Effects of NaCl or nutrient-induced salinity on
growth, yield, and composition of eggplants
grown in rockwool
D. Savvas
*, F. Lenz
Institut fuÈr Obst-und GemuÈsebau der UniversitaÈt Bonn, Auf dem HuÈgel 6, 53121 Bonn, Germany
Accepted 3 September 1999
Abstract
The effects of increasing the salt concentration of a basic nutrient solution from 2.1 up to 4.7 dS mÿ1by providing either additional amounts of nutrients or 25 mmol lÿ1NaCl on growth,
yield, fruit quality and mineral composition of eggplants were investigated. The extra nutrients used to raise the electrical conductivity were added either at the same ionic concentration ratio as in the basic nutrient solution or at an increased ratio of K to total cation concentration.
The vegetative growth and the number of ¯owers per plant were not in¯uenced by any of the salinity treatments. In contrast, the fresh fruit yield of eggplant was signi®cantly reduced to the same extent in all salinity treatments. The yield depression was a result of a decline in mean fruit weight, whereas the number of fruits per plant was not affected. However, recalculation of the data on dry weight basis revealed no signi®cant differences between the treatments. The percentage of eggplant fruits graded Class 1 was signi®cantly reduced at 4.7 dS mÿ1, whilst the kind of salts used
to induce salinity had no signi®cant effect on fruit quality. The increase of electrical conductivity up to 4.7 dS mÿ1
by the addition of extra nutrients did not result in a higher nutrient uptake, with the exception of P in roots, and P and organic N in the petioles of older leaves. In contrast, the concentrations of Mg and NO3±N were reduced in some plant parts when salinity was increased by
the addition of extra nutrients, regardless of the proportions of cations in the nutrient solution. All salinity treatments reduced the concentration of Mg in the leaves to the same degree, thus indicating that this salt effect is not ion speci®c.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Eggplant; Growth responses; Hydroponics; Mineral composition; Nutrient solution; Salinity
*Corresponding author. Current address: Faculty of Agricultural Technology, T.E.I. of Epirus,
PO Box 110, 47100 Arta, Greece. Tel.:30-681-77-468; fax:30-681-77-468.
E-mail address: savvas@teiep.gr (D. Savvas).
1. Introduction
The soilless cultivation of eggplants has expanded considerably over the last two decades. Whereas, initially, open hydroponic systems were mostly involved, recent environmental regulations against groundwater pollution and the require-ment to minimize water and fertilizer consumption have led to the recycling of nutrient solutions. However, if the tap water used to prepare the nutrient solution has a high salt concentration (usually NaCl but also Ca and Mg bicarbonates and sulphates) the reuse of the drain water may result in salt accumulation in the nutrient solution, accompanied by depletion of other nutrients such as K (Sonneveld, 1981; Savvas and Manos, 1999). If modern automatic equipment is used to control the recycling of the drain solution, this process might be retarded by progressively increasing the target electrical conductivity (EC) of the nutrient solution which is given as a preset value to the control system (Savvas and Manos, 1999). However, this would expose the plants gradually to a salt stress. Moreover, if tap water with a high salt content is used, the total salt concentration of the nutrient solution supplied to the plants will essentially be higher than the recommended target value of EC for the crop. Hence, the question is raised, as to how the eggplants respond to a moderate increase of EC in the nutrient solution. The responses of hydroponically grown eggplants to increasing EC in the nutrient solution due only to the presence of NaCl have been well documented (Savvas and Lenz, 1994b, 1996). However, the detrimental effects of salinity on plants may be either indiscriminate (osmotic), if the total salt concentration determines the extent of growth restriction, or ion speci®c, if the kind of salts being in excess in the nutrient solution is crucial for the plant response (Bernstein, 1975; Shannon and Grieve, 1999). Therefore, when studying the in¯uence of increasing salinity on growth and development of eggplant in soilless culture systems, it is important to compare nutrient solutions having equal electrical conductivities but different ionic composition.
