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Saline water irrigation effects on soil salinity distribution and some physiological responses of field grown Chemlali olive
Journal of Environmental Management 113 (2012) 538e544
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Saline water irrigation effects on soil salinity distribution and some physiological responses of field grown Chemlali olive Chedlia Ben Ahmed a, b, *, Salwa Magdich a, b, Bechir Ben Rouina b, Makki Boukhris a, Ferjani Ben Abdullah a a b
Laboratory of Environment and Biology of Arid Area, Department of Life Sciences, Faculty of Sciences of Sfax, Tunisia Laboratory of Improvement of Olive and Fruit Trees’ Productivity, Olive Tree Institute of Sfax, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2009 Received in revised form 3 September 2011 Accepted 8 March 2012 Available online 8 May 2012
The shortage of water resources of good quality is becoming an issue in arid and semi arid regions. Per consequent, the use of water resources of marginal quality is becoming an important consideration, particularly in arid regions in Tunisia, where large quantities of saline water are used for irrigation. Nevertheless, the use of these waters in irrigated lands requires the control of soil salinity and a comprehensive analysis even beyond the area where water is applied. The aim of this study was to investigate the effects of saline water irrigation on soil salinity distribution and some physiological traits of field-grown adult olive trees (Olea europaea L. cv. Chemlali) under contrasting environmental conditions of the arid region in the south of Tunisia. The plants were subjected, over two growing seasons, to two drip irrigated treatments: fresh water (ECe ¼ 1.2 dS m1, FW) and saline water (ECe ¼ 7.5 dS m1, SW). Saline water irrigation (SW) has led to a significant increase in soil salinity. Furthermore, these results showed that soil salinity and soil moisture variations are not only dependent on water salinity level but are also controlled by a multitude of factors particularly the soil texture, the distance from the irrigation source and climatic conditions (rainfall pattern, temperature average, .). On the other hand, salt treatment reduced leaf midday water potential (LMWP), relative water content and photosynthetic activity and increased the leaf proline content, and this increase was season-dependent. Indeed, LMWP in SW plants decreased to 3.71 MPa. Furthermore, the highest level of proline in SW plants was registered during summer period (2.19 mmol/mg Fw). The proline accumulation recorded in stressed plants has allowed them to preserve appropriate leaf water status and photosynthetic activity. More to the point, this olive cultivar seems to be more sensible to soil salinity during the intense growth phase. Such tendencies would help to better manage water resources for irrigation, particularly under actual climatic conditions of water scarcity. For example, in the case of the availability of different water qualities, it would be better to preserve those of high quality for olive irrigation during the intense vegetative growth phase, in coincidence with high salt sensitive period, and those of low quality for irrigation during partial growth and plant rest phases. What’s more, the urgent use of saline water for irrigation should not be applied without taking into consideration the different surroundings conditions where it is used, particularly the water salinity level, the soil type, the adopted irrigation system, the degree of the crop salt tolerance, the plant growth phase and the climatic conditions of the experimental site. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Irrigation Olea europaea L. Saline water Photosynthetic activity Proline accumulation Arid climate
1. Introduction Traditionally, olive tree was cultivated under rain-fed conditions. In recent decades, the olive plantation has been extended to irrigated lands. However, in arid and semi arid regions, as those in
* Corresponding author. Laboratory of Environment and Biology of Arid Area, Department of Life Sciences, Faculty of Sciences of Sfax, Sfax, Tunisia. Tel.: þ216 74 276 400/274 923; fax: þ216 74 274 437. E-mail address: [email protected] (C. Ben Ahmed). 0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2012.03.016
Tunisia, the limited water availability and the increased need for good water quality for urban use restrict the use of fresh water for irrigation. So, large quantities of marginal water, as saline water, are used for olive tree irrigation. Salt stress inhibits photosynthesis by reducing water potential (Parida and Das, 2005). This reduction has been explained by the dehydration of cellular membrane which reduces the permeability of CO2 by the hydroactive stomatal closure (Loreto et al., 2003). The maintenance of an appropriate plant water status under stressed conditions is essential for continued growth of the plant. This process can be achieved by stomatal regulation (Chartzoulakis,
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2005; Tattini et al., 2008) and accumulation of compatible solutes in either leaves or roots (Ashraf et al., 2008). These osmolytes play a great role in facilitating water retention in the cytoplasm and activation of water uptake to the growing tissues. Proline was known as the main important osmolyte compound accumulated under salinity conditions (Ashraf and Foolad, 2007). Nowadays, the controlled use of marginal water (saline water, treated waste water) to improve the qualitative characteristics of horticultural products is becoming more and more important, particularly under actual conditions of limited water resources and rainfall scarcity in arid regions. For fruit trees, there is some evidence that saline water could improve yield. Indeed, in “Picual” olive tree, Melgar et al. (2009) found that, for most of the different crop seasons, the long term irrigation with saline water resulted in an increase of olive oil content linearly with the water salinity level. The same authors showed that salinity did not affect the growth parameters and leaf salt ions concentrations did not reached the toxicity threshold. Although, there is some evidence that olive tree is moderately tolerant to salinity conditions and the tolerance level is cultivar and plant-age dependent (Chartzoulakis, 2005; Loreto et al., 2003; Tattini et al., 2008), most of the investigations have been carried out on young plants, grown in pots and/or under controlled conditions and little information is known on olive responses to salinity conditions under field locale, particularly, under contrasting environmental circumstances of arid region in Tunisia. The objectives of this investigation were to determine the effects of saline water used for irrigation, over two successive growing seasons under natural environmental conditions, on soil salinity distribution (with soil depth and around the irrigation source) and on some physiological traits of adult olive cv. Chemlali. 2. Materials and methods 2.1. Plant material, treatments and climatic conditions Twelve-year- old olive trees (Olea europaea L. cv. Chemlali), planted on a sandy soil at a density of 625 trees ha1 at Sfax, Tunisia (34 43N, 10 41E), were used in 2005 and 2006 crop seasons. Ten trees from two adjacent rows (total 20 trees per treatment), with four replications of 5 trees each, were selected to be similar in potential yield and canopy. The plants were subjected to the following treatments: irrigation with fresh water (FW, 1.2 dS m1 ECe); and saline water (SW, 7.5 dS m1 ECe). The water used was either that supplied by the Tunisian National Water Carrier, or saline water from the local reservoir situated in the area of the Olive Tree Institute in Sfax. The fresh and saline water used were characterized by 145 and 600 mg/l Naþ, 326 and 1169 mg/l Cl, 280 and 520 mg/l Kþ, 94 and 261 mg/l Ca2þ, 57 and 102 mg/l Mg2þ, respectively. The amount of water supplied to olive trees was estimated according to the Penman-Monteith-FAO equation (Doorenbos and Pruitt, 1977) as described by Ben Ahmed et al. (2007). The
