Effect of Irrigation Depths and Salinity Levels on the Growth and Production of Forage Palm Orelha de Elefante Mexicana

Abstract

Although the adaptation of forage palm to the Brazilian semi-arid, it may be influenced by soil and climatic conditions of this region, irregular rainy periods, high annual evapotranspiration and soils with a low water retention capacity. These factors may reduce crop production during dry seasons, including forage. The present research aimed at analyzing the effect of irrigation with different water depths and levels of salinity on Orelha de Elefante Mexicana cultivar. The study was carried out in pots in the Federal University of Campina Grande, from September 2017 to December 2018. Experimental design was randomized blocks in a factorial scheme 4 x 4, with 4 replications. Four irrigation water depths were applied (25, 50, 75, and 100%), as a function of water retention capacity of soil and four levels of electrical conductivity: 0.60; 3.00; 5.40 and 7.80 dS m−1. Morphometric and production variables were evaluated. Plant growth was not affected by irrigation water depth and levels of salinity, except the thickness of secondary cladode. Primary cladodes showed the greatest average values (4.03 cladodes) for 376.00 mm depth. The other variables evaluated did not present significant effects under treatments. Saline water did not affect the total production of the cultivar.

1. Introduction

Brazilian semi-arid is characterized by irregular rainy periods, high annual evapotranspiration and soils with a low water retention capacity, limiting livestock activities in this region [1]. These conditions affect the production during dry seasons, including the reduction of forage for animal feed. In this context, Orelha de Elefante Mexicana (Haworth) Haworth) is an important forage palm that can mitigate the effects of low performance of the livestock. Thus, the efficiency of soil water use by Opuntia species is around 100 and 150 liters of water for each kilogram of dry matter produced, while grasses need 250 and 350 liters to produce the same quantity of dry matter [2].

Forage palm species are cactus with a great exploitation potential in the Brazilian northeast, constituting an important resource during periods of drought, due to its high potential of phytomass production in semi-arid region [3]. Despite forage palm adaptation to the region, local meteorological conditions influence plant development, since hydric deficit may cause a reduction of water content and hydric potential, resulting in loss of turgescence, closure of stomata and reduction of growth, which, consequently, promote a decrease in the final production. Thus, irrigation practice is very important to the production system [4].

The usage of poor water quality has been an alternative for producers in the northeast region to minimalize water scarcity in plants. However, it is important to highlight that the available water in several Brazilian semi-arid regions has high soluble salt contents. In this context, palm water needs may modify, changing water absorption process and evapotranspiration due to salt accumulations in soil, contributing to its degradation [5]. According to Ribeiro, Moreira, Seabra Filho and Menezes [6], salinity is one of the abiotic stresses limiting agricultural production the most, since it presents negative effects on vegetal development.

Crops that are sensible to saline water show the need of studies that aim at analyzing viable technologies to producers, in order to minimize salt effects on plants.

Thus, the present research aimed to evaluate the effect of water depths and levels of saline water on the growth and production of forage palm Orelha de Elefante Mexicana (O. stricta (Haworth) Haworth).

2. Material and methods

2.1 Localization and characterization of the experimental area

The study was conducted open to sky at the experimental area of the Federal University of Campina Grande (UFCG), in the municipality of Campina Grande (7°12′52,56”S; 35°54′22,26”O and 532 m of altitude), state of Paraíba, from September 26, 2017 to December 11, 2018, totalizing 442 days. According to Köppen climate classification, the region has a mesometric, sub-humid, Csa climate, dry season (4 to 5 months) and rainy season (autumn to winter).

During the experiment, climate conditions were monitored by the automatic weather station at Brazilian National Institute of Meteorology (INMET), (7.22°S; 35.90°O and 546 m of altitude), located approximately 1200 m of distance (horizontal line) from the experimental area (Figure 1).

Figure 1.

Mean air temperature conditions (mean temperature), relative humidity of air (mean temperature), total precipitation (Ptotal) and reference evapotranspiration (ET₀) of the region in analysis during the research.