This paper reports some results concerning the responses of eggplants grown on rockwool to a moderate increase in the EC of the supplied nutrient solution, induced either by NaCl or by providing additional amounts of nutrients at two different cation proportions.
2. Materials and methods
Seedlings of eggplant (Solanum melongenaL.) cv. `Leanda' raised on rockwool cubes (0.7 l) were transferred to 32 uncovered rockwool slabs (90 cm
15 cm7.5 cm) in a heated glasshouse as soon as they had formed ®ve true
polyethylene ®lm. In each channel, ®ve plants were placed. The eight channels were arranged in four rows spaced 120 cm apart. Additional plant rows were placed on the four sides around the experimental installation to prevent margin effects. All plants were trained with three stems. The minimum day/night temperatures were set at 21/198C prior to the beginning of ¯owering and 21/168C thereafter. No measures were taken to improve fruit set.
In each channel (experimental unit) an individual tank was provided to supply the plants with nutrient solution via a drip irrigation system. The nutrient solution was automatically supplied to the plants at a rate of 20 l/h per channel using a pump and a special timer for irrigation scheduling. In each channel, the nutrient solution was continuously recirculating during the day to maintain a constant nutrient and water status in the root zone, whereas at the night no solution was supplied to the plants.
Four different nutrient solution treatments were applied to the eight experimental units, so that each treatment was duplicated. In particular, there was a basic nutrient solution (BNS) suitable for eggplants (Voogt, 1986) having an EC of 2.1 dS mÿ1
and three saline nutrient solutions having the same EC (4.7 dS mÿ1
). The EC was raised up to 4.7 dS mÿ1
by adding to the basic solution either nutrients at the same ionic concentration ratio as in the BNS, or nutrients at an increased K/(KCaMg) ratio, or 25 mmol lÿ1 NaCl. The increased K/
(KCaMg) ratio (meq/meq) was 0.473 whilst the standard value was 0.40.
The compositions of the four nutrient solutions are given in Table 1.
Table 1
Composition of the four nutrient solutions used as experimental treatmentsa Nutrient Basic nutrient
3 15.50 34.10 34.10 15.50
H2POÿ4 1.50 3.30 3.30 1.50
Fe 15.00 33.00 33.00 15.00
Mn 10.00 22.00 22.00 10.00
Zn 5.00 11.00 11.00 5.00
B 25.00 55.00 55.00 25.00
Cu 0.75 1.65 1.65 0.75
Mo 0.50 1.10 1.10 0.50
a
The concentrations of macronutrients, NaCl, and micronutrients are given in mM, mM and
All nutrient solutions were prepared using rain water. The amount of nutrient solution consumed by the plants was replenished regularly during each day. Every 10 days the concentration of N, P, K, Ca, and Mg was determined and adjusted in all nutrient solutions. Moreover, at fortnightly intervals the nutrient solutions were renewed completely in all tanks. In each experimental unit, 14 l of nutrient solution per plant were in recirculation during the day. This ratio proved to be suf®cient to prevent considerable changes in the nutrient ratios in short periods of less than 10 days. However, in the NaCl-salinity treatment, the Na concentration in the nutrient solution was measured and adjusted by adding NaCl twice weekly. The eggplant seedlings were planted on 15 March and the exposure to salinity began 10 days later. The ®rst harvest took place on 2 May and the experiment was terminated on 2 October. During the whole growing period, the opening ¯owers per plant in each treatment were counted weekly and marked to avoid a double registration. Ripening fruits were harvested twice weekly, weighed, and graded to determine the percentage of yield graded Class 1. Grading was performed in accordance with the European Community standards, whilst a fruit weight of 175 g was the lowest size accepted for Class 1. Since at each ¯ower level of eggplant, besides one basal ¯ower, some additional ¯owers may also be produced (Nothmann et al., 1979; Passam and Khah, 1992), the ¯owers and the harvested fruits were recorded either as basal or as additional ones. Moreover, the proportion of small fruit yield (<175 g) and of fruits affected by the physiological disorder internal fruit rot (Savvas and Lenz, 1994a), was also registered. At three different dates, fruit samples were dried at 658C to determine their dry matter content and this was used to estimate the total fruit dry weight per plant for each salinity treatment. During the whole growing period, the senescent leaves of two plants per experimental unit were removed weekly and, after measuring their area using a Licor 3100, they were dried at 658C to determine their dry weight. At the end of the growing period, these plants were harvested, separated into roots, stem and leaves, weighed and dried at 658C to constant weight to determine the total root, stem and leaf dry weight per plant, respectively. Prior to drying, the leaf area was measured. Thus, the total leaf areas and leaf dry weights per plant were estimated by adding the areas and the dry weights of the senescent leaves that had been removed during the growing period to those for the leaves harvested at crop termination. The roots were separated from the rockwool slabs using a HCl solution (1.8%) as proposed by Brouwer and Van Noordwijk (1978). To prevent harvesting of unripe fruits at crop termination, all ¯owers appearing during the last 3 weeks of the experiment were removed.
