Physiological Indices and Growth of Castor Bean Irrigated With Waters of Different Cationic Nature

It was proposed, in the present study, to evaluate the gas exchange and the growth of the castor bean cv. BRS Energia due to the isolated or mixed cationic nature of irrigation water. The study was conducted in drainage lysimeters under greenhouse conditions, using an Eutrophic Greyish Argissolo with a sandy-loam texture in the municipality of Campina Grande, Brazil. A randomized block design was used with six cationic composition of irrigation water (S1-control, S2-Na, S3-Ca, S4-Na+Ca, S5-K and S6-Na+Ca+Mg with four replicates, each composed of five plants. The plants under the control treatment were submitted to irrigation with low salinity water (ECw = 0.6 dS m) and the remaining treatments were irrigated with ECw of 4.5 dS m prepared with salts of different cations in chloride form. The gas exchanges and the growth of the castor bean cv. BRS Energy were determined at 100 days after sowing. The gas exchanges and the growth of the castor cv. BRS Energy were more sensitive to the variation in the electrical conductivity of the water compared to the cationic nature of the water, being the least deleterious effect observed in the plants irrigated with potassic water. The plants irrigated with water of potassium composition obtained the highest values for stomatal conductance, transpiration and rate of assimilation of CO2; at 100 days after sowing; the castor bean cv. BRS Energia showed sensitivity to the presence of sodium and calcium salts in irrigation water.


Introduction
The castor bean crop (Ricinus communis L.), a species of the Euphorbiaceae family, stands out as a profitable agricultural option for cultivation in the Northeast of Brazil, due to its characteristics of xerophilism and heliophilia and adaptation to various edaphoclimatic conditions (Ribeiro et al., 2009).Castor oil, obtained by the extraction of seeds, has several applications in the industrial area, and can be used in the manufacture of paints, dyes, disinfectants, germicides, low temperature lubricating oils, glues and adhesives, and as a byproduct cake after obtaining the oil, which has great potential as an organic manure, which can be used in crop fertilization (Silva & Martins, 2009;Silva et al., 2012).
In semiarid areas of the Brazilian Northeast, the occurrence of water scarcity and irregularity in the distribution of rainfall is common, and evapotranspiration is higher than precipitation in most part of the year.Therefore, the assured establishment of crops in this region depends necessarily on irrigation.However, most of the available water resources have high salt concentrations (Nobre et al., 2011).In the water intended for irrigation, the cationic composition depends on geological nature (lithologic composition), water source and path traveled, being easily found waters rich in sodium, calcium and magnesium, and in some cases, potassium (Medeiros et al., 2003).
Quantitative and qualitative scarcity of water resources has led to the search for alternatives for efficient water use and rational utilization of waters considered as of inferior quality since their use is almost obligatory in the agriculture of semi-arid regions (Alves et al., 2011).In this context, several authors have studied the effect of irrigation with waters of different salinity levels on castor bean cultivation (Cavalcanti et al., 2005;Alves et al., 2012;Nobre et al., 2013;Santos et al., 2013;Lima et al., 2014aLima et al., , 2014b)).
However, these studies were limited to the use of different levels of water salinity on the cultivation of this oilseed crop.Thus, conducting research that seeks to identify the cation that provides the greatest or least deleterious effect on the crop is extremely important for its establishment in the Brazilian semi-arid region.Lima et al. (2016) in a study evaluating the morphophysiology of the castor bean cv.BRS Energy at 80 days after sowing and oil content, as a function of the irrigation with waters of different electrical conductivity and cationic compositions [control-ECw = 0.6 dS m -1 , Na + , Ca 2+ , Na + +Ca 2+ ; K + and Na + +Ca 2+ +Mg 2+ (ECw = 4.5 dS m -1 )], concluded that gas exchange, growth and oil yield were more affected by ECw variation than by cationic nature of water.Sousa Júnior et al. ( 2017) evaluating the gas exchange and production of sunflower cv.BRS 324 under irrigation with water of different cationic nature and salinity [control-ECw = 0.5 dS m -1 , NaCl, CaCl 2 , KCl and NaCl+CaCl 2 + MgCl 2 (ECw = 5.0 dS m -1 )] also verified that the greatest decrease occurred in gas exchanges due to the variation in the ECw of the irrigation water.
Since most crops exploited in the semi-arid region are classified as sensitive to moderately sensitive to salinity, there emerges the urgent need to better characterize these waters to establish adequate management for plants, soil and irrigation systems (Medeiros et al., 2003).In this context, the aim of this study was to evaluate the gas exchange and the growth of the castor bean cv.BRS Energy as a function of salinity and cationic composition of irrigation water 100 days after sowing during the fruiting phase of the crop.

