- Email: [email protected]
Irrigation with brackish water under desert conditions IV. Salt tolerance studies with lettuce (Lactuca sativa L.)
Agricultural Water Management, 11 (1986) 303-311
303
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Irrigation w i t h B r a c k i s h Water u n d e r Desert Conditions IV. Salt T o l e r a n c e Studies w i t h Lettuce
( L a c t u c a s a t i v a L. ) D. PASTERNAK 1, Y. DE MALACH 2, I. BOROVIC', M. SHRAM ' and C. AVIRAM~
'Rudolf and Rhoda Boyko Institute for Agriculture and Applied Biology, The Institutes for Applied Research, Ben -Gurion University of the Negev, P.O. Box 1025, Beer- Sheva 841 I0 (Israel) ~Rarnat Negev Regional Council, Doar Na Halutsa 85515 (Israel) 3The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sed Boqer 84990 (Israel) ~The Division of Vegetables, The Ministry of Agriculture, Hakiriah, Tel Aviv 61070 (Israel) (Accepted 17 January 1986)
ABSTRACT Pasternak, D., De Malach, Y., Borovic, I., Shram, M. and Aviram, C., 1986. Irrigation with brackish water under desert conditions. IV. Salt tolerance studies with lettuce (Lactuca sativa L.). Agric. Water Manage., 11: 303-311. In a field trial, three Romaine lettuce cultivars and seven iceberg lettuce cultivars were drip irrigated from day 20 after planting with water of four levels of salinity (electrical conductivity of 1.2, 3.5, 8.2 and 10.5 dS/m). The Romaine lettuce cultivars were far more tolerant to salinity than the iceberg cultivars. There were no specific salt-tolerant cultivars within either of the two cultivar groups. The slope of the linear regression of relative yield vs. salinity was - 5 . 6 for the iceberg lettuce cultivars, which is half the value reported elsewhere for lettuce. Iceberg lettuce appears to be more sensitive to salinity at later than at early growth stages. There was no apparent osmotic adaptation of lettuce to salinity. The osmotic potential of the leaves decreased to - 1.3 MPa about a month after planting and then gradually increased to - 0 . 6 MPa.
INTRODUCTION
Lettuce is becoming an increasingly important crop in the arid regions of Israel, where it is grown during the winter season for export to Europe. The local water sources are brackish, and lettuce is considered to be a moderately salt-tolerant species (Ayers et al. 1951; Maas and Hoffman, 1977). A preliminary experiment (Pressman et al., 1980) demonstrated that the yield of iceberg lettuce was not affected by sprinkler irrigation with water hav0378-3774/86/$03.50
© 1986 Elsevier Science Publishers B.V.
304 ing an electrical conductivity of 4.4 dS/m. In addition, the product quality was not adversely affected by this level of salinity ( Mizrahi and Pasternak, 1985). A detailed field study was therefore carried out to: (a) assess the salt tolerance of lettuce; (b) determine the range of salt tolerance among lettuce cultivars; and (c) identify physiological parameters related to salt tolerance mechanisms in lettuce. MATERIALSAND METHODS The trial was carried out at the Ramat Negev Experimental Station. The soil is a torrifluvent sandy loam containing 70-80% sand, 10-15% clay and 10-20% silt. Climatic data for the region are presented in Pasternak et al. (1984). Seven iceberg lettuce cultivars and three Romaine lettuce cultivars were tested with irrigation water of four levels of salinity. The iceberg lettuce cultivars were: Cal K-60, Vanmax, Morangold, Climax, Salinas, King Gordon, and Great Lakes Mesa. The Romaine lettuce cultivars were Roman 9, Cos, and Roman Yellow. The electrical conductivity (ECi) of the irrigation water was 1.2, 3.5, 8.2 and 10.5 dS/m. These salinity levels were obtained by adding a mixture of NaCl and CaCl2 (weight ratio 3 : 1) to fresh water from the Israeli National Water Carrier in a system described by Pasternak et al. (1985). The treatments were replicated five times in a randomized split-plot block design. There were 200 plots, each containing one cultivar planted in 6-m-long, 1.96-m-wide beds. There were four rows of plants per bed. Distances between plants were 0.25 m, so that there were 8 plants per m 2. The Salinas cultivar was planted in two 6-m-long beds. One of the two was used for sampling plants for dry weight and physiological parameters, as described below. The field size was 2700 m 2. Before being planted, the field was irrigated with 100 mm of water to assure proper salt leaching. Then the field was dressed with dry pelletted cow manure at 5000 kg/ha, ammonium sulfate at 500 kg/ha, and superphosphate at 400 kg/ha. The field was planted with seedlings on 26.10.81. Harvesting ended on 23.3.82, some 148 days later. The field was drip-irrigated. There were two dripper laterals per bed, the distance between emitters was 0.50 m, and the discharge rate was 2 1/h. Irrigation was carried out daily, the quantity of water being determined from evaporation (E) data obtained with a USWB Class A pan. For the first 15 days after planting, the amount of water applied was 0.6E, from day 16 to 36 it was 0.9E, and from day 36 until harvest it was 1.2E. These high rates of water application were recommended by the Israeli agricultural extention service, based on the results of other irrigation trials with lettuce. The trial was carried out in the winter, which is the rainy season. Rainwater leaches salts from the
