. This approach, which seems very logical, was based on observations when the salt distribution in the soil was at steady state. Under these conditions, the roots may not explore the more saline soil volumes. However, under most practical field conditions, root growth and spatial and temporal variations in salinity occur simultaneously. The variations in salinity are, therefore, in part, imposed on already-developed root systems. The final c o t t o n root distribution (Fig. 2) for two qualities of water supplied by drippers spaced 5 0 c m from the plant axiQ 2 is the outcome o f non-steady-state conditions. It is evident that with nonsaline water most of the roots were evenly distributed between the plant stem and the dripper. When nonsaline soil was irrigated with saline water there was a high-salt front which moved, during the irrigation season, near the soil surface from the dripper towards the plant row. The larger horizontal c o m p o n e n t of root distribution below the plant

ECiw 8.0dSn~ I

0 15

45

I

ECiw I.OdSm I

123 6O

75

50

25

DISTANCE

0 75

FROM

50

EMITTER

25

0

(cm)

Fig. 2. Influence o f water quality on root distribution of fully mature cotton plants irrigated through drippers spaced 5 0 c m from the crop row. The n u m b e r e d contours indicate percent of roots in the soil by weight 32.

CROP MANAGEMENT AND SALINITY

257

row and its larger fraction at 3 0 c m depth were the result of root growth that by-passed the region o f the high-salt front. However, with the progress of the irrigation season the salt front moved further into the root zone and the major part o f the root system was exposed to this salinity. Such variations in salinity, which are imposed on alreadydeveloped root systems, suggest that the mean root-zone salinity is the important factor. Recent analysis of experiments in pots, lysimeters, and fields showed that the effective salinity is the temporal and spatial mean in the root z o n e 24'33 With flood and sprinkler irrigation, the roots occupy the entire soil volume to the rooting depth. With furrow and drip irrigation, only the wetted part of the soil volume occupied by roots should be considered as the root zone. The observations that the mean salinity is the effective one can be explained if the water status in the entire root zone determines the plant water status and yield. It is well d o c u m e n t e d that plants need to maintain full transpiration for full growth 1~ A higher rate o f water uptake by the roots exposed to lower salinity will compensate for the reduced water uptake by the roots exposed to higher salinity. F o r constant root resistance the plant water potential will decrease, resulting in reduced growth. The weighted mean water salinity approach also does not account for the influence that water uptake distribution may have on the matric c o m p o n e n t of the soil water potential. This point will be extended in the discussion on the effects of the interval between irrigations.

Timing of leaching To maintain a steady-state level o f salinity, leaching is necessary at every irrigation. Accepting the temporal and spatial mean root-zone salinity as the effective salinity may lead to a modified leaching practice. The adoption of L R as excess water to be added with every irrigation is most undesirable when saline water is introduced into a field o f lower salinity than the maximum allowed for the crop and salinity build-up occurs. Such management may aggravate the salinity effect as it will result in faster salinization. For a crop that is irrigated only a few times it may also result in a higher ECe value 4s (Fig. 3). N o t only the mean but also the maximum permissible salinity must be considered. Bernstein and Pearson 6 studied the response o f t o m a t o and pepper plants to various salinity regimes. They showed that pepper was less tolerant o f constant salinity, but could tolerate larger fluctuations in salinity than tomatoes. A build-up o f salinity in the root zone results from water uptake by the plants in the entire root zone and

258

MEIRI AND PLAUT

I0

I

I

1 LOW LF

0.0- 0.3 HIGH LF

TE

0.5- 0.6

t

U) m v

e.- . . . I~

I

/ I i I I

0

1

0 DAYS

I

l

20 OF

I

40 SALINE

60

IRRIGATION

Fig. 3. Salt accumulation in the soil (mean, O - 9 0 c m ) as a function of time, for two leaching fractions (LF) 4s.