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irrigation was delivered using a drip system with 4 drip nozzles (two per side), of 4 l h1 per tree set in a line along the rows (at 0.5 m from the trunk). Without taking rainfall into account, total water supplied to mature olive trees was 4000 m3/ha/year. The sandy soil of the experimental orchard (90.5% sand, 4.5% clay and 5% silt) was characterized by an organic matter of 1.1%, 13.4% CaCO3, 1.3% N, pH of 7.6, a field capacity (measured at 33 KPa) of 11.8% and a wilting point (measured at 1500 KPa) of 5.9%. The region is characterized by an arid climate of Mediterranean type. The annual rainfall and temperature averages over a 52-year period were of 250 mm and 23 C, respectively. In both crop seasons, precipitation was virtually absent during summer and it was of 218.5 and 285.5 mm, respectively in the first and the second crop seasons. The mean temperatures were of 25.6 and 25.1 C, respectively and maximum temperatures were of 38 and 37 C, respectively. The evapotranspiration rates were of 1413 and 1271 mm in 2004 and 2005, respectively. Climatic conditions of the experimental site during the trial period are summarized in Table 1. In Tunisia, the different seasons periods are as following: the spring season: from March to May; the summer season: from June to August; the autumn: from September to November; and winter season: from December to February. 2.2. Soil moisture and salinity measurements At the middle of each month along the different seasons (spring, summer, autumn and winter), three soil samples per treatment were taken from the surface (0e30 cm) until a depth of 1.2 m with a layer of 0.3 m. On these samples, the soil moisture (%) and the electrical conductivity (ECe) of the saturated phase were determined. Values represented here correspond to the means of the different measurements along each season. The ECe was determined also at different distance (0, 0.15, 0.3 and 0.6 m) from the irrigation source. Values of soil moisture for each soil depth represent the means of the measurements for the different distances from the irrigation source. 2.3. Physiological parameters The measurements of photosynthetic activity (Pn) and relative water content (RWC) were carried out on leaves selected from the median part of the shoots. Pn was measured on well-exposed four leaves per plant from five plants per treatment from 10:00 to 13:00 pm using a portable gas exchange system (Li-CorInc e 6200, Lincoln, Nebraska USA). The RWC was determined as following (Gucci et al., 1997):
RWC ð%Þ ¼ ½ðFw DwÞ=ðTw DwÞ 100 where Fw is the fresh weight, Dw the dry weight and Tw the turgid weight of leaf samples. Predawn and midday leaf water potentials (LPWP and LMWP) were measured, on leaves taken from the median part of the same
Table 1 Principal seasonal climatic conditions (Air temperature (T), precipitation (P), and photosynthetic active radiations (PAR)) of the experimental site during the trial period. Spring
Summer
2004/2005
Crop season T ( C) P (mm) PAR (mmol m2 s1)
28 2.45 31.5 3.48 1013 56.5
36.6 2.87 1 0.42 1418.6 59.3
2005/2006
T ( C) P (mm) PAR
28.2 1.89 47.8 5.78 945.6 45.91
36.3 2.45 1.2 0.49 1414 67.8
Autumn
Winter
30.6 2.12 56.5 4.91 970.3 28.3
23.2 3.56 72.8 5.28 746.7 47.81
29.8 2.63 61.2 5.28 1076.6 44.92
23.6 2.13 58.3 3.48 775.8 37.82
Values of each season are means of averages for the three respective months (SD). In Tunisia: Spring: MarcheAprileMay; Summer: JuneeJulyeAugust; Autumn: SeptembereOctobereNovember and Winter: DecembereJanuaryeFebruary.
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shoots served for leaf relative water content, by a Scholander Pressure Chamber (pms-1000, Oregon, Corvallis, USA) (Gucci et al., 1997). To minimize water loss during the transfer of the leaf to the chamber, leaves were enclosed immediately, after excision, in a black plastic bag.
end of the irrigation period. Furthermore, the lower level of soil salinity registered at a soil depth of 1.2 m, in comparison to the layer of 0.3 m, let us suggest that salts are transported and accumulated at higher depth, and that the rainfall occurring generally in autumn and winter was too sufficient to assure the leaching of salts, accumulated during summer season, as reported by Melgar et al. (2009) in “Picual” olive orchard located in southern Cordoba province, Spain. This process seems to be facilitated by the sandy soil texture (90.5% sand) allowing the free water circulation through the soil. Indeed, an essential factor determining soil salinity and crop salt response is soil texture. In fact, olive tree responses to drought conditions, which were in some extent similar to those under salt stress, are soil type dependent (Ben Rouina et al., 2007). The same authors showed that the effects of water deficit on Chemlali olive tree were more deleterious on clay than on sandy soils. This could be explained by the fact that the water holding capacity is soil texture dependent and sandy soil, the most abundant soil type in arid region in Tunisia, has lower waterholding capacity than a medium textured soil. It is well known that sandy soil loses more water than a clay soil which can lead to a more rapid increase of soil solution concentration. Besides, the irrigation source location (at 0.5 m from the trunk) and the drip system used for irrigation seem to play a great role in the distribution of soil salinity, via the exclusion of salts outside the root zone, and per consequent the maintenance of plant water status at acceptable level. Indeed, soil salinity distribution is the result of the interaction of water salinity level, irrigation source location, soil texture and climatic conditions. In addition, the active root zone of olive could affect the soil salinity distribution. Indeed, the high ability of olive tree to accumulate salts in their active roots, generally localized at a depth superior than 0.3 m, allows the decrease of soil salinity level. This strategy is commonly developed by the salt stressed plants to decrease the osmotic potential in the roots, via accumulation of inorganic salts, in order to activate water retention and transport from the soil to the plant (Munns and Tester, 2008; Parida and Das, 2005; Tattini et al., 2008) The differential pattern in soil salinity among the two treatments was accompanied with that of soil moisture (Fig. 3). Indeed, the use of saline water for irrigation resulted in a decrease of soil moisture levels, in comparison to values recorded in FW treatment, but at different extent among seasons. In fact, the layer with lower ECe showed the higher soil moisture values (Fig. 3). Besides, during both crop years, the largest decrease of soil moisture was registered in summer season in coincidence with harmful climatic conditions. Differences in soil moisture values between the two treatments were significant for the levels recorded in summer, autumn and winter of the first crop year and
2.4. Proline content determination Leaf samples (collected from the median part of selected shoots) for proline content determination were frozen immediately in liquid nitrogen. Free proline content was determined according to Bates et al. (1973) using L-proline for the standard curve (0e50 mg/ ml). 2.5. Statistical analysis Statistical analyses were performed using the SPSS 10.0 Windows. The treatments means were compared using least significant difference (LSD) test at 5% level and seasons and soil depths means were compared using Tukey’s test calculated at 5% level. At least three replicates were used for each laboratory and field tests. 3. Results and discussion 3.1. Soil moisture and salinity distribution The soil salinity variation was different among the two salinity treatments with significantly more important values under high soil salinity level (p < 0.05), but at different extent among soil layers and crop seasons. In fact, during the second trial year, the ECe of the soil saturated paste at the upper (0e0.3 m) and deep soil layers (0.9e1.2 m) of SW treatment was at more than 4 times higher than the levels recorded in FW one. On the other hand, in spring and autumn 2006, the ECe of the different soil layers and distances from the irrigation source of SW treatment was 5 times higher than those registered in the case of FW treatment, with except for the soil sample taken in autumn at 1.2 m soil depth, at 0.6 m from the irrigation source. Furthermore, the highest salt accumulation was registered at the soil layer of 0.3 m, and there is a slight decrease of ECe through the soil depth (Figs. 1 and 2). Moreover, salts were more accumulated during summer season and salt distribution through the soil depth was affected by autumnewinter rainfall. The seasonal variation of the soil salinity level in the 1.2 m depth showed that it was significantly lower during autumnewinter period than that of the spring-summer one (p < 0.05). These results are in agreement with the findings of Melgar et al. (2009) showing that salts were leached by the rainfall occurring at the
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Fig. 1. Changes in soil salinity (ECe) at different distance from the irrigation source for the different deep layers (described above) in FW (A) and SW (B) treatments during 2005 crop season. d1, d2, d3 and d4 represent the distance from the irrigation source (0, 0.15, 0.3 and 0.6 m, respectively). H1, H2, H3 and H4 represent the different soil depths (0.3, 0.6, 0.9 and 1.2 m, respectively). Values are the means of three soil samples measurements per month for each season (n ¼ 9).