K, and Mg was determined by atomic absorption spectrophotometry (Perkin-Elmer, 373) after digestion with HClO4 and HNO3 at 2008C. Phosphorus was estimated colorimetrically as phosphomolybdate blue complex at 880 nm from the same extract. Organic nitrogen was determined colorimetrically at 630 nm after Kjeldahl digestion. Nitrate was reduced to nitrite by cadmium and determined colorimetrically at 540 nm after formation of a diazo complex. The colorimetrical determinations of P, Kjeldahl-N and NO3±N were performed by methods similar to those described by KuÈnsch et al. (1977), using a ``Segmented Flow Analyzer System'' type SFAS 5100 of SKALAR.
The data were subjected to single factor analyses of variance, and when a signi®cant F-test was obtained, means were separated using Duncan's MRT (p0.05).
3. Results
Increasing the nutrient solution salinity from 2.1 up to 4.7 dS mÿ1
had no signi®cant in¯uence on the leaf area per plant, which on average was 1154 dm2. Moreover, the numbers of basal and additional ¯owers per plant (112 and 123, respectively) were not signi®cantly affected by salinity, regardless of the salts used to raise the EC. In contrast, the total fresh fruit weight per plant was signi®cantly reduced by 22±23% in all salinity treatments, whilst the salinity source had no speci®c effect on the yield depression (Table 2). The reduction of yield could be attributed to the lower mean weight of the basal fruits. The number of fruits per plant and the mean weight of additional fruits were unaffected by the salinity treatments.
Table 2
Effect of increasing the salt concentration of the nutrient solution using either additional nutrients at two different cation ratios (meq/meq) or NaCl on fruit yield and yield components in eggplants grown in rockwoola Basic nutrient solution (BNS)b 14.09a 0.25a 263.4a 54.3a 53.6a 4.50a
K/(KCaMg)0.40c 10.91b 0.27a 208.0b 52.4a 52.5a 5.25a K/(KCaMg)0.47c 11.23b 0.44a 229.5c 69.7a 49.0a 6.50a
BNS25 mM NaClc 11.31b 0.31a 212.4bc 55.1a 53.3a 5.25a
aIn each column, values followed by the same letter do not differ signi®cantly at
p0.05.
b
2.1 dS mÿ1
.
The only effect of the treatments on the root, stem, and leaf dry weights was the greater root dry weight in the plants exposed to NaCl-salinity (Table 3). Moreover, no signi®cant differences between the treatments in the total fruit dry weight per plant could be found, although the salinity had depressed the fruit fresh weight.
The exposure of eggplant to moderate salinity reduced the percentage of yield graded Class 1 and increased the proportion of undersized fruit yield (<175 g). In contrast, there was no signi®cant effect on the incidence of internal fruit rot (Table 4).