305 soil. Thus, in order to maintain a steady-state ECe for each of the four salinity treatments, we had to re-salinize the soil immediately after each shower, using the four salt solutions and applying approximately the same amount of water as fell during the shower. To achieve good field establishment, the field was irrigated with fresh water ( E C = 1.2 d S / m ) for the first 20 days after planting. On day 21 each plot was salinized with its assigned salt solution. On this day, 50 mm of the various solutions were applied, to assure immediate and equal salinization of the whole soil profile occupied by the roots. Liquid fertilizer was added to the irrigation water daily, using a complete fertilizer solution (Nutricol-1). The concentrations of N, P and K in the irrigation water were 100, 18 and 120 ppm, respectively. Fertilization was started on the day of planting and ended 80 days after planting. The electrical conductivity of the soil saturation extract was determined five times during the experiment, the first time just before planting and the last time at the end of the last harvest. On each sampling occasion, the ECe was determined in each of the five replicated plots over three depth intervals. Electrical conductivity was sampled immediately below the drippers where the soil is most leached. In doing so we assumed that plants respond to the lowest salt concentration in the root medium (Lunin and Gallatin, 1965 ) and not to the mean total root zone EC~, as accepted by others (U.S. Salinity Laboratory Staff, 1954). The osmotic potential of the leaf sap was determined by a modification of the Shimshi and Livne (1967) method (see Pasternak et al., 1985), on the ,. _mrmost fully expanded leaf, eight times during the season. The same leaves were also sampled for chloride concentration eight times during the season. The leaves were dried and ground, then extracted with HNO3. The C1- content of the filtered extract was determined with a Buchler-Cotlove Chloridometer. The dry weight of eight heads per plot was determined eight times during the season. RESULTS
The electrical conductivity of the saturated soil extract on four dates during the season is given in Table I. Generally, the salinity levels in the soil were low, relative to the water salinity, and were fairly constant with time. The decline in the ECe towards the end of the experiment was probably due to overestimation of water consumption in the salinized plot (A. Meiri, Institute of Soil and Water, Agricultural Research Organization, Bet Dagan, Israel, personal communication, 1984), with a consequent use of a higher leaching fraction. As can be seen by comparing Tables II and III, the Romaine lettuce cultivars are much more tolerant to salinity than the iceberg lettuce group. Irrigation water with an ECi of 10.5 d S / m reduced the yield of Romaine lettuce by only
306 TABLE I Electrical conductivity(dS/m), of the saturated soil extract sampledunder the drippers, averaged over the depth intervals 5-15, 15-30 and 30-60 cm, determined four times during growth Irrigation water ECI (dS/m)
1.2 3.5 8.2 10.5
Date 26.11.81 (31 D A P )
20.12.81 (55 D A P )
15.2.82 (112 D A P )
23.3.82 (148 D A P )
2.1 3.9 5.8 7.7
1.6 4.2 6.2 8.4
1.7 3.3 7.4 8.8
1.3 2.4 5.2 6.0
Values are means of 15 d e t e r m i n a t i o n s (3 d e p t h s x 5 replications). S t a n d a r d errors were 3-7% of means. DAP = days after planting. T A B L E II Fresh yield of three Romaine lettuce cultivars {kg per l 0 m 2) as affected by irrigation water salinity Irrigation water ECi (dS/m )
1.2 3.5 8.2 10.5 Mean
Cultivar
Mean
Roman9
Cos
67.7 64.5 52.8 58.3 60.8
57.7 63.4 62.9 44.5 57.1
a a a a A
Roman Yellow a a a b A
50.5 56.6 55.6 48.8 52.9
a a a a B
58.6 61.5 57.1 50.5
a a a b
Values in columns with same small letters and in row with same capital letters do not differ at P = 5 %. T h e r e was no significant interaction between salinity levels a n d cultivars. T A B L E III Fresh yield of seven iceberg lettuce cultivars ( kg per 10 m -~} as affected by irrigation water salinity ECi (dS/m)
1.2 3.5 8.2 10.5 Mean
Cultivar
Mean
Cal K-60
Vanmax
Morangold
Climax
Salinas
King Gordon
Great Lakes Mesa
41.8 40.8 30.7 27.1 35.1
41.3 40.3 30.5 27.5 34.9
37.2 42.3 31.9 26.7 34.5
41.1 42.1 29.5 25.5 34.5
40.7 35.1 26.1 21.8 30.9
37.8 34.6 25.9 20.3 29.6
31.7 30.9 24.0 17.1 25.9
a a a b A
a a b b A
a a a b A
a a b b A
a a b b AB
a a b b B
a a b c C
38.8 38.0 28.3 23.7
a a b b
Values in columns with same small letters a n d in row with same capital letters do not differ at P=5%. T h e r e was no significant interaction between salinity levels and cultivars.
