A. CONSIDERATIONS ABOUT THE OPTIMUM ELECTRICAL CONDUCTIVITY AND pH IN THE NUTRIENT SOLUTION
EC is an index of salt concentration that informs about the total amount of salts in a solution. Hence, EC of the nutrient solution is a good indicator of the amount of fertilizer available to the plants in the root zone (Nemali and Van Iersel, 2004). When plants absorb nutrients and water from the solution, the total salt concentration, that is the EC of the solution changes, and measurements of EC level are easy, fast and economic, hence, can be carried out daily by growers. Thus, fertigation management is currently based on the control of EC and pH in order to correct a preset nutrient solution prepared according to previous experience. This is a practical method but it is important to note that EC does not inform about the concentration of specific ions in the solution, hence, this way of managing nutrient solution may lead to nutrient imbalances.
The ideal EC range for soilless crops is between 1.5 and 2.5dS/m. However, the effect of salinity on crops is specific on the species and cultivar (Greenway and Munns, 1980). In general, EC > 2.5 dS/m may lead to salinity problems whereas EC < 1.5 dS/m may lead to nutrient deficiencies. In greenhouse culture, the high input of fertilizers is the main cause of the salinity problems (Li, 2000). In addition, a high EC may also be caused by the presence of specific ions such as Na+ and Cl~ in the solution. In order to avoid salinity problems, growers add fresh water to reduce EC. However, in some regions there is the added problem of having irrigation water of bad quality, that is with high content of Na+ and/or Cl. In that case, the addition of fresh water to the nutrient solution would not alleviate the problem of salinity and the use of cultivars with salinity tolerance may be the solution. Nevertheless, the amount and the frequency of fertigation may be managed in order to avoid salinity problems (Sonneveld and Voogt, 2009). High irrigation frequency and long irrigation events resulting in high leaching fractions may delay the rate of salt accumulation in the root zone, thereby mitigating the deleterious salinity effects (Lieth and Oki, 2008; Savvas et al., 2007).
In some cases, though, it may be advisable to use a high EC to improve the quality of the produce. For example, the quality of flavouring and health-promoting compounds in hydroponically grown tomatoes improves with increasing electrical conductivity in the nutrient solution (De Pascale et al., 2003; Krauss et al., 2007).
On the other hand, pH is a measure of the acidity or basicity of a solution and determines the availability of essential elements to plants. pH is an essential parameter to control in soil and soilless system, but in the latter, its correction should be done on daily basis because of the lower buffering capacity of soilless systems (Urrestarazu, 2004). In fertigation, pH should be such that it does not damage plant roots and allows all essential nutrients to be dissolved in the nutrient solution to prevent the formation of precipitates that block the irrigation systems and decrease nutrients availability to plants. The optimum nutrient solution pH depends on the plant but, in general, it ranges between 5.5 and 6.5, in which the maximum number of elements is at their highest availability for plants (Taiz and Zeiger, 2002, p. 79). For review of the management of pH in soilless systems see Urrestarazu (2004).
The change of pH in nutrient solutions is mainly related to the uptake of cation and anion species and especially to the uptake of nitrate and ammonium (Mengel and Kirkby, 2001). Three possible transport systems have been ascribed to nitrate uptake (i.e. 1NO3~/2H+ symport, 1NO3~/ 2OH~ antiport and 1NO3~/2HCO3~ antiport) but, in any case, the result is the alkalinization of the nutrient solution (Touraine, 2004). In contrast, uptake of NH4+ is mainly driven by facilitated diffusion in response to the electropotential difference, and results in a decrease of pH in the nutrient solution (Mengel and Kirkby, 2001). Actually, the incorporation of NH4 in the nutrient solution as a source of N (5-10%) has been used as a tool to regulate pH (Adams, 2004). In addition to nutrient uptake, pH may change due to release of protons by nitrification and excretion of protons by roots. Padgett and Leonard (1993) reported that conversion of NH4+ to NO3~ by nitrifying organisms is of significant importance in NH4+-based solutions in soilless systems. Moreover, roots release organic and inorganic compounds into the nutrient solution, thus reducing its pH (Mengel and Kirkby, 2001). For example, protons are pumped out of the plasmalemma of root cells by means of H+-ATPase pumps, providing the driving force for nutrient uptake. Plants can also excrete organic acids, which may be related to the nutrient status of plants, mainly to the P status, with the aim of increasing the availability of nutrients to plants (Mengel and Kirkby, 2001).