Characterization of the Experimental Area and Treatments
The experiment was conducted in drainage lysimeters in a greenhouse at the Center of Technology and Natural Resources of the Federal University of Campina Grande (CTRN/UFCG), located in the municipality of Campina Grande, PB, Brazil, at the local geographic coordinates 7º15′18″ S, 35º52′28″ W and mean altitude of 550 m.It should be emphasized that this work is a continuation of results reported earlier by Lima et al. (2016 on growth variables at 80 days of cultivation and oil content at 100 days after sowing. Treatments consisted of six cationic compositions of irrigation water (S 1 -control; S 2 -Na + ; S 3 -Ca 2+ ; S 4 -Na + +Ca 2+ ; S 5 -K + and S 6 -Na + +Ca 2+ +Mg 2+ ), in such a way to have equivalent proportions of 1:1 for Na:Ca and 7:2:1 for Na:Ca:Mg, respectively.Plants in the control treatment (S 1 ) were irrigated using water with electrical conductivity (ECw) of 0.6 dS m -1 , whereas the plants in other treatments (S 2 ; S 3 ; S 4 ; S 5 and S 6 ) were irrigated using water with ECw of 4.5 dS m -1 , prepared with different cations, in chloride form.The experiment was set up in randomized block design with six treatments and four replicates, totaling 24 experimental plots, each one composed of five plants.

Cultivar, Installation and Conduction of the Experiment
Castor bean cv.BRS Energia was studied.This cultivar stands out for its physiological vigor, precoce cycle (120 to 150 days), small size, semi-indiscriminate fruits (allows a single harvest), conical clusters with a mean size of 60 cm, oil content in the seeds on average 48% (Silva et al., 2009).
Plants were grown in drainage lysimeters with capacity for 100 L (50 cm high, 30 cm bottom diameter and 33 cm upper diameter), perforated at the base to allow drainage and attached to a 4-mm-diameter drain.The tip of the drain inside the lysimeter was involved with nonwoven geotextile (Bidim OP 30) and a plastic container was placed below each drain to collect the drained water.
The lysimeters were filled with a 2 kg layer of crushed stone (no.zero), followed by 54 kg of soil material (properly pounded to break up clods and homogenized), and 76 kg of the same soil mixed with 1.70 kg of earthworm humus per lysimeter to reach 1% of organic matter based on total weight.The soil used in the experiment was collected in the 0-30 cm layer (A horizon) of an Eutrophic Grey Argisol, from the district of São José da Mata (Campina Grande-PB).
Water salinity levels were obtained by dissolving sodium chloride (NaCl), calcium chloride (CaCl 2 •2H 2 O), magnesium chloride (MgCl 2 •6H 2 O) and potassium chloride (KCl) in the irrigation water according to the preestablished treatments in water from the local supply system.The amount of salt to be added was calculated according to the equation of Richards (1954), considering the relationship between ECw and concentration of salts (10*mmol c L -1 = 1 dS m -1 ).
Before sowing, the water volume required for the soil to reach field capacity was determined by capillary saturation followed by drainage.After the soil was brought to field capacity, using the respective waters according to the treatment, sowing was carried out using 10 seeds of 'BRS Energia' castor bean per lysimeter, at 2 cm depth and equidistantly distributed.At 10 days after sowing (10 DAS), thinning was performed to leave only one plant per recipient.
The soil was kept at field capacity by daily irrigations, applying in each lysimeter the water corresponding to the treatment.The applied water volume was determined according to the water needs of the plants, estimated by water balance: water volume applied minus water volume drained in the previous irrigation, plus a leaching fraction of 0.10, according to studies conducted previously (Nobre et al., 2013).
Plant height was obtained considering the distance from collar to apical meristem.Stem diameter was measured at 5 cm from plant collar.Leaf area was obtained by measuring the midrib length of all leaves, following the methodology described by Severino et al. (2005), according to Equation 1: S = Σ0.26622× P 2.4248  (1) where, S: total leaf area of the plant (cm 2 ); P: leaf midrib length (cm).
The phytosanitary control during the conduction of the research consisted of weeding, superficial scarification of the soil before each irrigation event and plant stewardship, in order to avoid the tipping of the same.The insecticides of the chemical group Neonicotinoid (5.4 g L -1 ), fungicide of the chemical group Triazole (7.0 g L -1 ) and acaricide (3.5 g L -1 ) belonging to the chemical group Abamectin were applied.