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Fig. 2. Changes in soil salinity (ECe) at different distance from the irrigation source for the different deep layers (described above) in FW (A) and SW (B) treatments during 2006 crop season. d1, d2, d3 and d4 represent the distance from the irrigation source (0, 0.15, 0. 3 and 0.6 m, respectively). H1, H2, H3 and H4 represent the different soil layers (0.3, 0.6, 0.9 and 1.2 m, respectively). Values are the means of three soil samples measurements per month for each season (n ¼ 9).
summer and autumn of the second crop year (p < 0.05), but at different extent among soil layers and seasons. The higher ECe value observed at the upper soil level (0.3 m depth), in comparison to the different soil layers (0.6, 0.9 and 1.2 m), was due to the higher evaporation occurring in the surface as reported by Aragüés et al. (2005). In fact, salts are more accumulated in dry layers. On the other hand, the horizontal variation of soil salinity displayed that salts are less accumulated in the drip zone (0e0.15 m), in comparison to the more outlying ones (0.3 and 0.6 m). 3.2. Leaf water relations The saline water used for irrigation has altered leaf water status of stressed plants. Indeed, these latter showed lower values of relative water content (RWC) and leaf water potentials (LPWP and LMWP) than those of FW with statistically significant differences between them (Fig. 4). The RWC in stressed plants ranged from 73 to 83% (Fig. 5A). Leaf predawn water potential varied between 1.12
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and 0.65 MPa and from 2.36 to 1.27 MPa, respectively in FW and SW-treated plants. Leaf midday water potential varied from 1.99 to 1.21 MPa and between 3.71 and 2.31 MPa, respectively. These results are in concordance with previous findings suggesting that the reductions of leaf water potential and relative water content are among the earliest responses of olive tree under stressed conditions (Chartzoulakis, 2005; Tattini and Traversi, 2009). Indeed, as soon as the plants subjected to salt stress conditions perceived water deficit, they reduce progressively their leaf water potential in such a way to maintain a water potential gradient, between the soil and the plant, favourable for water uptake and adsorption. Such mechanism would to help the plants to preserve appropriate leaf water status (Gucci et al., 1997; Sofo et al., 2005). Moreover, the deleterious effects of salt stress were reinforced by the severe environmental conditions characterizing the experimental site. Indeed, in both treatments, over the two crop seasons, the lowest values of RWC and leaf water potentials were recorded during summer period in coincidence with high air temperature and high light intensity. Nevertheless, stressed plants tend to restore better levels of water status. Indeed, the occurrence of more moist times (autumn period) was accompanied with better plant water status. This behaviour displayed, at the same time, the resistance of the Chemlali olive tree to salt stress and its ability for recovery under better climate conditions (low air temperature, high precipitation, .) (Table 1) as has been reported in Plantago coronopus by Koyro (2006). In addition, the decrease of leaf water potential to 3.7 MPa under salt stress conditions, represent for the Chemlali olive tree, an adaptive mechanism to activate water retention and uptake, more than a merely negative consequence of it. Besides, the important variation of plant water status over the two crop seasons justified that the plant water status in olive tree is not only dependent on water quality treatment but also on the different environmental conditions characterizing the experimental site (Table 1) and the olive vegetative growth phase.
g rin
05 Su
5 5 6 6 6 5 6 r0 r0 n0 n0 g0 er 0 er 0 nte nte rin tum tum mm mm Sp Wi Wi Au Au Su
Fig. 3. Changes in soil moisture at different deep layers in FW (A) and SW (B) treatments during 2005 and 2006 crop seasons. H1, H2, H3 and H4 represent the different soil depths (0.3, 0.6, 0.9 and 1.2 m, respectively). At each soil depth, values of soil moisture represent the means for the different distances from the irrigation source. Values are the means of three soil samples measurements per month for each season (n ¼ 9).
Salt stress has enormously altered photosynthetic activity of olive tree (Fig. 5A). Furthermore, for both treatments, the maximums of photosynthesis were noticed during spring period coinciding with the intense vegetative growth phase of the olive tree. These values were of 21.05 and 8.85 mmol m2 s1, respectively in FW and SW with a relative reduction of Pn in SW-treated plants of 57%. The reduction of net photosynthetic activity in salt stressed olive plants could be due also, in accordance to Tattini et al. (2008),
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Fig. 4. Changes in leaf predawn and midday water potentials (LPWP, on the left) (LMWP, on the right) in FW and SW treatments during 2005 and 2006 crop seasons. Values are the means of five replicates per month for each season (SE).
to the energy dissipated for management of the distribution and allocation of toxic ions at the different plant cellular levels to avoid salt damage. Several papers showed that the effects of salinity on CO2 assimilation rates in olive tree are cultivar dependent (Chartzoulakis, 2005; Loreto et al., 2003). Results of this study showed that the olive response and tolerance to salinity conditions depend on its vegetative growth phase. In the Chemlali olive tree, the most sensitive period to salt stress is the intense vegetative growth phase (spring and autumn) during which the relative reductions of Pn rates were the most important. The less sensitive periods coincide with the rest phases (winter and summer) adapted by the olive tree to avoid damaging its survival mechanisms by harsh environmental conditions (winter cold, high temperature, low air humidity, etc.). During autumn period, the olive tree cv. Chemlali established better photosynthetic activity. This response testified the rapid recovery of the olive tree activity after a stress period when better climatic conditions occurred. On the other hand, these behaviours let us suggest that water resources management for irrigation should take into consideration the vegetative growth phase of the crop and the climatic conditions of the experimental site. Indeed, in the case of the availability of different water resources quality, as in the case of Tunisia, it would be so important to preserve those of high quality for olive irrigation during the intense vegetative growth phase (sensitive phase) and those of low quality during the olive rest phase (less sensitive period). 3.4. Proline accumulation Salt stress-induced caused a significant increase of proline content in leaves of stressed plants (Fig. 5B). The highest
A
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RWC SW
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accumulation was observed during summer season in coincidence with the lowest values of RWC and leaf water potential. During the second crop season, averages of proline content in leaves of SWtreated plants were more than four times higher than those in FW. As well the longer the salt stress was, the higher the proline content was. These results were in accordance with the idea of Ashraf and Foolad (2007) indicating that proline is known to accumulate in large quantities in higher plants in response to environmental stresses. As has been reported by Hasegawa et al. (2000), and in order to accomodate the ionic balance in the vacuoles, the salt stressed olive trees tend to accumulate proline at high levels to improve water uptake to actively growing tissues. The same reports have been noticed by several papers indicating that many plants accumulate proline as a non toxic and protective osmolyte under saline conditions (Munns and Tester, 2008). Similarly, the accumulation of proline in the Chemlali olive tree constitutes other adaptive mechanism to harmful stress. As well for photosynthesis, proline content in both treatments was not maintained at a stable rate during the trial period. These patterns testified the effects of environmental conditions on osmolyte compounds in the olive tree, as well under stressed as under non stressed conditions. Recently, the improvement of plant tolerance to salinity by fertigation or exogenous application of osmolytes keeps more consideration. For vetiver plants, Edelstein et al. (2009) showed that the plant response to P, K and N fertilization is dose-dependent and a 2 mL/L fertilizer treatment which contained 118, 15.2, and 72.2 mg/L N, P and K, respectively, was found to be the optimal fertilizer level for obtaining maximum total dry weight of plant foliage under salinity conditions during two harvest times. The same authors added that the highest transpiration rates per plant were found at 2 mL/L fertilizer level. In young 2,5
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e mm
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Fig. 5. Changes in relative water content (RWC) and net photosynthesis (Pn) (A) and proline content (B) in FW and SW treatments during 2005 and 2006 crop seasons. Values for RWC are means of five replicates (SE), those of Pn are for twenty measurements per month for each season (SE) and those of proline are means of nine replicates (SE).