The K and Ca concentrations of roots, fruits and leaves of eggplant were not in¯uenced by the increase of total nutrient concentration up to 4.7 dS mÿ1
, regardless of the cation ratio in the nutrient solution. The addition of 25 mmol lÿ1 NaCl to the nutrient solution reduced the K concentration only in the roots from 20.5 to 11.3 mg gÿ1
dry wt. The Ca concentration in the petioles of old leaves was decreased by the NaCl salinity from 20.6 to 16.8 mg gÿ1
dry wt. The roots of
Table 3
Effect of increasing the salt concentration of the nutrient solution by providing either additional nutrients at two different cation ratios (meq/meq) or NaCl on the dry weights of roots, stem, leaves and fruits per plant in eggplants grown in rockwoola
Nutrient solution treatment Root (g) Stem (g) Leaves (g) Fruits (g) Basic nutrient solution (BNS)b 127.8a 571.4a 481.8a 895.3a
K/(KCaMg)0.40c 123.8a 556.9a 462.8a 824.8a K/(KCaMg)0.47c 132.9ab 518.5a 424.2a 801.0a
BNS25 mM NaClc 153.4b 566.3a 458.4a 854.3a
aIn each column, values followed by the same letter do not differ signi®cantly at
p0.05.
b
2.1 dS mÿ1
.
c4.7 dS mÿ1.
Table 4
Effect of increasing the salt concentration of the nutrient solution by providing either additional nutrients at two different cation ratios (meq/meq) or NaCl on fruit quality parameters in eggplants grown in rockwoola
Nutrient solution treatment Yield graded Class 1 (% w/w)
Small fruit (<175 g) yield (% w/w)
Fruits affected by IFR (%) Basic nutrient solution (BNS)b 91.2a 7.6a 0.0a
K/(KCaMg)0.40c 76.9b 19.9b 1.42a K/(KCaMg)0.47c 84.2c 14.2b 0.47a
BNS25 mM NaClc 82.0bc 15.8b 0.66a
aIn each column, values followed by the same letter do not differ signi®cantly at
p0.05.
b
2.1 dS mÿ1
.
plants supplied with BNS contained 18.4 mg Ca gÿ1
dry wt. and this value did not differ signi®cantly from those found in the salinity treatments. However, the root Ca concentration of the plants exposed to NaCl salinity (15.5 mg gÿ1
dry wt.) differed signi®cantly from the values measured in the nutrient-induced salinity treatments which on average amounted to 20.8 mg gÿ1
dry wt. On average, the K concentrations were 39.7, 72.6, 61.7, 137.2, and 127.3 mg gÿ1
dry wt. in the fruits, laminae of old and young leaves, and petioles of old and young leaves, respectively. Similarly, the Ca concentrations were 1.09, 49.5, 17.9, and 11 mg gÿ1
dry wt. in the fruits, laminae of old and young leaves, and petioles of young leaves, respectively.
Increasing the EC up to 4.7 dS mÿ1
, resulted in signi®cantly lower Mg concentrations in the leaf laminae of eggplant, even when the Mg concentration in the nutrient solution was more than doubled (Table 5). When the salinity was achieved by providing NaCl or extra nutrients at an increased K/Mg ratio, the Mg concentration was lowered also in the petioles of old leaves. The Mg concentration in the roots and fruits was not in¯uenced by any salinity treatment. Raising the salinity of the nutrient solution by supplying extra nutrients increased the concentration of organic N in the mature petioles from 23.0 to 25.2 mg gÿ1
dry wt. but had no effect on the concentrations in the other tissues, which were 23.5, 25.0, 30.1, 51.2 and 24.2 mg gÿ1
dry wt. in the roots, fruits, old leaf laminae, young leaf laminae and young petioles, respectively.