evaporation mostly near the surface. The maximum salinity at the b o t t o m of the root-zone depends on the plants' ability to absorb water against the soil water osmotic potential. Thus, water uptake will stop when the osmotic potential o f the soil solution decreases to the minimum water potential that can develop in the plant roots. A conclusion based on observations with alfalfa was that this salinity value is that corresponding to zero yield on the crop response curve. Reduction in LF cannot increase salinity above this value 3,49 (Fig. 4). The evaporation from the soil surface may further concentrate the soil solution, resulting in a high salinity at the soil surface under furrow or drip irrigation. This high surface salinity may cause salinity shock when the surface salts are flushed through the root zone 2. Under management for maintaining steady-state salinity, it was first recommended to allow - at the b o t t o m o f the root-zone, where maximum salinity exists - the salinity corresponding to 50% yield reduction on the crop response curve. Recently, however, it was shown that leaching can be decreased to allow for a maximum salinity corresponding to zero yield. This reduced LR is the minimum leaching requirement 49. Under non-steady state, when leaching is delayed s' 16, this zero yield salinity can be stored in some portion o f the deeper soil volume that was previously occupied by roots with no significant yield reduction. Thus, leaching may be intermittent. When rainfall occurs it may take care of part of the leaching. The amount of irrigation water

CROP MANAGEMENT AND SALINITY

259

15

45 o C3 _J

I05

135 0

I

1

I0

20

__

I

30

40

ECsw d S m - I Fig. 4. Effect of reducing LF below salt balance limits for water of 2 dS m- ; on the build-up of salinity in the r o o t zone. Numbers near lines indicate the LF .9 .

available at specific seasons may also favor intermittent heavy leachings. Allowing higher salt accumulation in the root zone will result in higher salt concentrations in the drainage water and more effective leaching 7. The irrigation dose under saline conditions should satisfy both the ET and the LR. Since LR is usually a small fraction of the irrigation, a small error in the estimate of ET may introduce a considerable error in the intended extent o f leaching. The ET for a given crop under given conditions can be adjusted to evaporation demand by using a k n o w n pan evaporation factor or calculations based on meteorologic data. It is well documented that salinity reduces transpiration. Therefore, the estimation of ET according to these methods should be modified for saline conditions. A second way to estimate ET is to use the soil moisture deficit from field capacity (FC). This m e t h o d may introduce an error if a constant FC is assumed for different irrigation doses. For frequent irrigations it was shown both experimentally 2~ and by calculations 8 that the increase in irrigation depth intended to cause higher LF also results in an increase in the soil moisture content. This change in soil moisture content was shown theoretically 9 to be large enough to m o d i f y the yield response to salinity under non-steady-state conditions (Fig. 5). A n o t h e r error when using ET estimates based on soil moisture

260

MEIRI AND PLAUT

1.0

?

2.o

Q.

0.9 II Q.

>>-

0.8

0.7

0

2

4

E'-Ce

6

8

I0

12

( dSm -I )

Fig. 5. Relative crop yield (Y/Yp = T/Tp) as a function of electrical conductivity (ECe) and relative water application (numbers labeling the curve) 9.

deficit results from the fact that water is taken up during infiltration and redistribution. A comparison between t h e actual and estimated (from the soil moisture deficit) leaching volumes in a pot experiment 3s (Fig. 6) illustrates this error. The data indicate an increased underestimation o f the leaching with increased intended leaching. It is interesting to note that some drainage occurred in treatments that were aimed at having no leaching. The suppression o f ET by salinity applies also to this level o f irrigation. Therefore, the error will be larger at low salinity, when drainage is underestimated, and at high salinity, when drainage is overestimated. Table 1 presents data from a field plot experiment by Meiri and Shalhevet 37 that show the greater efficiency of intermittent leaching in the control o f soil salinity. The o u t c o m e o f these considerations is the approach already proposed by Ayers and Westcot2: 'under non-steady state, delay leaching and apply it when needed.' We would like to recommend the non-steadystate approach to leaching under most conditions. With such an approach one may reduce the amount of water needed for leaching and do the leaching at the best time, according to the salt tolerance o f the crop at different growth stages and the availability of water. Smaller drainage volumes result in increased salt precipitation and reduced soil

CROP MANAGEMENT AND SALINITY

E E (.9 Z

I

261

400

300

0 W _J

yo,2

200

a i,i

n-

I00

W

0

I00

200

INTENDED

300 LEACHING

400

500 (ram)

Fig. 6. Actual a m o u n t of leaching water compared with the intended leaching, assuming no water uptake during leaching, for two water qualities 3s.