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Chemlali olive tree grown under different salinity levels supplied or not with exogenous proline, Ben Ahmed et al. (2010a,b) showed that the proline supply at 50 mM has been found to be more effective in improving photosynthetic activity and some antioxidant defense enzymes activities of stressed plants than the proline supply at 25 mM. The same authors added that proline supply under or nor salinity conditions determines, in a great part, the salt ions distribution pattern in both leaves and roots of Chemlali olive tree. In fact, the proline supplement allowed a reduction in toxic ions (Naþ, Cl) translocation from the roots to the aerial parts in favour of that of Kþ and Ca2þ.
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vegetative growth cycle, in coincidence with high salt-sensitive plant-period, and those of low quality for irrigation during the plant rest phase in concurrence with low sensitive plant-period, at least under the described experimental conditions. In conclusion, the urgent use of saline water for irrigation should not be applied without taking into consideration the different surroundings conditions where it is used, particularly the water salinity level, the soil type, the irrigation system adopted, the degree of the crop salt tolerance, the plant growth phase and the climatic conditions of the experimental site. Acknowledgements
4. Summary and conclusion Saline water (EC ¼ 7.5 dSm1) used for olive tree (cv. Chemlali) irrigation induced soil salinity increase, altered leaf water relations and photosynthetic activity of the plants. On the other hand, these results confirmed the fact that the olive tree response to salinity circumstances under field conditions was not only dependent on the water salinity level, but also on the different surroundings environmental conditions (climatic conditions, olive growth cycle, soil type, irrigation system, etc.). Further, these findings demonstrate that for better management of saline water use for irrigation, the irrigation with such water resources quality using a drip irrigation system on a sandy soil type could help in the prevention of soil salinization risk, at least under the described experimental conditions. In fact, although the soil salinity increases in summer period, the scarce rainfall occurring generally after this period, in association with the sandy soil texture and the drip irrigation system, the continuous salt leaching and the free circulation of water and air along the soil depth have been preserved. Consequently, no salt toxicity symptoms were recorded and the olive tree has been able to recover its activity at better level under favourable climatic conditions during autumn, in comparison to levels registered in summer period. Indeed, the surroundings experimental conditions (sandy soil texture, drip irrigation system, and rainfall pattern) seem to play an important role in the maintenance of plant water status and photosynthetic performances of the Chemlali olive tree during the experimental period at appropriate levels. More to the point, the saline water used for irrigation in this study has also improved both quantitative and qualitative characteristics of virgin olive oil in the case of Chemlali olive cultivar (Ben Ahmed et al., 2009). Indeed, the increment of total phenols and phenolic compounds contents in the case of SW- treated olive plants (Ben Ahmed et al., 2009) could be involved in the antioxidative mechanisms developed by the olive tree in response to oxidative stress induced by salt stress conditions as has been suggested by Foyer et al. (1997). In the case of Wiesman et al. (2004), the higher polyphenol contents recorded in oils of saline irrigated plants has been explained by the acceleration of maturation of the olives which could account for the higher levels of phenols. Furthermore, as it is known, the salt stress could result in both water deficit and salts accumulation. Per consequent, the increase of phenols contents in SW- treated plants might be due to the effects of water deficit on the activation of Phenylalanine Ammonia-Lyase (PAL), a key enzyme in the biosynthetic pathway of phenolic compounds, which is directly involved in the accumulation of polyphenols in the virgin olive oil (Romero et al., 2002; Tovar et al., 2001). Moreover, periods of severe conditions could influence PAL activity in olive fruit (Tovar et al., 2002). What’s more, the dependence of olive tree response to salinity on its growth cycle would to help also in the management of water resources for irrigation. For example, in the case of the availability of water resources with different quality, it would be better to preserve those of high quality for irrigation during the intense
Authors would to express their debt to the editor and to two anonymous reviewers for their effort to improve a first draft of the manuscript. Authors thank Mr Soua N. for his technical assistance. References Aragüés, R., Puy, J., Royo, A., Espada, J.L., 2005. Three-year field response of young olive trees (Olea europaea L., cv. Arbequina) to soil salinity: trunk growth and leaf ion accumulation. Plant Soil 271, 265e273. Ashraf, M., Athar, H.R., Harris, P.J.C., Kwon, T.R., 2008. Some prospective strategies for improving crop salt tolerance. Adv. Agron. 97, 45e109. Ashraf, M., Foolad, M.R., 2007. Roles of glycine betaine and proline in improving plant abiotic stress resistance. Environ. Exp. Bot. 59, 206e216. Bates, L.S., Waldren, R.P., Teare, I.K., 1973. Rapid determination of free proline for water stress studies. Plant Soil 39, 205e208. Ben Ahmed, Ch, Ben Rouina, B., Boukhriss, M., 2007. Effects of water deficit on olive trees cv. Chemlali under field conditions in arid region in Tunisia. Sci. Hortic. 133, 267e277. Ben Ahmed, Ch., Ben Rouina, B., Sensoy, S., Boukhriss, M., 2009. Saline water irrigation effects on fruit development, quality and phenolic composition of virgin olive oils, Cv Chemlali. J. Agric. Food Chem. 57 (7), 2803e2811. Ben Ahmed, Ch., Ben Rouina, B., Sensoy, S., Boukhriss, M., Ben Abdullah, F., 2010a. Exogenous proline effects on photosynthetic performance and antioxidant defense system of young olive tree. J. Agric. Food Chem. 58, 4216e4222. Ben Ahmed, Ch., Ben Rouina, B., Sensoy, S., Boukhriss, M., Ben Abdullah, F., 2010b. Exogenous proline effects on water relations and ions contents in leaves and roots of young olive. Amino Acids. doi:10.1007/s00726-010-0677-1. Ben Rouina, B., Trigui, A., d’Andria, R., Boukhris, M., Chaïeb, M., 2007. Effects of water stress and soil type on photosynthesis, leaf water potential and yield of olive trees (Olea europaea L. cv Chemlali Sfax). Aust. J. Exp. Agron. 47, 1484e1490. Chartzoulakis, K., 2005. Salinity and olive: Growth, salt tolerance, photosynthesis and yield. Agric. Water Manag. 78, 108e121. Doorenbos, J., Pruitt, W.O., 1977. Guidelines for Predicting Crop Water Requirements, FAO Irrigation and Drainage. Paper No 24, second ed. FAO, Rome. Edelstein, M., Plaut, Z., Dudai, N., Ben-Hur, M., 2009. Vetiver (Vetiveria zizanioides) responses to fertilization and salinity under irrigation conditions. J. Environ. Manag. 91, 215e221. Foyer, C.H., Lopez-Delgado, H., Dat, J.F., Scott, I.M., 1997. Hydrogen-peroxide and glutathione-associated mechanisms of acclamatory stress tolerance and signalling. Physiol. Plant 100, 241e254. Gucci, R., Lombardini, L., Tattini, M., 1997. Analysis of leaf water relations in leaves of two olive (Olea europaea) cultivars differing in tolerance to salinity. Tree Physiol. 17, 13e21. Hasegawa, P.M., Bressan, R.A., Zhu, J.K., Bohnert, H.J., 2000. Plant cellular and molecular responses to high salinity. Annu. Rev. Plant Physiol. Plant Mol. Biol. 51, 463e499. Koyro, H.W., 2006. Effect of salinity on growth, photosynthesis, water relations and solute composition of the potential cash crop halophyte Plantago coronopus (L.). Environ. Exp. Bot. 56, 136e146. Loreto, F., Centritto, M., Chartzoulakis, K., 2003. Photosynthetic limitations in olive cultivars with different sensitivity to salt stress. Plant Cell Environ. 26, 595e601. Melgar, J.C., Mohamed, Y., Serrano, N., Garcia-Galavis, P.A., Navarro, C., Parra, M.A., Benlloch, M., Fernandez-Escobar, R., 2009. Long term responses of olive trees to salinity. J. Agric. Water Manag. 96, 1105e1113. Munns, R., Tester, M., 2008. Mechanisms of salinity tolerance. Annu. Rev. Plant Biol. 59, 651e681. Parida, A.K., Das, A.B., 2005. Salt tolerance and salinity effects on plants. Ecotoxic. Environ. Safety 60, 324e349. Romero, M.P., Tovar, M.J., Girona, J., Motilva, M.J., 2002. Changes in the HPLC phenolic profile of virgin olive oil from young trees (Olea europaea L. Cv. Arbequina) grown under different deficit irrigation strategies. J. Agric. Food Chem. 50, 5349e5354. Sofo, A., Dichio, B., Xiloyannis, C., Masia, A., 2005. Antioxidant defences in olive trees during drought stress: changes in activity of some antioxidant enzymes. Funct. Plant Biol. 32, 45e53. Tattini, M., Melgar, J.C., Traversi, M.L., 2008. Responses of Olea europaea to high salinity: a brief ecophysiological e review. Adv. Hort. Sci. 22 (3), 159e173.