307 3O
25
o L2 d S / m • 10.5 d S / m
020 I.-
-1-
i~J
15
]:
sallnlzation
s
I0
/ r
~
20
~
J
30
I
40 DAYS
I
50 AFTER
I
I
60 70 PLANTING
I
80
I
90
I
I00
Fig. 1. Change with time in dry weight of individual heads of Salinas lettuce irrigated with 1.2 dS/m water ( O ) and with 10.5 dS/m water ( • ). Values are means of five samplings, each sample consisting of eight heads of lettuce harvested from a 1-m ~ plot. Vertical bars represent twice the standard errors.
14% (average of three cultivars), but with the iceberg lettuce cultivars, the yield reduction was 40% (average of seven cultivars). Dry matter production in the Salinas cultivar followed a biphasic curve ( Fig. 1). During the first 63 days after planting, the average growth per head in controls was 0.2 g/day. Thereafter, the average growth increased to about 0.5 g/day per head. During the first phase of growth, the production of dr)" matter was completely unaffected by water with a salinity of 10.5 dS/m. In the second phase, however, lettuce irrigated with saline water grew at half the rate of that receiving fresh water. The osmotic potential of leaves of very young lettuce seedlings was ca. - 0.8 MPa ( Fig. 2a ). Two weeks later it declined to about - 1.3 MPa, and thereafter it increased slowly and steadily to a value of - 0 . 6 MPa some 100 days after planting. Soluble metabolites (Fig. 2b ) and electrolytes (Fig. 2c ) contributed about equally to variations in the total osmotic potential. There was no significant difference in osmotic potential between lettuce irrigated with brackish or with fresh water. Ayers et al. {1951) and Shannon et al. (1983) showed that chloride and sodium are the salt ions most readily taken up by lettuce. Figure 3 shows that the chloride concentration was higher in the leaves of plants irrigated with brackish than in those receiving fresh water. The chloride concentration in the
308
-,.,
sallln|
zllt Ion
"-'°'"
SE
o
T•
J'
/"
-|,1
-
I.I
-I.!
Z-I.A
I
1
w "l'!
L
t
I
b.motoboUlto
L SE
1
I
~
r,
~ -|.!
@
- O,|
¢ . electrolyte
- O.II ....
I
zo
1
~o
l ,o DAYS
I
so AFTER
I
J
Do 7o PLANTING
I
Do
oo
,DO
Fig. 2. Variation with time of: (a) total osmotic potential; (b) osmotic potential due to metabolites; and (c) osmotic potential due to electrolytes, of leaves of Salinas lettuce irrigated with 1.2 dS/m water ( 0 ) and with 10.5 dS/m water ( • ).
leaves increased between days 40 and 70 and then declined again to the same levels as at the beginning of the experiment. DISCUSSION
Perhaps the most significant outcome of this study is that the Romaine lettuce cultivars are far more resistant to salinity than the iceberg type. This finding suggests that it should be possible to genetically increase the salt tolerance of iceberg lettuce. Recent studies by Shannon et al. {1983) and Shannon and McCreight {1984) also demonstrated a wide variation in salt tolerance among lettuce cultivars (some 85 local American lines and 115 introduced lines were tested). However, these workers did not detect the higher salt tolerance of Romaine
309
-~
1.6
w
1.4
,u
1.2
..J
Z
• 0 ),(
I.II
Z uJ Z 0
O.O
0.4 3: 0.2
I 40
I 50
I Ill DAYS
I 70 AFTER
"
I IIII
I 90
I I00
PLANTING
Fig. 3. Variation with time, of chlorideconcentration in leavesof Salinas lettuce irrigated with 1.2 dS/m water ( O ) and with 10.5dS/m water ( • }. Verticalbars represent twice the standard errors. lettuce cultivars, probably because their trials were done at only one level of salinity. It therefore appears that in studies designed to select for salt tolerance, one cannot escape the necessity of using a range of salinities and of expressing yield in both relative and absolute terms. Expression of yield in absolute values only is not enough, due to the inherent differences in weight between groups of cultivars (Tables II and III). No significant regression curve for yield vs soil salinity could be obtained for the Romaine lettuce cultivars studied here, since salinity had very little effect on their yield. For the iceberg lettuce, however, the correlation was significant (r= -0.75) and the salt tolerance equation could be expressed as Y= 100 5.6 ( ECe - 1.98 ), which means that the relative yield of iceberg lettuce declines by 5.6% with every unit increase in ECe above a threshold of 1.98 dS/m. The slope of the regression line ( - 5 . 6 ) is about half that of the salt tolerance equation given for lettuce by Maas and Hoffman (1967). This puts iceberg lettuce (and of course also Romaine lettuce) in the category of salt-resistant crops as defined by the latter authors. A reasonable explanation for the above discrepancy is that the present trial was conducted with drip irrigation - - a system more suitable for application of brackish water t h a n sprinkler or furrow irrigation (Bernstein and Francois, 1975), which were used in the salt tolerance studies described by Maas and Hoffman (1967). Within each group of lettuce cultivars tested here, there was no single outstanding salt-tolerant cultivar.