2.2 Experimental design and treatments

Experimental design was randomized blocks in a factorial scheme 4x4, with 4 replications, totalizing 64 experimental parcels. Four irrigation water depths were applied (L1 = 25%, L2 = 50%, L3 = 75% and L4 = 100%) as a function of soil water depletion taking into consideration the value of soil water retention capacity and four levels of electrical conductivity: S1 = 0.60 dS m⁻¹; S2 = 3.00 dS m⁻¹; S3 = 5.40 dS m⁻¹ and S4 = 7.80 dS m⁻¹; applied on forage palm Orelha de Elefante (O. stricta (Haworth) Haworth).

The research was carried out in 120 L pots open to the sky, with a space of 1.30 m between rows and 1.00 m between plants and one plant per pot. The pots were used as drainage lysimeters.

A layer of crushed stone (2.40 kg) was put at the bottom of each pot, covered with a texture fabric, a layer of coarse sand (2.10 kg) and 170 kg of soil (0.268 m³). Soil analysis was performed in the Irrigation and Salinity Laboratory (LIS) of The Federal University of Campina Grande. Physical characterization (Table 1), Fertility (Table 2) and Salinity (Tables 3 and 4) were evaluated.

Grain size			Texture classification	Soil density	Particles density	Porosity	Humidity (% dry soil basis)
% Sand	Silt	Clay	—		g cm^−3	%	0.10 atm	15.00 atm
82.79	11.86	5.35	Loam Sandy	1.58	2.72	41.91	7.50	2.85

Table 1.

Physical characterization of soil.

Ca	Mg	Na	K	SB	H	Al	CEC	O.M.	Available P	pH H2O (1:2.5)
	meq 100 g^−1 of soil							%	mg 100 g^−1	—
1.07	2.41	0.20	0.13	3.81	0.56	0.0	4.37	1.15	0.57	6.75

Table 2.

Chemical characterization of soil (fertility).

SB—sum of basis; CEC—cation exchange capacity; O.M—organic matter.

CE	Chloride	Carbonate	Bicarbonate	Sulphate	Ca	Mg	K	Na
mmhos cm^−1	meq L^−1
0.53	5.00	0.00	0.22	—	1.50	3.50	3.76	2.56

Table 3.

Chemical characterization of soil (salinity).

SP	SAR	ESP	Salinity	Soil classification
%	—	—	—	—
22.17	1.62	4.57	Non-saline	Normal

Table 4.

Initial chemical characterization of soil (salinity).

SP—saturation percentage; SAR—sodium absorption ratio; ESP—exchangeable sodium percentage.

2.3 Plant material and fertilization

The cladodes of forage palm (Orelha de Elefante Mexicana) evaluated in the experiment were obtained from the Experimental station Lagoa Bonita, National Institute of Semi-arid (INSA), located in the countryside of the municipality of Campina Grande – PB. Secondary cladodes with a homogeneous height were used. After cutting cladodes, they remained during 15 days in shadow to shed moisture and heal injures. Cladodes were treated with bordeaux mixture 48 hours before sowing, in order to prevent fungi and bacteria [7, 8]. Planting was performed at a 45° angle to avoid the fall of the rackets, considering the wind factor in east-west.

Fertilizations followed the recommendations of Novais, Neves and Barros [9] for experiments using urea, potassium chloride (KCl) and monoammonium phosphate (MAP), keeping equal doses in every pot. Fertilization was divided in 30% as basal dressing and the difference was divided and applied every month during the experiment.

2.4 Irrigation

Treatments were carried out at 108 days after sowing and concluded at the end of the cycle (442 days after sowing), totalizing 334 days of application, period necessary for establish forage palm.

Water depths were determined based on water retention capacity of soil (D1 = 25%, D2 = 50%, D3 = 75% and D4 = 100% of WRC), with a variable irrigation frequency and determined by the depletion of soil water content that corresponded to the water depths to be replaced. Water retention capacity was determined by water availability in soil, according to Salassier, Soares & Mantovani [10]:

$$\mathrm{AWC}=\left(\mathrm{FC}–\mathrm{WP}\right)\mathrm{x}\rho$$E1

$$\mathrm{WRC}=\mathrm{AWC}\mathrm{x}\mathrm{Z}$$E2

Where, AWC: Available water capacity in soil (mm cm⁻¹ of soil); FC – field capacity (% of weight); WP: wilting point (% of weight); ρ_b – bulk density (g cm⁻³); WRC – water retention capacity (mm) and Z – effective root system depth (mm).