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Tattini, M., Traversi, M.L., 2009. On the mechanism of salt tolerance in olive (Olea europaea L.) under low or high Ca2þ supply. Environ. Exp. Bot. 65, 62e71. Tovar, M.J., Romero, M.P., Girona, J., Motilva, M.J., 2002. Phenylalanine ammonialyase activity and concentration of phenolics in developing olive (Olea europaea L. Cv. Arbequina) fruit grown under different irrigation regimes. J. Sci. Food Agric. 82, 892e898.
Tovar, M.J., Romero, M.P.J., Motilva, M.J., 2001. Changes in the phenolic composition of olive oil from young trees (Olea europaea L. cv. Arbequina) grown under linear irrigation strategies. J. Agric. Food Chem. 49, 5502e5508. Wiesman, Z., Itzhak, D., Ben Dom, N., 2004. Optimisation of saline water level for sustainable Barnea olive and oil production in desert conditions. Sci. Hortic. 100, 257e266.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Saline water irrigation effects on soil salinity distribution and some physiological responses of field grown Chemlali olive Chedlia Ben Ahmed a, b, *, Salwa Magdich a, b, Bechir Ben Rouina b, Makki Boukhris a, Ferjani Ben Abdullah a a b
Laboratory of Environment and Biology of Arid Area, Department of Life Sciences, Faculty of Sciences of Sfax, Tunisia Laboratory of Improvement of Olive and Fruit Trees’ Productivity, Olive Tree Institute of Sfax, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2009 Received in revised form 3 September 2011 Accepted 8 March 2012 Available online 8 May 2012
The shortage of water resources of good quality is becoming an issue in arid and semi arid regions. Per consequent, the use of water resources of marginal quality is becoming an important consideration, particularly in arid regions in Tunisia, where large quantities of saline water are used for irrigation. Nevertheless, the use of these waters in irrigated lands requires the control of soil salinity and a comprehensive analysis even beyond the area where water is applied. The aim of this study was to investigate the effects of saline water irrigation on soil salinity distribution and some physiological traits of field-grown adult olive trees (Olea europaea L. cv. Chemlali) under contrasting environmental conditions of the arid region in the south of Tunisia. The plants were subjected, over two growing seasons, to two drip irrigated treatments: fresh water (ECe ¼ 1.2 dS m1, FW) and saline water (ECe ¼ 7.5 dS m1, SW). Saline water irrigation (SW) has led to a significant increase in soil salinity. Furthermore, these results showed that soil salinity and soil moisture variations are not only dependent on water salinity level but are also controlled by a multitude of factors particularly the soil texture, the distance from the irrigation source and climatic conditions (rainfall pattern, temperature average, .). On the other hand, salt treatment reduced leaf midday water potential (LMWP), relative water content and photosynthetic activity and increased the leaf proline content, and this increase was season-dependent. Indeed, LMWP in SW plants decreased to 3.71 MPa. Furthermore, the highest level of proline in SW plants was registered during summer period (2.19 mmol/mg Fw). The proline accumulation recorded in stressed plants has allowed them to preserve appropriate leaf water status and photosynthetic activity. More to the point, this olive cultivar seems to be more sensible to soil salinity during the intense growth phase. Such tendencies would help to better manage water resources for irrigation, particularly under actual climatic conditions of water scarcity. For example, in the case of the availability of different water qualities, it would be better to preserve those of high quality for olive irrigation during the intense vegetative growth phase, in coincidence with high salt sensitive period, and those of low quality for irrigation during partial growth and plant rest phases. What’s more, the urgent use of saline water for irrigation should not be applied without taking into consideration the different surroundings conditions where it is used, particularly the water salinity level, the soil type, the adopted irrigation system, the degree of the crop salt tolerance, the plant growth phase and the climatic conditions of the experimental site. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Irrigation Olea europaea L. Saline water Photosynthetic activity Proline accumulation Arid climate
1. Introduction Traditionally, olive tree was cultivated under rain-fed conditions. In recent decades, the olive plantation has been extended to irrigated lands. However, in arid and semi arid regions, as those in
* Corresponding author. Laboratory of Environment and Biology of Arid Area, Department of Life Sciences, Faculty of Sciences of Sfax, Sfax, Tunisia. Tel.: þ216 74 276 400/274 923; fax: þ216 74 274 437. E-mail address: [email protected] (C. Ben Ahmed). 0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2012.03.016
Tunisia, the limited water availability and the increased need for good water quality for urban use restrict the use of fresh water for irrigation. So, large quantities of marginal water, as saline water, are used for olive tree irrigation. Salt stress inhibits photosynthesis by reducing water potential (Parida and Das, 2005). This reduction has been explained by the dehydration of cellular membrane which reduces the permeability of CO2 by the hydroactive stomatal closure (Loreto et al., 2003). The maintenance of an appropriate plant water status under stressed conditions is essential for continued growth of the plant. This process can be achieved by stomatal regulation (Chartzoulakis,