310 Iceberg lettuce appears to be more salt tolerant in the earlier than the latter stages of its growth ( Fig. 1 ). This is contrary to the behavior of many other crop species (Lunin et al., 1963; Pasternak et al., 1985). The increased sensitivity of lettuce to salinity with ontogeny might be partly explained by developmental changes in leaf osmotic potential. The osmotic potential of young lettuce is about - 1.3 MPa and increases to a value of - 0.6 MPa 100 days after planting (Fig. 2a). It is difficult, however, to distinguish between cause and effect in this case. The increased sensitivity to salinity coincided with the period of rapid growth. It is quite possible that the rapid growth resulted in dilution of metabolites and electrolytes, increasing the osmotic potential and lowering the salt resistance of the plant tissues. The changes in salt tolerance with time could not be explained by changes in the internal chloride concentration, because the leaf chloride concentration decreased markedly after day 70 from planting (Fig. 3), at a time when the lettuce started suffering from salinity (Fig. 1 ). Ayers et al. (1951) also concluded that in lettuce, salt-induced growth retardation is not related to increased concentrations of salt ions in the leaves. They applied 6 levels of salinity and found that the leaf chloride concentration doubled between the first and second levels of salinity but did not change with further increases in salinity. Growth, however, was continuously reduced with increasing salinity. In the present study there was no apparent osmotic adaptation to increased salinity in the irrigation water (Fig. 2). This finding is different from that of Shannon et al. (1983), who demonstrated a clear decrease in the leaf osmotic potential of lettuce with increasing salinity of the root medium. The major difference between Shannon et al.'s trials and ours was that theirs were conducted in sand culture inside a greenhouse while ours were conducted in the open field. The finding that lettuce is more salt sensitive at later growth stages needs further verification. If found correct, it can become a basis for devising specific irrigation management practices of the kind Pasternak et al. {1985) suggested for maize, or a basis for deciding upon the best stage of growth for selection of lettuce cultivars for salt resistance. ACKNOWLEDGEMENTS The trials were partially supported by the Division of Vegetables of the Israeli Ministry of Agriculture. The authors are grateful to Ms. Hasia Klotz and Ms. Esther Katz for their help in data analysis and to Dr. Marjorie A. Tiefert for help with the manuscript.
311 REFERENCES Ayers, A.D., Wadleigh, C.H. and Bernstein, L., 1951. Salt tolerance of six varieties of lettuce. Proc. Am. Soc. Hortic. Sci., 57: 237-242. Bernstein, L. and Francois, L.E., 1975. Effects of frequency of sprinkling with saline waters compared with daily drip irrigation. Agron. J., 67: 185-190. Lunin, J. and Gallatin, M.H., 1965. Zonal salinization of the root system in relation to plant growth. Proc. Soil Sci. Soc. Am., 29: 608-612. Lunin, J., Gallatin, M.H. and Butchelder, A.R., 1963. Saline irrigation of several vegetable crops at various growth stages. 1. Effects on yields. Agron. J., 55: 107-110. Maas, E.V. and Hoffman, G.J., 1977. Crop salt tolerance - - current assessment. J. Irrig. Drain. Div. ASCE, 103 (IR2): 115-134. Mizrahi, Y. and Pasternak, D., 1985. Effect of salinity on quality of various agricultural crops. Plant Soil, 89: 301-307. Pasternak, D., De Malach, Y. and Borovic, I., 1984. Irrigation with brackish water under desert conditions. I. Problems and solutions in production of onions (Allium cepa L. ). Agric. Water Manage., 9: 225-235. Pasternak, D., De Malach, Y. and Borovic, I., 1985. Irrigation with brackish water under desert conditions. II. Physiological and yield response of maize (Zea mays) to continuous irrigation with brackish water and to alternating brackish-fresh-brackish water irrigation. Agric. Water Manage., I0: 47-60. Pressman, E., Aloni, E., De Malach, Y., Pasternak, D., Aviram, H., Tsiplevitch, I. and Aharoni, I., 1980. The effect of brackish water irrigation on three leafy vegetables. Hassadeh, 61:476-478 (in Hebrew). Shannon, M.C. and McCreight, J.D., 1984. Salt tolerance of lettuce introduction. HortScience, 19: 673-675. Shannon, M.C., McCreight, J.D. and Draper, J.H., 1983. Screening tests for salt tolerance in lettuce. J. Am. Soc. Hortic. Sci., 108: 225-230. Shimshi, D. and Livne, A., 1967. The estimation of the osmotic potential of plant sap by refractometry and conductimetry: a field method. Ann. Bot. N.S., 31: 505-511. U.S. Salinity Laboratory Staff, 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Handbook 60, U.S. Goverment Printing Office, Washington, DC, 160 pp.