Variation of water storage was determined based on Lopes et al. [11] at 0.15 m:

$$\text{∆TWS}=\left({\mathrm{\theta }}_2–{\mathrm{\theta }}_1\right)\mathrm{x}\mathrm{z}$$E3

Where, ∆TWS – water storage variation (mm d⁻¹); θ₂ – average humidity at the end time (cm³ cm⁻³) corresponding to the day; θ₁ – average humidity at the initial time (cm³ cm⁻³) corresponding to humidity of the previous day; z – depth for balancing.

For the evaluation of the reduction in moisture as the function of water retention capacity perceptual, daily collections of soil samples were performed at 15 and 30 cm (corresponding to the radicular zone of great distribution of forage palm O. stricta (Haworth) Haworth), in order to determine moisture using electric oven [12].

Based on the physical analysis of soil, soil-water characteristic curve was determined and adjusted by van Genuchten model using the computer program Soil Water Retention Curve fit (SWRT fit), considering granulometry (%) and density particles values (g cm⁻³) [13]. Thus, water content in soil (humidity in a given volume cm³ cm⁻³) was determined as a function of humidity, obtained by gravimetry method, using an electric oven in the radicular system depth.

Water was prepared by adding commercial sodium chloride (without iodine), calcium chloride and magnesium chloride in the proportions 7:2:1, respectively, in order to increase electrical conductivity of water, according to the methodology proposed by Richards [14]. The dilutions were performed in four 500 L polyethylene pots, in which, every pot corresponded to a different saline level. The water used came from the Water and Sewerage Company of Paraíba (CAGEPA). Chemical water analyses were performed in the Irrigation and Salinity Laboratory (LIS/UFCG) (Tables 5 and 6).

Ph	EC	Ca	Mg	Na	K
—	dS m^−1		mg L^−1
6.70	0.03	1.20	2.40	0.70	0.0

Chloride	Bicarbonate	Carbonate	Sulphate	Hardness	Alkalinity
		mg L^−1
0.0	20.13	0.0	0.0	13.20	16.50

Table 5.

Chemical parameters of rainy water used in the experiment.

Ph	EC	Ca	Mg	Na	K
—	dS m^−1		mg L^−1
7.40	0.37	16.00	8.00	19.00	32.70

Chloride	Bicarbonate	Carbonate	Sulphate	Hardness	Alkalinity
		mg L^−1
46.79	75.64	0.0	35.04	73.12	62.00

Table 6.

Chemical parameters of tap water used in the experiment.

2.5 Growth variables

Growth variables were analyzed at the end of the cycle, at 334 days after treatments application or 442 days after sowing, according to the methodology proposed by Borges et al. [15] and consisted of: length of primary cladode (PCL, cm) and secondary cladode (SCL, cm); width of primary cladode (PCW, cm) and secondary cladode (SCW, cm); perimeter of primary cladode (PCP, cm) and secondary cladode (SCP) and thickness of primary cladode (PCT, mm) and secondary cladode (SCT, mm). A measure type was used for height, width, length and perimeter; and a digital caliper for thickness, 0.05 mm precision.

Primary (PCA cm²) and secondary (ACS, cm²) cladode areas were estimated considering cladode width and length, following the methodology proposed by Santos et al. [16] for forage palm (Opuntia):

$$\mathrm{CA}=\mathrm{CL}\mathrm{x}\mathrm{CW}\mathrm{x}0.693$$E4

CA – cladode area (cm²); CL – cladode length (cm); CW – cladode width (cm) and; 0.693 – correction factor as a function of cladode.

2.6 Production variables

At 442 days after sowing, or 334 days after treatments application, production evaluations were performed. Number of primary cladodes (NPC), secondary cladodes (NSC) and total number of cladodes (TNC) per plant were obtained by direct counting, according to Borges et al. [15].

2.7 Statistical analyses

Growth and production data were submitted to the distribution normality test (Shapiro–Wilk test) at 5% probability. The variables that did not demonstrate distribution normality were altered using quadratic root. After normality test, variance analysis by F test at 1 and 5% probability was used for isolate irrigation depths factors and for the interaction depths versus salinity.