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2005; Tattini et al., 2008) and accumulation of compatible solutes in either leaves or roots (Ashraf et al., 2008). These osmolytes play a great role in facilitating water retention in the cytoplasm and activation of water uptake to the growing tissues. Proline was known as the main important osmolyte compound accumulated under salinity conditions (Ashraf and Foolad, 2007). Nowadays, the controlled use of marginal water (saline water, treated waste water) to improve the qualitative characteristics of horticultural products is becoming more and more important, particularly under actual conditions of limited water resources and rainfall scarcity in arid regions. For fruit trees, there is some evidence that saline water could improve yield. Indeed, in “Picual” olive tree, Melgar et al. (2009) found that, for most of the different crop seasons, the long term irrigation with saline water resulted in an increase of olive oil content linearly with the water salinity level. The same authors showed that salinity did not affect the growth parameters and leaf salt ions concentrations did not reached the toxicity threshold. Although, there is some evidence that olive tree is moderately tolerant to salinity conditions and the tolerance level is cultivar and plant-age dependent (Chartzoulakis, 2005; Loreto et al., 2003; Tattini et al., 2008), most of the investigations have been carried out on young plants, grown in pots and/or under controlled conditions and little information is known on olive responses to salinity conditions under field locale, particularly, under contrasting environmental circumstances of arid region in Tunisia. The objectives of this investigation were to determine the effects of saline water used for irrigation, over two successive growing seasons under natural environmental conditions, on soil salinity distribution (with soil depth and around the irrigation source) and on some physiological traits of adult olive cv. Chemlali. 2. Materials and methods 2.1. Plant material, treatments and climatic conditions Twelve-year- old olive trees (Olea europaea L. cv. Chemlali), planted on a sandy soil at a density of 625 trees ha1 at Sfax, Tunisia (34 43N, 10 41E), were used in 2005 and 2006 crop seasons. Ten trees from two adjacent rows (total 20 trees per treatment), with four replications of 5 trees each, were selected to be similar in potential yield and canopy. The plants were subjected to the following treatments: irrigation with fresh water (FW, 1.2 dS m1 ECe); and saline water (SW, 7.5 dS m1 ECe). The water used was either that supplied by the Tunisian National Water Carrier, or saline water from the local reservoir situated in the area of the Olive Tree Institute in Sfax. The fresh and saline water used were characterized by 145 and 600 mg/l Naþ, 326 and 1169 mg/l Cl, 280 and 520 mg/l Kþ, 94 and 261 mg/l Ca2þ, 57 and 102 mg/l Mg2þ, respectively. The amount of water supplied to olive trees was estimated according to the Penman-Monteith-FAO equation (Doorenbos and Pruitt, 1977) as described by Ben Ahmed et al. (2007). The
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irrigation was delivered using a drip system with 4 drip nozzles (two per side), of 4 l h1 per tree set in a line along the rows (at 0.5 m from the trunk). Without taking rainfall into account, total water supplied to mature olive trees was 4000 m3/ha/year. The sandy soil of the experimental orchard (90.5% sand, 4.5% clay and 5% silt) was characterized by an organic matter of 1.1%, 13.4% CaCO3, 1.3% N, pH of 7.6, a field capacity (measured at 33 KPa) of 11.8% and a wilting point (measured at 1500 KPa) of 5.9%. The region is characterized by an arid climate of Mediterranean type. The annual rainfall and temperature averages over a 52-year period were of 250 mm and 23 C, respectively. In both crop seasons, precipitation was virtually absent during summer and it was of 218.5 and 285.5 mm, respectively in the first and the second crop seasons. The mean temperatures were of 25.6 and 25.1 C, respectively and maximum temperatures were of 38 and 37 C, respectively. The evapotranspiration rates were of 1413 and 1271 mm in 2004 and 2005, respectively. Climatic conditions of the experimental site during the trial period are summarized in Table 1. In Tunisia, the different seasons periods are as following: the spring season: from March to May; the summer season: from June to August; the autumn: from September to November; and winter season: from December to February. 2.2. Soil moisture and salinity measurements At the middle of each month along the different seasons (spring, summer, autumn and winter), three soil samples per treatment were taken from the surface (0e30 cm) until a depth of 1.2 m with a layer of 0.3 m. On these samples, the soil moisture (%) and the electrical conductivity (ECe) of the saturated phase were determined. Values represented here correspond to the means of the different measurements along each season. The ECe was determined also at different distance (0, 0.15, 0.3 and 0.6 m) from the irrigation source. Values of soil moisture for each soil depth represent the means of the measurements for the different distances from the irrigation source. 2.3. Physiological parameters The measurements of photosynthetic activity (Pn) and relative water content (RWC) were carried out on leaves selected from the median part of the shoots. Pn was measured on well-exposed four leaves per plant from five plants per treatment from 10:00 to 13:00 pm using a portable gas exchange system (Li-CorInc e 6200, Lincoln, Nebraska USA). The RWC was determined as following (Gucci et al., 1997):
RWC ð%Þ ¼ ½ðFw DwÞ=ðTw DwÞ 100 where Fw is the fresh weight, Dw the dry weight and Tw the turgid weight of leaf samples. Predawn and midday leaf water potentials (LPWP and LMWP) were measured, on leaves taken from the median part of the same
Table 1 Principal seasonal climatic conditions (Air temperature (T), precipitation (P), and photosynthetic active radiations (PAR)) of the experimental site during the trial period. Spring
Summer
2004/2005
Crop season T ( C) P (mm) PAR (mmol m2 s1)
28 2.45 31.5 3.48 1013 56.5
36.6 2.87 1 0.42 1418.6 59.3
2005/2006
T ( C) P (mm) PAR
28.2 1.89 47.8 5.78 945.6 45.91
36.3 2.45 1.2 0.49 1414 67.8
Autumn
Winter
30.6 2.12 56.5 4.91 970.3 28.3
23.2 3.56 72.8 5.28 746.7 47.81
29.8 2.63 61.2 5.28 1076.6 44.92
23.6 2.13 58.3 3.48 775.8 37.82
Values of each season are means of averages for the three respective months (SD). In Tunisia: Spring: MarcheAprileMay; Summer: JuneeJulyeAugust; Autumn: SeptembereOctobereNovember and Winter: DecembereJanuaryeFebruary.
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shoots served for leaf relative water content, by a Scholander Pressure Chamber (pms-1000, Oregon, Corvallis, USA) (Gucci et al., 1997). To minimize water loss during the transfer of the leaf to the chamber, leaves were enclosed immediately, after excision, in a black plastic bag.