B. DIAGNOSIS OF PLANT STRESS CAUSED BY ELECTRICAL CONDUCTIVITY AND pH IN THE NUTRIENT SOLUTION
The use of solutions with too low EC and the incorrect management of pH may lead to nutrient deficiencies, which have been reviewed earlier. In this section, we will discuss about ways of detecting salinity stress in plants. Depending on whether high EC is due to the use of highly concentrated solutions or due to the use of water with high levels of Na+ and Cl, the responses of plants are twofold: First, the presence of high levels of salts in the soil solution reduces the ability of the plant to take up water, which is referred to as the osmotic or water-deficit effect of salinity. Second, if excessive amounts of injurious ions (e.g. Na+ or Cl_) enter the plant in the transpiration stream, there may be injury to cells in the transpiring leaves, which is called the salt-specific or ion-excess effect of salinity (Greenway and Munns, 1980).
The osmotic effect of salinity induces metabolic changes in the plant identical to those caused by water stress (Munns, 2002). Specifically, the following effects have been observed in different crops under salinity stress: a decrease of biomass production and growth (Giuffrida et al., 2008; Shani and Ben-Gal, 2005; Soussi et al., 1998; Tavakkoli et al., 2008; Zhao et al., 2007; Zribi et al., 2009); a decrease of leaf area (Giuffrida et al., 2008; Netondo et al., 2004; Taiz and Zeiger, 2002, p. 614; Terry et al., 1983; Zhao et al., 2007); an increase of leaf abscission (Taiz and Zeiger, 2002, p. 614); a decrease of root growth (Rodriguez et al., 1997) but to a lesser extent than the reduction in leaf growth (Munns, 2002); a lower shoot/root ratio (Houimli et al., 2008; Meloni et al., 2004); a reduction in stomatal conductance (Netondo et al., 2004; Sultana et al., 1999; Terry et al., 1983; Zribi et al., 2009); an accumulation of ABA (He and Cramer, 1996); a decrease in CO2 assimilation (Maricle et al., 2007; Netondo et al., 2004), the effect in photosynthetic rate being less important than the effect in leaf enlargement (Terry et al., 1983); a decrease of water uptake (Giuffrida et al., 2008); a decrease in water potential (De Pascale et al., 2003; Zribi et al., 2009); a decrease in relative water content (Meloni et al., 2004); an increase in osmotic adjustment (De Pascale et al., 2003; Taiz and Zeiger, 2002, p. 612) due to accumulation of glycine betaine (Agastian et al., 2000; Meloni et al., 2004), proline (Agastian et al., 2000; Mattioni et al., 1997; Soussi et al., 1998) or sugars (Agastian et al., 2000; Soussi et al., 1998) among other compounds; down-regulation of photosynthetic electron transport (Netondo et al., 2004); a relative resistance of PSII primary photochemistry (Maricle et al., 2007; Zribi et al., 2009); an increased production of ROS (Cakmak, 2008); a stimulation of antioxidant enzymes such as SOD, APX, MDAR, CAT or GR (Esfandiari et al., 2007; Hernandez et al., 2000; Tanaka et al., 1999); a higher synthesis of antioxidant compounds like glutathione, carotenoids and lycopene (De Pascale et al., 2003; Ruiz and Blumwald, 2002); a decrease in RuBisCO activity (Miteva et al., 1992); a change in the ultrastructure of chloroplasts similar to that caused by water stress (Dubey, 1997); a lower translocation of photosynthates leading to an accumulation of carbohydrates in the photosynthesizing leaves (Dubey, 1997); an increase of leaf temperature (Kluitenberg and Biggar, 1992); a decrease of nutrient uptake (Dubey, 1997) and N content (Meloni et al., 2004); a decreased ATP synthesis (Dubey, 1997); a decrease of NR activity (Meloni et al., 2004); a reduced viability of reproductive organs (Munns, 2002); and, finally, a change in gene expression, similar to that caused by water stress (Taiz and Zeiger, 2002, p. 614). Therefore, the same methods can be used for diagnosis of any osmotic effect, either caused by water or by salinity stress.