Data that presented significant effects were adjusted by polynomial, linear and quadratic regression. Statistical analyses (Shapiro–Wilk and F test) were performed using Sisvar software, 5.6 version [17].

3. Results and discussion

3.1 Irrigation

The results concerning average frequency of irrigation as a function of depths and salinity levels are shown in Table 7.

Treatment	Frequency (days)	Treatment	Frequency (days)	Treatment	Frequency (days)	Treatment	Frequency (days)
L1S1	2	L2S1	5	L3S1	9	L4S1	10
L1S2	2	L2S2	6	L3S2	10	L4S2	10
L1S3	2	L2S3	5	L3S3	9	L4S3	13
L1S4	3	L2S4	6	L3S4	8	L4S4	12

Table 7.

Average frequency of irrigation as a function of depths and salinity levels.

L1S1, L1S2, L1S3 and L1S4 (25% water retention capacity - WRC and 0.60; 3.00; 5.40 and; 7.80 dS m⁻¹, respectively); L2S1, L2S2, L2S3 and L2S4 (50% of WRC and 0.60; 3.00; 5.40 and; 7.80 dS m⁻¹, respectively); L3S1, L3S2, L3S3 and L3S4 (75% of WRC and 0.60; 3.00; 5.40 and; 7.80 dS m⁻¹, respectively) and; L4S1, L4S2, L4S3 and L4S4 (100% of WRC and 0.60; 3.00; 5.40 and; 7.80 dS m⁻¹, respectively).

According to the results, average frequencies of irrigation varied between 2 and 13 days, considering treatments. In general, the lowest frequencies were obtained under the greatest saline levels (S3 = 5.40 dS m⁻¹ and S4 = 7.80 dS m⁻¹).

The lowest frequencies of irrigation using the highest saline waters may have occurred due to the salt effects on soil, altering its physical–chemical properties.

When comparing the same saline level to the other depths, it was observed a progressive reduction of irrigation frequencies, this occurred because irrigation was determined by water decay in soil as a function of water retention capacity. Thus, with the increase of water depths, it was necessary a longer period of time (lower frequency of irrigation) in order to promote the water decay in soil and the application of irrigation.

The total value irrigated per treatment (depth x salinity) are shown in Table 8.

Treatment	Irrigation (mm)	Treatment	Irrigation (mm)	Treatment	Irrigation (mm)	Treatment	Irrigation (mm)
L1S1	434.88	L2S1	375.42	L3S1	310.32	L4S1	352.76
L1S2	505.37	L2S2	344.51	L3S2	309.64	L4S2	385.57
L1S3	507.63	L2S3	415.31	L3S3	326.78	L4S3	299.22
L1S4	380.61	L2S4	347.39	L3S4	337.42	L4S4	317.95
Mean	457.12	L2S4	370.65	L3S4	321.04	L4S4	338.87

Table 8.

Total accumulated irrigation at the end of the cycle as a function of water depths and salinity levels.

In general, the greatest irrigation frequencies resulted in the highest values of accumulated irrigation depths (treatments with 25 and 50% of water retention capacity – WRC). Furthermore, the influence of salt in irrigation water was verified in the crop hydric consumption, demonstrated by treatments with 5.40 and 7.80 m⁻¹ that presented the lowest irrigation depths.

According Souza et al. [18], the accumulation of salts in soil is related to irrigation that compromises chemical, physical and biological properties of soil. Especially in semi-arid regions, where there is a predominance of evaporation over precipitation. Salts present in soils reduce the osmotic potential of its solution and may decrease water availability in plants.

3.2 Growth

3.2.1 Primary cladodes

The results of variance analysis (F test at 1 and 5% probability) for the variables of length, width, area, perimeter and thickness of primary cladodes at 334 days after treatment applications, corresponding to a total cycle of 442 days, are shown in Table 9.

Variation source	DF	Quadratic Average (QA)
		PCL1	PCW	PCA1	PCP	PCT
Depth	3	66.51ns	6.46ns	44623.12ns	48.01ns	1.45ns
Salinity	3	41.13ns	5.83ns	19373.73ns	51.82ns	3.67ns
Depth x Salinity	9	50.73ns	1.71ns	17987.23ns	18.07ns	2.56ns
Block	3	71.47ns	8.04ns	53399.78ns	103.92ns	0.45ns
Error	45	55.33	4.43	29940.33	49.86	2.08
CV (%)	—	9.80	8.68	12.83	8.91	9.28
General average	—	30.02	24.27	551.86	79.25	15.56

Table 9.