end of the irrigation period. Furthermore, the lower level of soil salinity registered at a soil depth of 1.2 m, in comparison to the layer of 0.3 m, let us suggest that salts are transported and accumulated at higher depth, and that the rainfall occurring generally in autumn and winter was too sufficient to assure the leaching of salts, accumulated during summer season, as reported by Melgar et al. (2009) in “Picual” olive orchard located in southern Cordoba province, Spain. This process seems to be facilitated by the sandy soil texture (90.5% sand) allowing the free water circulation through the soil. Indeed, an essential factor determining soil salinity and crop salt response is soil texture. In fact, olive tree responses to drought conditions, which were in some extent similar to those under salt stress, are soil type dependent (Ben Rouina et al., 2007). The same authors showed that the effects of water deficit on Chemlali olive tree were more deleterious on clay than on sandy soils. This could be explained by the fact that the water holding capacity is soil texture dependent and sandy soil, the most abundant soil type in arid region in Tunisia, has lower waterholding capacity than a medium textured soil. It is well known that sandy soil loses more water than a clay soil which can lead to a more rapid increase of soil solution concentration. Besides, the irrigation source location (at 0.5 m from the trunk) and the drip system used for irrigation seem to play a great role in the distribution of soil salinity, via the exclusion of salts outside the root zone, and per consequent the maintenance of plant water status at acceptable level. Indeed, soil salinity distribution is the result of the interaction of water salinity level, irrigation source location, soil texture and climatic conditions. In addition, the active root zone of olive could affect the soil salinity distribution. Indeed, the high ability of olive tree to accumulate salts in their active roots, generally localized at a depth superior than 0.3 m, allows the decrease of soil salinity level. This strategy is commonly developed by the salt stressed plants to decrease the osmotic potential in the roots, via accumulation of inorganic salts, in order to activate water retention and transport from the soil to the plant (Munns and Tester, 2008; Parida and Das, 2005; Tattini et al., 2008) The differential pattern in soil salinity among the two treatments was accompanied with that of soil moisture (Fig. 3). Indeed, the use of saline water for irrigation resulted in a decrease of soil moisture levels, in comparison to values recorded in FW treatment, but at different extent among seasons. In fact, the layer with lower ECe showed the higher soil moisture values (Fig. 3). Besides, during both crop years, the largest decrease of soil moisture was registered in summer season in coincidence with harmful climatic conditions. Differences in soil moisture values between the two treatments were significant for the levels recorded in summer, autumn and winter of the first crop year and
2.4. Proline content determination Leaf samples (collected from the median part of selected shoots) for proline content determination were frozen immediately in liquid nitrogen. Free proline content was determined according to Bates et al. (1973) using L-proline for the standard curve (0e50 mg/ ml). 2.5. Statistical analysis Statistical analyses were performed using the SPSS 10.0 Windows. The treatments means were compared using least significant difference (LSD) test at 5% level and seasons and soil depths means were compared using Tukey’s test calculated at 5% level. At least three replicates were used for each laboratory and field tests. 3. Results and discussion 3.1. Soil moisture and salinity distribution The soil salinity variation was different among the two salinity treatments with significantly more important values under high soil salinity level (p < 0.05), but at different extent among soil layers and crop seasons. In fact, during the second trial year, the ECe of the soil saturated paste at the upper (0e0.3 m) and deep soil layers (0.9e1.2 m) of SW treatment was at more than 4 times higher than the levels recorded in FW one. On the other hand, in spring and autumn 2006, the ECe of the different soil layers and distances from the irrigation source of SW treatment was 5 times higher than those registered in the case of FW treatment, with except for the soil sample taken in autumn at 1.2 m soil depth, at 0.6 m from the irrigation source. Furthermore, the highest salt accumulation was registered at the soil layer of 0.3 m, and there is a slight decrease of ECe through the soil depth (Figs. 1 and 2). Moreover, salts were more accumulated during summer season and salt distribution through the soil depth was affected by autumnewinter rainfall. The seasonal variation of the soil salinity level in the 1.2 m depth showed that it was significantly lower during autumnewinter period than that of the spring-summer one (p < 0.05). These results are in agreement with the findings of Melgar et al. (2009) showing that salts were leached by the rainfall occurring at the
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Fig. 1. Changes in soil salinity (ECe) at different distance from the irrigation source for the different deep layers (described above) in FW (A) and SW (B) treatments during 2005 crop season. d1, d2, d3 and d4 represent the distance from the irrigation source (0, 0.15, 0.3 and 0.6 m, respectively). H1, H2, H3 and H4 represent the different soil depths (0.3, 0.6, 0.9 and 1.2 m, respectively). Values are the means of three soil samples measurements per month for each season (n ¼ 9).
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Fig. 2. Changes in soil salinity (ECe) at different distance from the irrigation source for the different deep layers (described above) in FW (A) and SW (B) treatments during 2006 crop season. d1, d2, d3 and d4 represent the distance from the irrigation source (0, 0.15, 0. 3 and 0.6 m, respectively). H1, H2, H3 and H4 represent the different soil layers (0.3, 0.6, 0.9 and 1.2 m, respectively). Values are the means of three soil samples measurements per month for each season (n ¼ 9).
summer and autumn of the second crop year (p < 0.05), but at different extent among soil layers and seasons. The higher ECe value observed at the upper soil level (0.3 m depth), in comparison to the different soil layers (0.6, 0.9 and 1.2 m), was due to the higher evaporation occurring in the surface as reported by Aragüés et al. (2005). In fact, salts are more accumulated in dry layers. On the other hand, the horizontal variation of soil salinity displayed that salts are less accumulated in the drip zone (0e0.15 m), in comparison to the more outlying ones (0.3 and 0.6 m). 3.2. Leaf water relations The saline water used for irrigation has altered leaf water status of stressed plants. Indeed, these latter showed lower values of relative water content (RWC) and leaf water potentials (LPWP and LMWP) than those of FW with statistically significant differences between them (Fig. 4). The RWC in stressed plants ranged from 73 to 83% (Fig. 5A). Leaf predawn water potential varied between 1.12
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and 0.65 MPa and from 2.36 to 1.27 MPa, respectively in FW and SW-treated plants. Leaf midday water potential varied from 1.99 to 1.21 MPa and between 3.71 and 2.31 MPa, respectively. These results are in concordance with previous findings suggesting that the reductions of leaf water potential and relative water content are among the earliest responses of olive tree under stressed conditions (Chartzoulakis, 2005; Tattini and Traversi, 2009). Indeed, as soon as the plants subjected to salt stress conditions perceived water deficit, they reduce progressively their leaf water potential in such a way to maintain a water potential gradient, between the soil and the plant, favourable for water uptake and adsorption. Such mechanism would to help the plants to preserve appropriate leaf water status (Gucci et al., 1997; Sofo et al., 2005). Moreover, the deleterious effects of salt stress were reinforced by the severe environmental conditions characterizing the experimental site. Indeed, in both treatments, over the two crop seasons, the lowest values of RWC and leaf water potentials were recorded during summer period in coincidence with high air temperature and high light intensity. Nevertheless, stressed plants tend to restore better levels of water status. Indeed, the occurrence of more moist times (autumn period) was accompanied with better plant water status. This behaviour displayed, at the same time, the resistance of the Chemlali olive tree to salt stress and its ability for recovery under better climate conditions (low air temperature, high precipitation, .) (Table 1) as has been reported in Plantago coronopus by Koyro (2006). In addition, the decrease of leaf water potential to 3.7 MPa under salt stress conditions, represent for the Chemlali olive tree, an adaptive mechanism to activate water retention and uptake, more than a merely negative consequence of it. Besides, the important variation of plant water status over the two crop seasons justified that the plant water status in olive tree is not only dependent on water quality treatment but also on the different environmental conditions characterizing the experimental site (Table 1) and the olive vegetative growth phase.
g rin
05 Su
5 5 6 6 6 5 6 r0 r0 n0 n0 g0 er 0 er 0 nte nte rin tum tum mm mm Sp Wi Wi Au Au Su
Fig. 3. Changes in soil moisture at different deep layers in FW (A) and SW (B) treatments during 2005 and 2006 crop seasons. H1, H2, H3 and H4 represent the different soil depths (0.3, 0.6, 0.9 and 1.2 m, respectively). At each soil depth, values of soil moisture represent the means for the different distances from the irrigation source. Values are the means of three soil samples measurements per month for each season (n ¼ 9).