In contrast to organic N, the nitrate concentration was reduced in the roots and fruits by all the salinity treatments, whilst the NaCl salinity depressed the concentration of NO3±N also in the old laminae and petioles. The reductions of NO3±N in the laminae and petioles of young leaves were less consistent (Table 6). The NaCl salinity decreased slightly the P concentration in the roots and the laminae of old leaves (Table 7). In contrast, the salinity treatments obtained by
Table 5
Concentration of Mg (mg gÿ1
dry wt) in different plant parts of eggplants grown in rockwool as in¯uenced by increasing the salt concentration of the nutrient solution by providing either additional nutrients at two different cation ratios (meq/meq) or NaCla
Nutrient solution Roots Fruits Old laminae Basic nutrient solution (BNS)b 2.07a 2.61a 4.76a 5.00a 3.89a 1.77a K/(KCaMg)0.40c 2.05a 2.44a 3.80b 4.63a 3.50b 1.73a
K/(KCaMg)0.47c 1.96a 2.38a 3.63b 3.79b 3.39b 1.44a BNS25 mM NaClc 2.16a 2.65a 3.70b 3.86b 3.51b 1.59a
a
In each column, values followed by the same letter do not differ signi®cantly atp0.05.
b2.1 dS mÿ1. c
4.7 dS mÿ1
raising the nutrient concentration caused marked increases in the P contents of the roots and, to a lesser extent, in the petioles of old leaves.
4. Discussion and conclusions
The present results revealed that, in soilless culture, the vegetative growth and the yield of eggplant respond differently to moderate levels of salinity. Although the former was unaffected by the increased EC, the fruit fresh weight was depressed. Moreover, the effects of increasing the salinity by adding NaCl were similar to those obtained by raising the nutrient concentration, thus suggesting that the effects were osmotic in origin and not salt speci®c. Adams (1991) came to the same conclusion for tomatoes in the range 3±8 dS mÿ1
. The absence of any signi®cant differences in the number of ¯owers and fruits per plant indicates that,
Table 6
Concentration of NOÿ
3ÿN (mg g
ÿ1
dry wt) in different plant parts of eggplants grown in rockwool as in¯uenced by increasing the salt concentration of the nutrient solution by providing either additional nutrients at two different cation ratios (meq/meq) or NaCla
Nutrient solution Roots Fruits Old laminae Basic nutrient solution (BNS)b 2.48a 0.81a 15.6a 38.7a 6.95a 23.0a K/(KCaMg)0.40c 2.01b 0.46b 15.4a 39.3a 6.15b 22.3a
K/(KCaMg)0.47c 2.02b 0.65c 15.8a 39.6a 6.38a 22.6a BNS25 mM NaClc 1.32c 0.43b 13.1b 33.7b 6.01b 20.3a
a
In each column, values followed by the same letter do not differ signi®cantly atp0.05.
b2.1 dS mÿ1.
dry wt.) in different plant parts of eggplants grown in rockwool as in¯uenced by increasing the salt concentration of the nutrient solution by providing either additional nutrients at two different cation ratios (meq/meq) or NaCla
Nutrient solution Roots Fruits Old laminae Basic nutrient solution (BNS)b 5.81a 5.21a 4.71a 5.40a 5.04a 4.61a
K/(KCaMg)0.40c 9.18b 4.99a 4.72a 6.08b 5.00a 4.58a K/(KCaMg)0.47c 8.54c 5.11a 4.91a 6.53b 5.14a 4.65a
BNS25 mM NaClc 5.11d 5.41a 4.29b 5.43a 4.67a 4.82a
aIn each column, values followed by the same letter do not differ signi®cantly at
p0.05.
b
2.1 dS mÿ1
.
in eggplants, the setting of fruits is not in¯uenced by increasing the EC up to 4.7 dS mÿ1
, regardless of the salinity source.
The present study clearly demonstrated that the detrimental effects of moderate salinity on the yield of hydroponically grown eggplants are due only to a decreased mean fruit weight. This yield response of eggplant was observed also at higher EC values up to 8.1 dS mÿ1
obtained by adding NaCl to a BNS (Savvas and Lenz, 1994b). As suggested by Adams (1991) and Cuartero and Fernandez-MunÄoz (1999), the yield reduction observed in tomato at relatively low salinity levels is also caused by a decrease in mean fruit weight whilst the number of fruits per plant is not affected.
