EFFECTS OF NUTRIENT SOLUTION SALINITY ON THE PHYSIOLOGICAL PERFORMANCE OF MELON CULTIVATED IN COCONUT FIBER 1

Brackish waters represent great potential for profitable agricultural production; however, productive usage depends on the adoption of proper cultural practices as well as a culture tolerant of salinity, which can require some restrictions related to soil and crop production. Given the lack of information pertaining to hydroponic melon culture, the objective of this study was to investigate physiological changes promoted by the use of brackish water in the preparation of the nutrient solution for melon (Cucumis melo L., cv. AF 015) growth in coconut fiber substrate under greenhouse conditions in Mossoró-RN, a semiarid region of Brazil. The experimental design was completely randomized, with 12 treatments arranged in a 4 x 3 factorial scheme, with 4 salt concentration levels (1.1 control, 2.5, 4.0 and 5.5 dS m) and 3 exposition times (vegetative growth: 10-30 days after transplanting, DAT; flowering: 31 to 50 DAT; and fruiting and ripening: 51-70 DAT, which are the assessment phases of physiological maturation). Increasing salt concentrations in the nutrient solution reduced photosynthetic efficiency, stomatal conductance and transpiration, but increased the intercellular CO2 concentration in melon plants. A salt concentration in the low to intermediate range (2.5 dS m) resulted in the best water use efficiency by melon crops.


INTRODUCTION
In the fruit-producing region of Rio Grande do Norte, a semiarid region of Brazil, some of the water used for irrigation comes from deep artesian wells.Although it is a source of good quality water, high pumping costs make it economically prohibitive for agricultural use (SOUSA et al., 2009;MEDEIROS et al., 2014).Alternatively, irrigated water can be pumped at low costs from shallow wells in the Jandaíra limestone sediment, located on the north shore of the Rio Grande do Norte State and on the northeast of the Ceará State (VASCONCELOS; TEIXEIRA; ALVES NETO, 2010); however, it is considered brackish water and possesses a high salt concentration.The lower cost of pumping encourages the use of brackish water in this region, which can lead to degradation of vast areas of soil through salinization (MEDEIROS et al., 2017).
This problem could be reduced by mixing high and low quality water, which would increase the amount of water available to the crops.Dilution of brackish water reduces total salt concentration and allows irrigation of larger areas.However, the best dilution ratio must be determined in order to achieve better crop production and minimize environmental disturbance.Besides avoiding deleterious effects of high salinity on the plants and soil, such a management strategy could save fresh water and increase the availability of other water resources for agricultural use.Therefore, research on the rational use of these brackish waters becomes essential, as their uncontrolled use may increase the risk of soil salinization (DIAS et al., 2007;DIAS et al., 2011;TURHAN et al., 2014), especially in regions of high evapotranspiration rates and low rainfall (MEDEIROS et al., 2017).
Hydroponic systems can be advantageous when using brackish water due to the absence of matric potential, which allows increased crop tolerance to high salinity (MEDEIROS et al., 2007;SOARES et al., 2007).Moreover, hydroponic systems alleviate the problem of soil contamination and, as an extra benefit, provide people an alternative source of water for agricultural production in semiarid regions.
Salinity affects plant performance directly through water stress, toxicity caused by ions and nutritional imbalance (MEDEIROS et al., 2008;ARAGÃO et al., 2009;TERCEIRO NETO et al., 2014;PEREIRA et al., 2016), as well as indirectly by mediating inter-specific competition (ACOSTA-MOTOS et al., 2017).Thus, as a response to the damage caused by excess salt concentrations, many plants increase their tolerance mechanisms through the exclusion and/or compartmentalization of salt.Melon crops are considered moderately tolerant to salt stress, as was observed by Medeiros et al. (2008).In their study, Electric Conductivity (EC) values up to 3.31 dS m -1 did not affect melon yield.Conversely, a reduction in productivity is commonly observed when the culture is irrigated with water containing a high salt concentration (ARAGÃO et al., 2009;HUANG et al., 2011;MEDEIROS et al., 2011).However, little is known about physiological changes within melon crops grown under high salt concentration in hydroponic systems.Therefore, this work aimed to evaluate physiological changes among melon plants grown under greenhouse conditions in a nutrient solution prepared with brackish water and using coconut fiber as a substrate.