Salt stress has enormously altered photosynthetic activity of olive tree (Fig. 5A). Furthermore, for both treatments, the maximums of photosynthesis were noticed during spring period coinciding with the intense vegetative growth phase of the olive tree. These values were of 21.05 and 8.85 mmol m2 s1, respectively in FW and SW with a relative reduction of Pn in SW-treated plants of 57%. The reduction of net photosynthetic activity in salt stressed olive plants could be due also, in accordance to Tattini et al. (2008),
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0
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Fig. 4. Changes in leaf predawn and midday water potentials (LPWP, on the left) (LMWP, on the right) in FW and SW treatments during 2005 and 2006 crop seasons. Values are the means of five replicates per month for each season (SE).
to the energy dissipated for management of the distribution and allocation of toxic ions at the different plant cellular levels to avoid salt damage. Several papers showed that the effects of salinity on CO2 assimilation rates in olive tree are cultivar dependent (Chartzoulakis, 2005; Loreto et al., 2003). Results of this study showed that the olive response and tolerance to salinity conditions depend on its vegetative growth phase. In the Chemlali olive tree, the most sensitive period to salt stress is the intense vegetative growth phase (spring and autumn) during which the relative reductions of Pn rates were the most important. The less sensitive periods coincide with the rest phases (winter and summer) adapted by the olive tree to avoid damaging its survival mechanisms by harsh environmental conditions (winter cold, high temperature, low air humidity, etc.). During autumn period, the olive tree cv. Chemlali established better photosynthetic activity. This response testified the rapid recovery of the olive tree activity after a stress period when better climatic conditions occurred. On the other hand, these behaviours let us suggest that water resources management for irrigation should take into consideration the vegetative growth phase of the crop and the climatic conditions of the experimental site. Indeed, in the case of the availability of different water resources quality, as in the case of Tunisia, it would be so important to preserve those of high quality for olive irrigation during the intense vegetative growth phase (sensitive phase) and those of low quality during the olive rest phase (less sensitive period). 3.4. Proline accumulation Salt stress-induced caused a significant increase of proline content in leaves of stressed plants (Fig. 5B). The highest
A
RWC FW
RWC SW
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accumulation was observed during summer season in coincidence with the lowest values of RWC and leaf water potential. During the second crop season, averages of proline content in leaves of SWtreated plants were more than four times higher than those in FW. As well the longer the salt stress was, the higher the proline content was. These results were in accordance with the idea of Ashraf and Foolad (2007) indicating that proline is known to accumulate in large quantities in higher plants in response to environmental stresses. As has been reported by Hasegawa et al. (2000), and in order to accomodate the ionic balance in the vacuoles, the salt stressed olive trees tend to accumulate proline at high levels to improve water uptake to actively growing tissues. The same reports have been noticed by several papers indicating that many plants accumulate proline as a non toxic and protective osmolyte under saline conditions (Munns and Tester, 2008). Similarly, the accumulation of proline in the Chemlali olive tree constitutes other adaptive mechanism to harmful stress. As well for photosynthesis, proline content in both treatments was not maintained at a stable rate during the trial period. These patterns testified the effects of environmental conditions on osmolyte compounds in the olive tree, as well under stressed as under non stressed conditions. Recently, the improvement of plant tolerance to salinity by fertigation or exogenous application of osmolytes keeps more consideration. For vetiver plants, Edelstein et al. (2009) showed that the plant response to P, K and N fertilization is dose-dependent and a 2 mL/L fertilizer treatment which contained 118, 15.2, and 72.2 mg/L N, P and K, respectively, was found to be the optimal fertilizer level for obtaining maximum total dry weight of plant foliage under salinity conditions during two harvest times. The same authors added that the highest transpiration rates per plant were found at 2 mL/L fertilizer level. In young 2,5
30
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0 6 6 5 5 6 05 06 05 r0 r0 n0 n0 g0 er er ng me nte ri n i nt tum tum pri m mm u u S Sp u u Wi W A A S S
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e mm
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Fig. 5. Changes in relative water content (RWC) and net photosynthesis (Pn) (A) and proline content (B) in FW and SW treatments during 2005 and 2006 crop seasons. Values for RWC are means of five replicates (SE), those of Pn are for twenty measurements per month for each season (SE) and those of proline are means of nine replicates (SE).
C. Ben Ahmed et al. / Journal of Environmental Management 113 (2012) 538e544
Chemlali olive tree grown under different salinity levels supplied or not with exogenous proline, Ben Ahmed et al. (2010a,b) showed that the proline supply at 50 mM has been found to be more effective in improving photosynthetic activity and some antioxidant defense enzymes activities of stressed plants than the proline supply at 25 mM. The same authors added that proline supply under or nor salinity conditions determines, in a great part, the salt ions distribution pattern in both leaves and roots of Chemlali olive tree. In fact, the proline supplement allowed a reduction in toxic ions (Naþ, Cl) translocation from the roots to the aerial parts in favour of that of Kþ and Ca2þ.
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vegetative growth cycle, in coincidence with high salt-sensitive plant-period, and those of low quality for irrigation during the plant rest phase in concurrence with low sensitive plant-period, at least under the described experimental conditions. In conclusion, the urgent use of saline water for irrigation should not be applied without taking into consideration the different surroundings conditions where it is used, particularly the water salinity level, the soil type, the irrigation system adopted, the degree of the crop salt tolerance, the plant growth phase and the climatic conditions of the experimental site. Acknowledgements
4. Summary and conclusion Saline water (EC ¼ 7.5 dSm1) used for olive tree (cv. Chemlali) irrigation induced soil salinity increase, altered leaf water relations and photosynthetic activity of the plants. On the other hand, these results confirmed the fact that the olive tree response to salinity circumstances under field conditions was not only dependent on the water salinity level, but also on the different surroundings environmental conditions (climatic conditions, olive growth cycle, soil type, irrigation system, etc.). Further, these findings demonstrate that for better management of saline water use for irrigation, the irrigation with such water resources quality using a drip irrigation system on a sandy soil type could help in the prevention of soil salinization risk, at least under the described experimental conditions. In fact, although the soil salinity increases in summer period, the scarce rainfall occurring generally after this period, in association with the sandy soil texture and the drip irrigation system, the continuous salt leaching and the free circulation of water and air along the soil depth have been preserved. Consequently, no salt toxicity symptoms were recorded and the olive tree has been able to recover its activity at better level under favourable climatic conditions during autumn, in comparison to levels registered in summer period. Indeed, the surroundings experimental conditions (sandy soil texture, drip irrigation system, and rainfall pattern) seem to play an important role in the maintenance of plant water status and photosynthetic performances of the Chemlali olive tree during the experimental period at appropriate levels. More to the point, the saline water used for irrigation in this study has also improved both quantitative and qualitative characteristics of virgin olive oil in the case of Chemlali olive cultivar (Ben Ahmed et al., 2009). Indeed, the increment of total phenols and phenolic compounds contents in the case of SW- treated olive plants (Ben Ahmed et al., 2009) could be involved in the antioxidative mechanisms developed by the olive tree in response to oxidative stress induced by salt stress conditions as has been suggested by Foyer et al. (1997). In the case of Wiesman et al. (2004), the higher polyphenol contents recorded in oils of saline irrigated plants has been explained by the acceleration of maturation of the olives which could account for the higher levels of phenols. Furthermore, as it is known, the salt stress could result in both water deficit and salts accumulation. Per consequent, the increase of phenols contents in SW- treated plants might be due to the effects of water deficit on the activation of Phenylalanine Ammonia-Lyase (PAL), a key enzyme in the biosynthetic pathway of phenolic compounds, which is directly involved in the accumulation of polyphenols in the virgin olive oil (Romero et al., 2002; Tovar et al., 2001). Moreover, periods of severe conditions could influence PAL activity in olive fruit (Tovar et al., 2002). What’s more, the dependence of olive tree response to salinity on its growth cycle would to help also in the management of water resources for irrigation. For example, in the case of the availability of water resources with different quality, it would be better to preserve those of high quality for irrigation during the intense
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