On the other hand, salt-specific effects may result in toxicity, deficiency or changes in mineral balance. First, plant deficiency of several nutrients and nutritional imbalance (i.e. extreme ratios of Na+/Ca2+, Na+/K+, Ca2+/ Mg2+ and Cl~/NO3~ in plant tissues) may be caused by the higher concentration of Na+ and Cl~ in the nutrient solution derived from ion antagonism (Grattan and Grieve, 1998). For example, Ca2+ and K+ deficiencies have been observed under salt stress, which affects membrane integrity (Cramer et al., 1985) and root growth (Munns, 2002). Second, toxicity in plant cells may appear as a consequence of accumulation of Na+ and/or Cl~ in transpiring leaves. Plants are capable of compartmentalizing these ions in the vacuole up to a certain extent, but if the limit is exceeded, ions build up in the cytoplasm and inhibit enzyme activity, or they build up in the cell walls and dehydrate the cell, eventually causing cell death (Munns, 2002). The salt-specific effects of salinity depend on the concentration of salts, the duration of salinity exposure as well as on the plant species. Salt tolerant plants differ from salt-sensitive ones in having a low rate of Na+ and Cl_ transport to leaves and in the ability to compartmentalize these ions in vacuoles to avoid salt toxicity (Munns, 2002). Therefore, the resistance of salt-tolerant plants to salts is not a consequence of salt-resistant metabolism but of strategies that avoid salt injury (Taiz and Zeiger, 2002, p. 613).
The toxicity effects of salts have metabolic consequences. Photosynthesis may be inhibited when high concentrations of Na+ and/or Cl_ accumulate in chloroplasts (Plaut et al., 1989; Taiz and Zeiger, 2002, p. 613). For example, alterations in the photochemical activity have been observed under salinity in salt sensitive crop species (Muranaka et al., 2002). Accumulation of injurious ions in the cytoplasm inactivates enzymes, inhibits protein synthesis and damages chloroplasts and other cell organelles (Taiz and Zeiger, 2002, pp. 612-613). These effects are more important in older leaves as they have been transpiring the longest, hence, accumulating more ions (Munns, 2002). This results in a progressive loss of the older leaves with time and reduces the photosynthetic leaf area of the plant to a level that cannot sustain growth. The rate at which leaves die becomes the crucial issue determining the survival of the plant (Munns, 2002). Hence, vine mortality has been correlated with the increase in Na+ and Cl~ content of leaves (Shani and Ben-Gal, 2005).
To summarise, plant growth might be reduced by both the osmotic and the salt-specific effect of salinity, sometimes being difficult to determine which of the two effects is responsible for the growth reduction. For that reason, Munns et al. (1995) proposed a two-phase model of salt injury, where growth is initially reduced by osmotic stress and then by salt toxicity. According to the authors, the effect of salinity takes some time to develop and may become obvious over weeks, especially in the more sensitive species (Munns, 2002). This model has been proved in broccoli under salinity stress (Lopez-Berenguer et al., 2006). However, it is difficult to assess with confidence the relative importance of the two mechanisms on yield reduction because they overlap (Tavakkoli et al., 2008). In brief, diagnosis of salinity stress in plants can be evaluated by the same techniques used for water stress in addition to the measurement of the concentration of Na+ and Cl~ content in leaves. Special attention should be paid to the old leaves as they are the target of salt injury.
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