MATERIAL AND METHODS
Experiments were conducted in a greenhouse at the Department of Environmental and Technological Sciences of the Federal Rural University of Semiarid (UFERSA), located in Mossoró (latitude 5º 11´S, longitude 37º 20´W and altitude 18 m).The study design was completely randomized, using a 4 x 3 factorial scheme, with 4 levels of salinity in the nutrient solution (1.1 -control, 2.5, 4.0 and 5.5 dS m -1 ) and 3 phenological stages of melon plants (10-30 days after plant transplanting -DAT), flowering (31-50 DAT) and fruiting and ripening (51-70 DAT), with 3 replicates each.
The melon seeds (Cucumis melo L., cv.AF 015) were planted in polystyrene trays containing 128 cells filled with coconut fiber substrate.Within 13 days of sprouting, seedlings were transplanted into 12 L plastic bags containing a 2 cm gravel layer at the bottom, covered with nylon mesh and filled with 3.5 kg of coconut fiber (Golden Mix ®).The experimental units were arranged in six rows with 1.00 m x 0.50 m between plants.Fence posts were installed at the end of each plant row in order to guide plant growth.
The nutrient solution was prepared with water of different salinity levels: tap water (EC = 0.52 dS m -1 ), well water (EC = 3.7 dS m -1 ), mixed water and well water with NaCl added.After water preparation, basic nutrient solution fertilizers were added as recommended by Santos and Minami (2002).Briefly, 805 g of calcium nitrate, 334 g of potassium nitrate, 175 g of ammonium phosphate, 252 g of magnesium sulfate and 10 g of Quelatec were added to each 100 L of water.
All results are expressed as an average of three replicates.Treatment effects were determined by analysis of variance according to SISVAR.Regression analyses were performed to investigate the relationship between salt concentration levels and plant physiological parameters.