303
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Irrigation w i t h B r a c k i s h Water u n d e r Desert Conditions IV. Salt T o l e r a n c e Studies w i t h Lettuce
( L a c t u c a s a t i v a L. ) D. PASTERNAK 1, Y. DE MALACH 2, I. BOROVIC', M. SHRAM ' and C. AVIRAM~
'Rudolf and Rhoda Boyko Institute for Agriculture and Applied Biology, The Institutes for Applied Research, Ben -Gurion University of the Negev, P.O. Box 1025, Beer- Sheva 841 I0 (Israel) ~Rarnat Negev Regional Council, Doar Na Halutsa 85515 (Israel) 3The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sed Boqer 84990 (Israel) ~The Division of Vegetables, The Ministry of Agriculture, Hakiriah, Tel Aviv 61070 (Israel) (Accepted 17 January 1986)
ABSTRACT Pasternak, D., De Malach, Y., Borovic, I., Shram, M. and Aviram, C., 1986. Irrigation with brackish water under desert conditions. IV. Salt tolerance studies with lettuce (Lactuca sativa L.). Agric. Water Manage., 11: 303-311. In a field trial, three Romaine lettuce cultivars and seven iceberg lettuce cultivars were drip irrigated from day 20 after planting with water of four levels of salinity (electrical conductivity of 1.2, 3.5, 8.2 and 10.5 dS/m). The Romaine lettuce cultivars were far more tolerant to salinity than the iceberg cultivars. There were no specific salt-tolerant cultivars within either of the two cultivar groups. The slope of the linear regression of relative yield vs. salinity was - 5 . 6 for the iceberg lettuce cultivars, which is half the value reported elsewhere for lettuce. Iceberg lettuce appears to be more sensitive to salinity at later than at early growth stages. There was no apparent osmotic adaptation of lettuce to salinity. The osmotic potential of the leaves decreased to - 1.3 MPa about a month after planting and then gradually increased to - 0 . 6 MPa.
INTRODUCTION
Lettuce is becoming an increasingly important crop in the arid regions of Israel, where it is grown during the winter season for export to Europe. The local water sources are brackish, and lettuce is considered to be a moderately salt-tolerant species (Ayers et al. 1951; Maas and Hoffman, 1977). A preliminary experiment (Pressman et al., 1980) demonstrated that the yield of iceberg lettuce was not affected by sprinkler irrigation with water hav0378-3774/86/$03.50
© 1986 Elsevier Science Publishers B.V.
304 ing an electrical conductivity of 4.4 dS/m. In addition, the product quality was not adversely affected by this level of salinity ( Mizrahi and Pasternak, 1985). A detailed field study was therefore carried out to: (a) assess the salt tolerance of lettuce; (b) determine the range of salt tolerance among lettuce cultivars; and (c) identify physiological parameters related to salt tolerance mechanisms in lettuce. MATERIALSAND METHODS The trial was carried out at the Ramat Negev Experimental Station. The soil is a torrifluvent sandy loam containing 70-80% sand, 10-15% clay and 10-20% silt. Climatic data for the region are presented in Pasternak et al. (1984). Seven iceberg lettuce cultivars and three Romaine lettuce cultivars were tested with irrigation water of four levels of salinity. The iceberg lettuce cultivars were: Cal K-60, Vanmax, Morangold, Climax, Salinas, King Gordon, and Great Lakes Mesa. The Romaine lettuce cultivars were Roman 9, Cos, and Roman Yellow. The electrical conductivity (ECi) of the irrigation water was 1.2, 3.5, 8.2 and 10.5 dS/m. These salinity levels were obtained by adding a mixture of NaCl and CaCl2 (weight ratio 3 : 1) to fresh water from the Israeli National Water Carrier in a system described by Pasternak et al. (1985). The treatments were replicated five times in a randomized split-plot block design. There were 200 plots, each containing one cultivar planted in 6-m-long, 1.96-m-wide beds. There were four rows of plants per bed. Distances between plants were 0.25 m, so that there were 8 plants per m 2. The Salinas cultivar was planted in two 6-m-long beds. One of the two was used for sampling plants for dry weight and physiological parameters, as described below. The field size was 2700 m 2. Before being planted, the field was irrigated with 100 mm of water to assure proper salt leaching. Then the field was dressed with dry pelletted cow manure at 5000 kg/ha, ammonium sulfate at 500 kg/ha, and superphosphate at 400 kg/ha. The field was planted with seedlings on 26.10.81. Harvesting ended on 23.3.82, some 148 days later. The field was drip-irrigated. There were two dripper laterals per bed, the distance between emitters was 0.50 m, and the discharge rate was 2 1/h. Irrigation was carried out daily, the quantity of water being determined from evaporation (E) data obtained with a USWB Class A pan. For the first 15 days after planting, the amount of water applied was 0.6E, from day 16 to 36 it was 0.9E, and from day 36 until harvest it was 1.2E. These high rates of water application were recommended by the Israeli agricultural extention service, based on the results of other irrigation trials with lettuce. The trial was carried out in the winter, which is the rainy season. Rainwater leaches salts from the