Table 1. Influence of the number of leaching irrigations as a fraction of the total number of irrigations (R) on C1 accumulation in the root zone ( 0 - 6 0 cm) of flood-irrigated pepper for two water qualities (ECiw)(Meiri and Shalhevet 3~) ECiw (dSm- 1) 5.11

8.63

R

LF y

RCLC z

LF

RCLC

0:1 1:1 1 :7 1 : 13

0 0.31 0.15 0.21

1.00 0.76 0.47 0.36

0 0.26 0.24 0.23

1.00 0.82 0.42 0.68

y - Leaching fraction z - Relative chloride content of soil

weathering. Thus, the salt load of the drainage water is lower, and secondary salinization o f water resources is reduced (these aspects are not discussed in this review).

Special cultivation practices High salinity in the soil volumes closest to the plants during seedling emergence and the initial growth stages may affect the entire production. Therefore, leaching of the soil volume adjacent to the seed, which constitutes the entire root-zone for the seedling, is most important. This leaching may be most critical with furrow and drip irrigation,

262

MEIRI AND PLAUT

where high salinity levels build up near the soil surface at the wetting front 9,s0,52 Adjustment o f the soil surface contour and seeding or planting position (Fig. 7) according to the expected salt distribution is a rather simple solution with furrow irrigation 1,s~ With drip irrigation the same strategy is being used. Locating the emitters in a shallow slit in the soil alongside the seed rows will confine the water distribution and direct leaching of salts away from the plants (D. Pasternak and Y. De Malach, personal communication).

Irrigation interval Reduced growth under extended irrigation intervals and nonsaline conditions results mainly from a low matric potential o f the soil water. Under saline conditions, reduction in matric and osmotic potentials occur simultaneously. An experiment with guayule sl showed a stronger salinity effect with longer irrigation intervals. Therefore, increasing the irrigation frequency was r e c o m m e n d e d under saline conditions 1. Several experiments were conducted recently with alfalfa 3, eggplant 43, corn 44 and tall fescue 21 to study the effect o f irrigation interval on response to salinity. All experiments confirmed the conclusion that extended irrigation intervals may reduce absolute yields but did not support the second conclusion that it aggravates the relative salinity damage. With alfalfa the relative salinity damage was even smaller under a longer interval 3. SOIL SALINITY AT PLANTING TIME ( millimho= ) 4

16

8

SINGLE ROW BED

to germinate

DOUBLE ROW BED

SLOPING BED

Fig. 7. Various bed shapes and resulting salt accumulation zones, illustrating the best positions for planting'.

CROP MANAGEMENT AND SALINITY

263

Salinity reduces ET. Thus, in experimental comparisons o f the effect of irrigation interval with saline and nonsaline water, the salt-treated plots will retain more water than the controls between irrigations. This will moderate the rate of change in total soil moisture stress, resulting in reduced inhibitory effect o f the interval on growth 47. These effects are illustrated in Figs. 8 and 9. Extension of the irrigation interval results in a larger root zone and usually in reduced ET, and therefore a longer time and a larger amount of saline water will be required to salinize the root zone o f the crop. In addition, the larger amounts of water per irrigation and the lower residual moisture in the upper soil layers will displace the more concentrated salt solution to a deeper soil layer, thus allowing larger root-zone volumes o f lower salinity.

Multisource water supply operation Water o f various qualities from different sources can be applied separately or together. For separate applications, the types of water are allocated to different fields and the selection o f crops for each field is determined by their salt tolerance. Another option is to use the different sources for partial supply o f the crop water requirement. There are several operational practices, influenced by the expected crop response and practical operational limitations, which can be arranged in three groups: (a) To use the different sources at different

2.0

-

9

2

9 1.6 ~.~

DAYS

16 DAYS

-j.

-___,,.: E m /

1.2 0.8 .~

..i~ IDr

0.4' 0 0

L

i

L

l

4

8

12

16

TIME

FROM

IRRIGATION

(days)

Fig. 8. Change in total soil water potential (era) with time from irrigation for two irrigation intervals (2 and 16 days) and two irrigation water salinities ( 1.5 and 10.0 dS m-1 )43.