Statistical Analysis
At the end of this research, the data collected were analyzed through analysis of variance by 'F' test; when a significant difference for the source of variation occurred, the test of comparison of means and the contrasts between the means of the treatments (Tukey at 0.05 of probability) were performed using the statistical software SISVAR-ESAL (FERREIRA, 2011) .The following orthogonal contrasts were established: ŷ 1 (S 1 vs. S 2 ; S 3 ; S 4 ; S 5 ; S 6 ); ŷ 2 (S 2 vs. S 3 ); ŷ 3 (S 2 vs. S 6 ); ŷ 4 (S 2 vs. S 5 ); ŷ5 (S 5 vs. S 2 , S 3 , S 4 , S 6 ).
With regard to internal CO 2 concentration (Figure 1C), it was observed that castor bean plants irrigated with water containing Na + (S 2 ) were significantly different from those subjected to the control treatment (ECw = 0.6 dS m -1 ), and containing K + (S 5 ) or sodium + calcium + magnesium (S 6 ).However, no significant differences were observed in plants irrigated with water containing sodium (S 2 ), calcium (S 3 ) and calcium + sodium (S 4 ).The low internal CO 2 concentration observed in plants subjected to S 1 and S 5 possibly results from the higher values found for CO 2 assimilation rate (Figure 1D).
According to the mean estimation for Ci (Table 2), it can be noted that when castor bean plants irrigated using water of lowest ECw level (0.6 dS m -1 ) there was a reduction of 119.23 μmol m -2 s -1 in Ci, at 100 DAS, in comparison to those subjected to ECw of 4.5 dS m -1 (S 2 ; S 3 ; S 4 ; S 5 and S 6 ).However, in plants irrigated with saline water containing Na + (S 2 ) Ci increased by 69.50, 93.75 and 169.33 μmol m -2 s -1 , respectively, in comparison to the treatments S 3 , S 6 and S 5 .
On the other hand, comparing plants irrigated using water containing K + (S 5 ) with the other cations present in the irrigation water (S 2 ; S 3 ; S 4 and S 6 ), Ci decreased by 115.08 μmol m -2 s -1 .The higher internal CO 2 concentration found in the castor bean plants irrigated with S 2 , S 3 ; S 4 and S 6 water, is an indication that the carbon fixed in the substomatic camera was not being used in the synthesis of photoassimilates or may be associated to the changes in the kinetics of the enzyme Ribulose 1,5-bisphosphatocarboxylase oxidase in the carbon metabolism due to the sensitivity to the accumulation of salts in the leaves, especially Na + and Cl - (Habermann et al., 2003).
Regarding the CO 2 assimilation rates (Figure 1D), plants irrigated using water with the lowest saline level (S 1 ) and saline water containing K + (S 5 ) were significantly different from those subjected to the treatments S 2 , S 3 and S 4 .However, there was no difference in plants under treatment S 6 and S 2 , S 3 and S 4 .
As observed for E, the analysis of contrasts (Table 1) showed significant effects on CO 2 assimilation rate in the comparison between plants irrigated using water with ECw of 0.6 dS m -1 (S 1 ) and plants subjected to 4.5 dS m -1 (S 2 ; S 3 ; S 4 ; S 5 and S 6 ).The mean estimation (Table 2) demonstrates the superiority of A in plants irrigated with 0.6 dS m -1 water (mean value of 5.05 μmol m -2 s -1 ) in comparison to those subjected to ECw of 4.5 dS m -1 (S 2 ; S 3 ; S 4 ; S 5 and S 6 ).When plants were irrigated using water prepared with Na + (S 2 ), there was a reduction in A of 7.5 μmol m -2 s -1 in comparison to those receiving water containing K + (S 5 ).Additionally, when irrigation was performed using saline water containing K + (S 5 ) (Table 2), there was an increase of 7.17 μmol m -2 s -1 in CO 2 assimilation rate compared with plants subjected to the other cationic compositions (S 2 ; S 3 ; S 4 and S 6 ) of high salinity.
The decline observed in the CO 2 assimilation rate, especially in plants irrigated using water containing sodium, calcium, sodium + calcium and sodium + calcium + magnesium, may have resulted from the reduction in the amount of water absorbed and fixation of CO 2 by the plant due to stomatal closure (Figure 1A), due to decrease in the osmotic potential of the soil caused by the increase in ECw since water and CO 2 are two of the main limiting factors of photosynthesis, highlighting that higher diffusive resistance of the stomata reduces photosynthesis, especially by limiting gas conduction in the leaves (Kerbauy, 2008).
The plant height (PH), stem diameter (SD) and leaf area (LA) of the castor bean varied significantly (p < 0.01) between the different cationic compositions of irrigation water (Table 3).Cavalcanti et al. (2005)   ect on ) was 99.52 water containing K + , there were increments of 10.36 cm, 6.24 mm and 1758.76 cm 2 in PH, SD and LA, respectively, in comparison to plants in the treatments S 2 , S 3 , S 4 , S 5 and S 6 .According to Melo et al. (2014), the accumulation of potassium in the plant favors the formation and translocation of carbohydrates and the efficient use of water by the plant; in addition, the supply of potassium in the soil solution can stimulate the greater uptake and assimilation of nitrogen and phosphorus and, consequently, promote greater plant growth.

Table 2 .
Mean estimation for transpiration (E), stomatal conductance (gs), internal CO 2 concentration (Ci) and CO 2 assimilation rate (A) of castor bean plants, cv.'BRS Energia', irrigated with waters of different cationic compositions and electrical conductivity, at 100 days after sowing