Unlike the fruit fresh weight, the dry weights of fruits and vegetative parts and the leaf area were not signi®cantly reduced by the salinity. However, at higher EC levels (6.1 and 8.1 dS mÿ1
) obtained by adding NaCl to the BNS, the leaf area and the dry weight of leaves and stem per plant were also restricted, and the fruit dry weight was reduced almost as much as the growth of the vegetative organs, whereas the fruit fresh weight was even more severely depressed (Savvas and Lenz, 1994b). Consequently, the more detrimental effects of salinity on the yield than on the vegetative organs of eggplant should be attributed to a restriction of water accumulation in the fruit. As suggested by Johnson et al. (1992) the water ¯ow into the fruit under saline conditions is restricted by a lower water potential in the plant. This response is of osmotic origin and is, therefore, independent of the salts raising the EC of the nutrient solution in the root environment. A lower water accumulation in the fruit, which resulted in a higher dry matter content, was observed also in tomatoes when they were exposed to increased EC, irrespective of the salinity source (Ehret and Ho, 1986, Adams, 1991, Willumsen et al., 1996). As shown in Table 2, only the basal fruits were affected by salinity, whereas neither the number nor the mean weight of the additional fruits were reduced. Similar results were found also at higher EC values up to 8.1 dS mÿ1
(Savvas and Lenz, 1994b). According to Caro et al. (1991) the smaller the fruit size of tomatoes, the lower is the reduction in yield due to salinity. The smaller response of additional fruits to salinity may be explained, therefore, by their much lower mean fruit weight than that of basal fruits (Table 2), possibly because the ability of smaller fruits to acquire water is less severely affected by salinity.
Even a moderate level of salinity has an adverse effect on the fruit quality of eggplants, since the proportion of yield graded Class 1 is reduced as a result of a higher percentage of undersized fruits.
The absence of any ion speci®c effects when the EC of the nutrient solution was raised up to 4.7 dS mÿ1
though statistically signi®cant, are unlikely to have substantially in¯uenced the overall nutritional status of the plants. However, the increase in P concentration in the roots of plants exposed to nutrient-induced salinity was considerable. As suggested by Bernstein et al. (1974) and Nieman and Clark (1976), the P uptake is markedly increased and may result in P toxicity when high salinity is accompanied by enhanced P supply. These interactions between P and salinity would usually occur in soilless culture and not under ®eld conditions, since the phosphate concentrations in the soil solutions are usually orders of magnitude lower than in the hydroponic nutrient solutions (Grattan and Grieve, 1999). Nevertheless, as indicated by our results (Table 7), under conditions of moderate salinity and high P supply the eggplant seems to retain the excess P mainly in the roots, thus preventing the occurrence of P toxicity in the growing leaves.
Decreased nutrient concentrations in some plant parts of eggplant were mainly found in the NaCl salinity treatment. The effects of NaCl salinity on the mineral composition of eggplant in soilless culture were investigated in a previous study in the range 2.1±8.1 dS mÿ1
(Savvas and Lenz, 1996). However, it should be pointed out that the Mg concentration in the leaves, mainly in the laminae, is reduced almost to the same extent in all salinity treatments, regardless of the salts being in excess in the root environment. Sonneveld and Welles (1988) also reported lower Mg concentrations in leaves of tomatoes grown on rockwool at high EC levels induced by increasing the nutrient concentration in the nutrient solution. The decrease of leaf Mg concentration at high EC values, regardless of salinity source, indicates that the restricted Mg supply to the eggplant leaves under saline conditions is not a salt speci®c effect. Nevertheless, it remains unclear whether salinity affects directly the Mg uptake by the roots, the Mg translocation through the plant, or any physiological function in which Mg is involved, thereby forcing the plant to adjust the leaf Mg due to a reduced demand.
The depressing effects of NaCl salinity on the tissue NOÿ
3 concentration were
reviewed recently by Grattan and Grieve (1999). However, the reduced nitrate concentrations in the roots and fruits of eggplants exposed to nutrient-induced salinity indicate that the NOÿ
3 uptake and translocation into the plant may be
partly affected also by a reduced water uptake, as suggested by Lea-Cox and Syvertsen (1993) and not only by Clÿ
antagonism of NOÿ
3 uptake.
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