305 soil. Thus, in order to maintain a steady-state ECe for each of the four salinity treatments, we had to re-salinize the soil immediately after each shower, using the four salt solutions and applying approximately the same amount of water as fell during the shower. To achieve good field establishment, the field was irrigated with fresh water ( E C = 1.2 d S / m ) for the first 20 days after planting. On day 21 each plot was salinized with its assigned salt solution. On this day, 50 mm of the various solutions were applied, to assure immediate and equal salinization of the whole soil profile occupied by the roots. Liquid fertilizer was added to the irrigation water daily, using a complete fertilizer solution (Nutricol-1). The concentrations of N, P and K in the irrigation water were 100, 18 and 120 ppm, respectively. Fertilization was started on the day of planting and ended 80 days after planting. The electrical conductivity of the soil saturation extract was determined five times during the experiment, the first time just before planting and the last time at the end of the last harvest. On each sampling occasion, the ECe was determined in each of the five replicated plots over three depth intervals. Electrical conductivity was sampled immediately below the drippers where the soil is most leached. In doing so we assumed that plants respond to the lowest salt concentration in the root medium (Lunin and Gallatin, 1965 ) and not to the mean total root zone EC~, as accepted by others (U.S. Salinity Laboratory Staff, 1954). The osmotic potential of the leaf sap was determined by a modification of the Shimshi and Livne (1967) method (see Pasternak et al., 1985), on the ,. _mrmost fully expanded leaf, eight times during the season. The same leaves were also sampled for chloride concentration eight times during the season. The leaves were dried and ground, then extracted with HNO3. The C1- content of the filtered extract was determined with a Buchler-Cotlove Chloridometer. The dry weight of eight heads per plot was determined eight times during the season. RESULTS
The electrical conductivity of the saturated soil extract on four dates during the season is given in Table I. Generally, the salinity levels in the soil were low, relative to the water salinity, and were fairly constant with time. The decline in the ECe towards the end of the experiment was probably due to overestimation of water consumption in the salinized plot (A. Meiri, Institute of Soil and Water, Agricultural Research Organization, Bet Dagan, Israel, personal communication, 1984), with a consequent use of a higher leaching fraction. As can be seen by comparing Tables II and III, the Romaine lettuce cultivars are much more tolerant to salinity than the iceberg lettuce group. Irrigation water with an ECi of 10.5 d S / m reduced the yield of Romaine lettuce by only
306 TABLE I Electrical conductivity(dS/m), of the saturated soil extract sampledunder the drippers, averaged over the depth intervals 5-15, 15-30 and 30-60 cm, determined four times during growth Irrigation water ECI (dS/m)
1.2 3.5 8.2 10.5
Date 26.11.81 (31 D A P )
20.12.81 (55 D A P )
15.2.82 (112 D A P )
23.3.82 (148 D A P )
2.1 3.9 5.8 7.7
1.6 4.2 6.2 8.4
1.7 3.3 7.4 8.8
1.3 2.4 5.2 6.0
Values are means of 15 d e t e r m i n a t i o n s (3 d e p t h s x 5 replications). S t a n d a r d errors were 3-7% of means. DAP = days after planting. T A B L E II Fresh yield of three Romaine lettuce cultivars {kg per l 0 m 2) as affected by irrigation water salinity Irrigation water ECi (dS/m )
1.2 3.5 8.2 10.5 Mean
Cultivar
Mean
Roman9
Cos
67.7 64.5 52.8 58.3 60.8
57.7 63.4 62.9 44.5 57.1
a a a a A
Roman Yellow a a a b A
50.5 56.6 55.6 48.8 52.9
a a a a B
58.6 61.5 57.1 50.5
a a a b
Values in columns with same small letters and in row with same capital letters do not differ at P = 5 %. T h e r e was no significant interaction between salinity levels a n d cultivars. T A B L E III Fresh yield of seven iceberg lettuce cultivars ( kg per 10 m -~} as affected by irrigation water salinity ECi (dS/m)
1.2 3.5 8.2 10.5 Mean
Cultivar
Mean
Cal K-60
Vanmax
Morangold
Climax
Salinas
King Gordon
Great Lakes Mesa
41.8 40.8 30.7 27.1 35.1
41.3 40.3 30.5 27.5 34.9
37.2 42.3 31.9 26.7 34.5
41.1 42.1 29.5 25.5 34.5
40.7 35.1 26.1 21.8 30.9
37.8 34.6 25.9 20.3 29.6
31.7 30.9 24.0 17.1 25.9
a a a b A
a a b b A
a a a b A
a a b b A
a a b b AB
a a b b B
a a b c C
38.8 38.0 28.3 23.7
a a b b
Values in columns with same small letters a n d in row with same capital letters do not differ at P=5%. T h e r e was no significant interaction between salinity levels and cultivars.