264

MEIRI AND PLAUT

-.04

---I0

._I

0

_J laJ

0 0

2

TIME A F T E R

6 DROUGHT

8

IMPOSED (days)

Fig. 9. Corn leaf elongation as influenced by salinity and days after irrigation during an irrigation interval. Numbers labeling the lines indicate the osmotic potentials of the irrigation solutions. Reproduced from ref. 47 by permission of the American Society of Agronomy.

growth stages, so that water of higher salinity would not be used during sensitive growth stages. D. Pasternak and Y. De Malach (personal communication) showed that a relatively short period of supplying nonsaline water at the young seedling stage considerably increased the final yields o f corn and tomatoes irrigated with saline water. (b) Mixing o f the water in the distribution network to adjust its salt concentration to the crop. (c) Mixing o f the water in the soil, i.e. supplying the water from the different sources alternately in a proportion tolerated by the crop. Using potatoes and drip irrigation, Meiri et al.4~ compared these three strategies and found that, except for specifically sensitive growth stages, the crop responded to the average water salinity regardless of the mixing procedure. Increased yield on the field level - stand and row spacing A salinity level that causes yield reduction per plant does not necessarily reduce yield on the field level to the same extent. The field yield is the product of stand density and yield per plant. Conventional planting density was established under nonsaline conditions. With stunted plants an increase in stand may increase yield on the field level. C o t t o n was the first crop for which modified plant spacing under saline conditions was studied a7'26 (J.D. Rhoades, personal communication),

CROP MANAGEMENT AND SALINITY

265

and the expected economic benefit was calculated is. For this row crop, a change in stand can be obtained by changing inter-row and/or intra-row spacing. Data from Keren e t al. 26 (Table 2) showed that reduced intra-row spacing resulted in strong competition between plants and had no effect on the yield per area. However, reduced interrow spacing did not cause plant competition and therefore increased the yield per meter 2 . Table 2. Influence of inter-row and intra-row spacing on the yield of seed cotton (Acala SJ-2) sprinkler irrigated with saline water 26 Yield

Inter-row spacing (cm)

Intra-row spacing (cm)

Stand (plants/m 2)

(g/m 2)

(g/plant)

75 75 96.5 95.5

12.5 9.1 12.5 9.1

10.7 14.7 8.3 11.4

597 616 486 503

56 42 59 44

Reduced salinity damage at the plant level This implies a change in the yield vs. salinity curve, in response to modified management or environmental conditions. Four types o f modification of this curve are presented in Fig. 10: (1)simultaneously changing the threshold (a) and the slope (b) and maintaining the zero yield salinity (Yo) constant; (2) simultaneously changing a, b, and Yo; (3) changing only b and Y0 ; and (4) changing a and Yo. In case 1 the constant Y0 indicates an osmotic effect and constant m a x i m u m salt tolerance by the crop. That is, Y0 corresponds to the critical, lower water potential that the plant can resist and thus the maximum salinity level against which it can absorb water when the water supply is ample. This critical value was already considered as the upper limit for root-zone salinization under reduced leaching 3,49. Changing conditions to influence the water demand and the water stress level in the plant should not change the Y0 at which there is no water uptake. The a, on the other hand, is the plant stress level at which production starts to be affected and is a function o f the rates o f water uptake and transpiration. Therefore, the a value should be sensitive to environmental conditions that influence water demand. In case 2 the changes in a, b and Y0 indicate a change in the critical m a x i m u m tolerance to salt stress. This may indicate a change in tolerance to osmotic stress or to a specific toxic effect. Cases 3 and 4 may be considered modifications of case 2. All these types o f modifications can be expected as a result o f changing management practices. They may be useful in analysis o f

266

MEIRI AND PLAUT

1.0

"\\\~\\\ t~ ...l b.I

0.5 -

-

x,\~\\ I9

\\N~\\ 3

\ x ~ '\\, ~

\\ ~X\\\x

W F-_.l b.I tic

1.0 \\\\

0.5

\\\

-

2

-

\\\ \\

x~

ECe

N

"XN\ ~ \ \ \ \

x

\\

ECe

Fig. 10. Schematic presentation of types of crop response to salinity after a change in environmental conditions and management.

the effects o f management on the response and should be considered when management decisions are made. Climate

Greater salinity damage in a warm, dry climate than in a cool, wet one was reported 31 . This may indicate a more severe salinity stress with an increase in the atmospheric demand for water. Temperature- and humidity-controlled glasshouse experiments were used to study these climatic factors' effects on salt tolerance o f root crops 23, cereals 19, and beans 22. The influence o f relative air humidity on the responses o f corn and barley to salinity (Table 3) is expressed as a higher a and similar Y0 values at the higher relative humidity for both crops (case 1). A comparison of data from two experiments with muskmelons which differed in many aspects - one with vat. Galia grown in a glasshouse in Israel in the spring and irrigated daily 36, the other with varieties grown outdoors in California in the mid-summer and irrigated twice a week 46 - also shows a lower a value under the more severe water stress and only a small difference in Y0 (case 1)(Table 3). Meiri et al. 34 studied the effect of reduced transpiration demand on salt tolerance by shading muskmelons grown in a temperature-controlled glasshouse in the spring. Again, in this experiment, mainly the a value was affected (Table 3). However, contrary to the results reported above, the un-shaded plants had a higher salt tolerance than the shaded ones. Apparently the reduced