RESULTS AND DISCUSSION
The photosynthetic rates of melon plants were reduced (R 2 = 0.98) with increasing electrical conductivity of the nutrient solution.The reduction was 14% between the lowest (1.1 dS m -1 ) and highest (5.5 dS m -1 ) salinity levels of the nutrient solution (Figure 1A).However, intracellular CO 2 concentrations increased (R 2 = 0.94) with increasing nutrient solution salinity levels (Figure 1B), which is generally ascribed to salt-tolerant species.Reductions were also observed in stomatal conductance (R 2 = 0.88) and transpiration rates (Figures 1 C and D).Photosynthesis, the physicochemical process by which plants, algae and photosynthetic bacteria use light energy to drive the synthesis of organic compounds, involves three phases: first, atmospheric CO 2 enters the substomatal chamber by diffusion; second, light energy is absorbed through a photochemical reaction where water molecules are broken down to release oxygen and electrons.Both oxygen and electrons will be used to activate the production of ATP (Adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate); in the third phase, CO 2 is synthesized into organic molecules through the activity of the enzyme RuBisCo (D-ribulose 1,5-bisphosphate carboxylase/oxygenase) (TAIZ; ZEIGER, 2010).In the present study, there were no observed changes in the first phase of the process.However, although carbon was detected in the substomatic chamber and available for the photosynthetic process, it was not synthesized during the carboxylation phase.This is probably due to plant stress caused by salinity levels higher than 2.5 dS m -1 (Figure 1A), and suggests restricted function during the following phases of photosynthesis.For instance, based on the equation shown in Figure 1A, the highest photosynthetic rate (17.28 µmol CO 2 m -2 s -1 ) was estimated for the lowest level of salinity (1.1 dS m -1 ).Conversely, the lowest photosynthetic rate was observed for the highest level of salinity (5.5 dS m -1 ).Huang et al.
(2011) found similar results when melon plants were grown with and without water stress, they observed photosynthetic rates of 15.7 µmol CO 2 m -2 s -1 (highest value) under normal growing conditions and 9.6 µmol CO 2 m -2 s -1 in plants growing under water stress conditions.A linear reduction in the photosynthetic rate was also observed by Bezerra et al. (2005) after growing young dwarf cashew plants with increasing levels of salinity in the irrigation water.Several plant species were also affected by the photosynthetic rate due salt stress (NEGRÃO et al., 2017).Pereira et al. (2004) observed decreasing photosynthetic rates of 11.8, 24.1 and 49.4%, respectively, in tomato plants grown in 50, 100 or 150 mM NaCl solutions.
As opposed to the reduced photosynthetic rate, higher values of intercellular CO 2 , (222 µmol CO 2 mol -1 to 233 µmol CO 2 mol -1 ) were observed with increasing salinity levels (Figure 1B).According to Sivakumar, Sharmila and Saradhi (2000), who also observed an accumulation of CO 2 in the intercellular spaces of many plant species, this happens when CO 2 does not enter the carboxylation-dependent photosynthetic phase.In melon plants, Mavrogianopoulos, Spanakis and Tsikalas (1999) observed intercellular CO 2 concentrations of approximately 300 µmol CO 2 mol -1 when plants were grown in a solution enriched with 400 µmol mol -1 of CO 2 .They also confirmed that high salt levels in growing media for melon plants lead to a reduction in photosynthetic rates, stomatal conductance and internal CO 2 concentration.
The findings of the present study, demonstrate that salinity levels correlate to photosynthetic rates and intracellular CO 2 concentrations of melon plants, suggesting the presence of a specific type of tolerance mechanism in these plants.The photosynthetic rate was still reasonably high when the level of salinity was around 4.00 dS m -1 (Figure 1A).Therefore, the melon plant might either exclude the toxic Na + ions from the shoots or compartmentalize it into its specific parts, avoiding severe growth reductions (MUNNS;TESTER, 2008).Furthermore, reduced photosynthetic rate was related to the total salt concentration and ionic composition of the nutrient solution.High salt concentrations increase the osmotic potential, reducing water availability for plants.Low water potential leads to osmotic stress and can irreversibly affect the enzymes and photosynthetic ion transport system in the plasma membrane (PAPAGEORGIOU et al., 1998).Storey and Walker (1998) reported that a reduction in photosynthesis due to salt stress can result from reduced cell turgor pressure, Calvin cycle inhibition and K + or Cl -deficiency.This is because electron transport in the chloroplasts is salt indifferent.
Transpiration rates of melon plants were reduced with increasing salt levels in the nutrient solution and values ranged from 4.32 mmol H 2 O m -2 s -1 to 3.97 mmol H 2 O m -2 s -1 for salt levels of 1.0 and 5.5 dS m -1 , respectively (Figure 1D).A similar value (4.2 mmol H 2 O m -2 s -1 ) was reported by Huang et al. (2011) when they cultivated melons under water stress in China.These results are also in agreement with those found for stomatal conductance, which decreased with increasing salt concentrations (ANDRADE JÚNIOR et al., 2011;FERNANDES et al., 2010).The reduction in this variable is a reflection of stomatal behavior in response to water vapor loss.According to Vieira Júnior et al. ( 2008), 95% of the total water absorbed by the plant is used in the transpiration process to maintain thermal equilibrium.
The relationship between photosynthesis and transpiration rate is related to water use efficiency and indicates the capacity of the plant to assimilate CO 2 despite water vapor loss.In this study, an increase in water use efficiency was observed for salt concentrations from 1.1 to 2.5 dS m -1 in the irrigation water.However, with salt concentrations of 4 and 5.5 dS m -1 there was a reduction in water use efficiency, indicating that the CO 2 fixation/water loss ratio was favored by intermediate salinity levels (2.5 dS m -1 ) (Figure 2).Huang et al. ( 2011) studied saline stress in melon plants, and observed a photosynthetic water use efficiency value of 1.78 μmol CO 2 mmol H 2 O for plants growing under water stress and a value of 3.27 μmol CO 2 mmol H 2 O for plants growing in control treatment.In our study, melon plants benefited from a slightly higher salinity level (2.5 dS m -1 ), which caused an increase of approximately 4% in water use efficiency (Figure 2); suggesting that plants submitted to small levels of stress may be able to maximize water use through mechanisms to increase CO 2 assimilation.This is a desirable trait in plants that are tolerant to salinity.Our results differ from those of Huang et al. (2011), they they reported a linear relationship between increased water stress and reductions in photosynthetic rate, stomatal conductance, intercellular CO 2 concentration and transpiration rate.

CONCLUSIONS
Increasing salinity levels in the nutrient solution reduced photosynthetic efficiency, stomatal conductance, transpiration rates and increased intercellular CO 2 concentrations in melon plants.Water use efficiency in melon plants was highest for a salt concentration of 2.5 dS m -1 in the nutrient solution.The per capita ratio and loss of water in the melon crop environment is more efficient under salt concentrations of the nutrient solution ranging from low to intermediate levels.

Figure 2 .
Figure 2. Water use efficiency as a function of salt concentration in nutrient solutionin melon plants (Cucumis melo L., cv.AF 015).