307 3O
25
o L2 d S / m • 10.5 d S / m
020 I.-
-1-
i~J
15
]:
sallnlzation
s
I0
/ r
~
20
~
J
30
I
40 DAYS
I
50 AFTER
I
I
60 70 PLANTING
I
80
I
90
I
I00
Fig. 1. Change with time in dry weight of individual heads of Salinas lettuce irrigated with 1.2 dS/m water ( O ) and with 10.5 dS/m water ( • ). Values are means of five samplings, each sample consisting of eight heads of lettuce harvested from a 1-m ~ plot. Vertical bars represent twice the standard errors.
14% (average of three cultivars), but with the iceberg lettuce cultivars, the yield reduction was 40% (average of seven cultivars). Dry matter production in the Salinas cultivar followed a biphasic curve ( Fig. 1). During the first 63 days after planting, the average growth per head in controls was 0.2 g/day. Thereafter, the average growth increased to about 0.5 g/day per head. During the first phase of growth, the production of dr)" matter was completely unaffected by water with a salinity of 10.5 dS/m. In the second phase, however, lettuce irrigated with saline water grew at half the rate of that receiving fresh water. The osmotic potential of leaves of very young lettuce seedlings was ca. - 0.8 MPa ( Fig. 2a ). Two weeks later it declined to about - 1.3 MPa, and thereafter it increased slowly and steadily to a value of - 0 . 6 MPa some 100 days after planting. Soluble metabolites (Fig. 2b ) and electrolytes (Fig. 2c ) contributed about equally to variations in the total osmotic potential. There was no significant difference in osmotic potential between lettuce irrigated with brackish or with fresh water. Ayers et al. {1951) and Shannon et al. (1983) showed that chloride and sodium are the salt ions most readily taken up by lettuce. Figure 3 shows that the chloride concentration was higher in the leaves of plants irrigated with brackish than in those receiving fresh water. The chloride concentration in the
308
-,.,
sallln|
zllt Ion
"-'°'"
SE
o
T•
J'
/"
-|,1
-
I.I
-I.!
Z-I.A
I
1
w "l'!
L
t
I
b.motoboUlto
L SE
1
I
~
r,
~ -|.!
@
- O,|
¢ . electrolyte
- O.II ....
I
zo
1
~o
l ,o DAYS
I
so AFTER
I
J
Do 7o PLANTING
I
Do
oo
,DO
Fig. 2. Variation with time of: (a) total osmotic potential; (b) osmotic potential due to metabolites; and (c) osmotic potential due to electrolytes, of leaves of Salinas lettuce irrigated with 1.2 dS/m water ( 0 ) and with 10.5 dS/m water ( • ).
leaves increased between days 40 and 70 and then declined again to the same levels as at the beginning of the experiment. DISCUSSION
Perhaps the most significant outcome of this study is that the Romaine lettuce cultivars are far more resistant to salinity than the iceberg type. This finding suggests that it should be possible to genetically increase the salt tolerance of iceberg lettuce. Recent studies by Shannon et al. {1983) and Shannon and McCreight {1984) also demonstrated a wide variation in salt tolerance among lettuce cultivars (some 85 local American lines and 115 introduced lines were tested). However, these workers did not detect the higher salt tolerance of Romaine
309
-~
1.6
w
1.4
,u
1.2
..J
Z
• 0 ),(
I.II
Z uJ Z 0
O.O
0.4 3: 0.2
I 40
I 50
I Ill DAYS
I 70 AFTER
"
I IIII
I 90
I I00
PLANTING
Fig. 3. Variation with time, of chlorideconcentration in leavesof Salinas lettuce irrigated with 1.2 dS/m water ( O ) and with 10.5dS/m water ( • }. Verticalbars represent twice the standard errors. lettuce cultivars, probably because their trials were done at only one level of salinity. It therefore appears that in studies designed to select for salt tolerance, one cannot escape the necessity of using a range of salinities and of expressing yield in both relative and absolute terms. Expression of yield in absolute values only is not enough, due to the inherent differences in weight between groups of cultivars (Tables II and III). No significant regression curve for yield vs soil salinity could be obtained for the Romaine lettuce cultivars studied here, since salinity had very little effect on their yield. For the iceberg lettuce, however, the correlation was significant (r= -0.75) and the salt tolerance equation could be expressed as Y= 100 5.6 ( ECe - 1.98 ), which means that the relative yield of iceberg lettuce declines by 5.6% with every unit increase in ECe above a threshold of 1.98 dS/m. The slope of the regression line ( - 5 . 6 ) is about half that of the salt tolerance equation given for lettuce by Maas and Hoffman (1967). This puts iceberg lettuce (and of course also Romaine lettuce) in the category of salt-resistant crops as defined by the latter authors. A reasonable explanation for the above discrepancy is that the present trial was conducted with drip irrigation - - a system more suitable for application of brackish water t h a n sprinkler or furrow irrigation (Bernstein and Francois, 1975), which were used in the salt tolerance studies described by Maas and Hoffman (1967). Within each group of lettuce cultivars tested here, there was no single outstanding salt-tolerant cultivar.