CROP M A N A G E M E N T A N D SALINITY

267 0

9....4

...-i

0'3

+

r~ o,-c~

0

o

0 ,---t ~ 0 .--~ ~ 1

t"l 0

'.0 0

C) r--

0"~ ,--~

o

c5 ,--t r~

e-,

[-..

~ ~

. ~

~

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o

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~ ~ ~

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.4

o.~

o

r..)

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268

MEIRI AND PLAUT

rate of photosynthesis (P) in the shaded plants had a stronger effect on the response to salinity than the expected benefit from the reduction in transpiration due to shading (cases 1 and 2). Increased light was also reported to be beneficial by Hellal and Mengel aS. I n c r e a s e d COz c o n c e n t r a t i o n

Increasing the COz concentration above normal may increase the rate of photosynthesis, which in turn may increase the salt tolerance. Enoch e t al. la actually showed a marked change in the response of t o m a t o to salinity at 8000 ppm CO2 as compared with 350 ppm (Fig. 11). Although the entire salinity range was not covered, the response curve at 8 0 0 0 p p m CO2 showed an increase in the Y0 value. This change may correspond to case 2 and indicates an increased salt tolerance. Higher salt tolerance under higher COz concentration was also found for beans, corn, Xanthium and Atriplex by Schwartz and Gale 42, and for roses by M. Zerony and J. Gale (this volume). G r o w t h stage

Changes in salt tolerance at different growth stages were recognized long ago. Plants were usually found to be more sensitive at the young seedling stage. Maas et al. a~ showed greater salt tolerance o f corn after tasseling. The higher tolerance shows up in larger a and Y0 and smaller b (Table 3). A similar response was found by D. Pasternak I

[.0 a ._1 I.U )..

1.1.1 >

-

I

I

I

I

I

I

-

p "" p"" "" m" " o ~-

8000

0.8--

0.6--

m

I._1 1.1.1 fie

0.4--

pm

0.2--

I

0 0

t 0.4

SALT Fig. 11. I n f l u e n c e o f C O 2 c a l c u l a t e d o n a relative basis).

I

I

I

0.8

I

I

I

1.2

1.6

CONCENTRATION ( 9 / I ) concentration

on

salt t o l e r a n c e

by

t o m a t o e s ~3 ( D a t a

re-

CROP MANAGEMENT AND SALINITY

269

and Y. De Malach (personal communication) for corn and processing tomatoes, which exhibit lower salt tolerance at early growth stages resulting from reduced a and increased b. These changes in response to salinity, which indicate a real change in salt tolerance, correspond to case 2 o f Fig. 10. Irrigation m e t h o d The specific leaf damage caused by canopy wetting during sprinkler irrigation was documented by Ehlig and Bernstein 12, Bernstein and Francois 2,4, and Maas et al. 27,28. Potatoes are one o f the most sensitive crops to such leaf damage. Recently, the quantitative response o f potatoes to salinity was studied by Meiri e t a / . 38,a9,4~ with drip and sprinkler irrigation. The data in Table 3 show that the specific sprinkling damage was expressed as a smaller a and Yo and larger b (case 2). This indicates that sprinkler irrigation had a specific toxic effect. Fertilization Recent reviews by Feigin 14 and Kafkafi 2s indicate various interactions between fertilization and salinity. A clear increase in salt tolerance under high fertilization was shown by Ravikovitch and Yoles 41 with millet and clover (Table 3). With clover there was no change in the threshold, while the slope decreased and Yo increased (case 3). With millet the threshold and Yo values increased, with no change i n the slope (case 4). The beneficial effect could be an overcoming o f nutritional imbalance or a reduction in toxicity, two responses that may result in similar changes in the curves. Acknowledgements The authors acknowledge the permission given by publishers and authors to reproduce figures and tables in this review.

References 1

2 3 4 5 6

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