310 Iceberg lettuce appears to be more salt tolerant in the earlier than the latter stages of its growth ( Fig. 1 ). This is contrary to the behavior of many other crop species (Lunin et al., 1963; Pasternak et al., 1985). The increased sensitivity of lettuce to salinity with ontogeny might be partly explained by developmental changes in leaf osmotic potential. The osmotic potential of young lettuce is about - 1.3 MPa and increases to a value of - 0.6 MPa 100 days after planting (Fig. 2a). It is difficult, however, to distinguish between cause and effect in this case. The increased sensitivity to salinity coincided with the period of rapid growth. It is quite possible that the rapid growth resulted in dilution of metabolites and electrolytes, increasing the osmotic potential and lowering the salt resistance of the plant tissues. The changes in salt tolerance with time could not be explained by changes in the internal chloride concentration, because the leaf chloride concentration decreased markedly after day 70 from planting (Fig. 3), at a time when the lettuce started suffering from salinity (Fig. 1 ). Ayers et al. (1951) also concluded that in lettuce, salt-induced growth retardation is not related to increased concentrations of salt ions in the leaves. They applied 6 levels of salinity and found that the leaf chloride concentration doubled between the first and second levels of salinity but did not change with further increases in salinity. Growth, however, was continuously reduced with increasing salinity. In the present study there was no apparent osmotic adaptation to increased salinity in the irrigation water (Fig. 2). This finding is different from that of Shannon et al. (1983), who demonstrated a clear decrease in the leaf osmotic potential of lettuce with increasing salinity of the root medium. The major difference between Shannon et al.'s trials and ours was that theirs were conducted in sand culture inside a greenhouse while ours were conducted in the open field. The finding that lettuce is more salt sensitive at later growth stages needs further verification. If found correct, it can become a basis for devising specific irrigation management practices of the kind Pasternak et al. {1985) suggested for maize, or a basis for deciding upon the best stage of growth for selection of lettuce cultivars for salt resistance. ACKNOWLEDGEMENTS The trials were partially supported by the Division of Vegetables of the Israeli Ministry of Agriculture. The authors are grateful to Ms. Hasia Klotz and Ms. Esther Katz for their help in data analysis and to Dr. Marjorie A. Tiefert for help with the manuscript.
311 REFERENCES Ayers, A.D., Wadleigh, C.H. and Bernstein, L., 1951. Salt tolerance of six varieties of lettuce. Proc. Am. Soc. Hortic. Sci., 57: 237-242. Bernstein, L. and Francois, L.E., 1975. Effects of frequency of sprinkling with saline waters compared with daily drip irrigation. Agron. J., 67: 185-190. Lunin, J. and Gallatin, M.H., 1965. Zonal salinization of the root system in relation to plant growth. Proc. Soil Sci. Soc. Am., 29: 608-612. Lunin, J., Gallatin, M.H. and Butchelder, A.R., 1963. Saline irrigation of several vegetable crops at various growth stages. 1. Effects on yields. Agron. J., 55: 107-110. Maas, E.V. and Hoffman, G.J., 1977. Crop salt tolerance - - current assessment. J. Irrig. Drain. Div. ASCE, 103 (IR2): 115-134. Mizrahi, Y. and Pasternak, D., 1985. Effect of salinity on quality of various agricultural crops. Plant Soil, 89: 301-307. Pasternak, D., De Malach, Y. and Borovic, I., 1984. Irrigation with brackish water under desert conditions. I. Problems and solutions in production of onions (Allium cepa L. ). Agric. Water Manage., 9: 225-235. Pasternak, D., De Malach, Y. and Borovic, I., 1985. Irrigation with brackish water under desert conditions. II. Physiological and yield response of maize (Zea mays) to continuous irrigation with brackish water and to alternating brackish-fresh-brackish water irrigation. Agric. Water Manage., I0: 47-60. Pressman, E., Aloni, E., De Malach, Y., Pasternak, D., Aviram, H., Tsiplevitch, I. and Aharoni, I., 1980. The effect of brackish water irrigation on three leafy vegetables. Hassadeh, 61:476-478 (in Hebrew). Shannon, M.C. and McCreight, J.D., 1984. Salt tolerance of lettuce introduction. HortScience, 19: 673-675. Shannon, M.C., McCreight, J.D. and Draper, J.H., 1983. Screening tests for salt tolerance in lettuce. J. Am. Soc. Hortic. Sci., 108: 225-230. Shimshi, D. and Livne, A., 1967. The estimation of the osmotic potential of plant sap by refractometry and conductimetry: a field method. Ann. Bot. N.S., 31: 505-511. U.S. Salinity Laboratory Staff, 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Handbook 60, U.S. Goverment Printing Office, Washington, DC, 160 pp.