Summary of variance analysis of primary cladode length (PCL), primary cladode width (PCW), primary cladode area (PCA), primary cladode perimeter (PCP) and primary cladode thickness (PCT) of forage palm Orelha de Elefante at 442 days of cycle.

^ns

Non-significant by F test (p > 0.05).

¹

Data transformed by quadratic root.

According to the data obtained (Table 9), there were not significant statistical differences (p > 0.05) for any growth variable evaluated. Thus, treatments did not influence the growth parameters of primary cladodes.

Average values were observed for length (PCL), width (PCW), area (PCA), perimeter (PCP) and thickness (PCT), as the following: 30.02 cm; 24.27 cm; 551.86 cm²; 79.25 cm and; 15.56 mm, respectively.

Donato et al. [19] state that morphometric characteristics of forage palm are rarely influenced by management, which was also verified in the present study. The results differed from two other authors, as Silva et al. [20] and Lima et al. [21]. These authors verified that the adoption of management practices on forage palm results in significant effects on the plant growth, by irrigating with different sowing spacing or by supplying fertilization.

Silva et al. [20] evaluated the growth of forage palm clones in semi-arid region conditions and its relations with meteorological variables and verified statistical differences for the average length of primary cladode with 27.73 cm for Orelha de Elefante Mexicana. This value is close to the one observed in this study for the same cultivar.

Lima et al. [21] studied morphological and productive characteristics of forage palm Gigante irrigated with saline water (5.25 dS m⁻¹) and submitted to intensive cutting, and obtained average length of 37.87 cm for the second cycle. Significant differences were not observed for this variable. The authors only verified significant effects for average width (20.95 cm), thickness (18.62 cm) and area (583.46 cm²). The aforementioned authors concluded that the differences observed for width and thickness of cladode may have resulted from the best efficiency of physiological and biochemical process of plant, as photosynthesis, respiration and transpiration, being influenced by management practices.

Pereira et al. [22] evaluated the growth of forage palm clones (Orelha de Elefante Mexicana, IPA Sertânia and Miúda), in the municipality of Serra Talhada in the state of Pernambuco, under drip irrigation with a permanent depth (7.50 mm) and three intervals of water application (7, 14 and 28 days). The authors concluded that irrigation promoted the best biometric increments for the evaluated clones. For Orelha de Elefante Mexicana were observed the following average values in the primary cladodes, in absolute terms: 23.80 cm of cladode length and 11.80 mm of cladode thickness. In relative terms, for the frequencies of 7, 14 and 28 days, these values were the following: 18.70 cm (7 days), 15.40 cm (14 days) and 16.20 cm (28 days) of cladode width; 68.60 cm (7 days), 59.00 cm (28 days) of cladode perimeter; and 285.00 cm² (7 days), 310.00 cm² (14 days) and 377.00 cm² (28 days) of cladode area. The total water received by the crop was equivalent to 558.00 mm (7 days), 475.00 mm (14 days) and 438.00 mm (28 days). Even with different irrigation frequencies performed in this research, these depth values are close to those verified by Pereira et al. [22], which did not present significant effects for primary cladodes as well as the present study.

Sarmento et al. [23] evaluated the influence of different irrigation frequencies on the growth and production of forage palm Orelha de Elefante Mexicana (O. stricta (Haworth) Haworth) submitted to different frequencies of irrigation (0, 7, 14 and 21 days) and verified an increasing linear effect in width and length of primary cladodes due to the increase in irrigation frequency. This did not occur to the thickness of primary cladode. Means observed for primary cladode length were the following: 31.16 cm (without irrigation); 32.83 cm (21 days); 34.97 cm (14 days) and; 36.07 (7 days). For average width: 23.86 (without irrigation); 26.46 cm (21 days); 26.73 (14 days) and; 28.90 (7 days). Thickness: 19.14 mm (0 days); 17.60 mm (21 days); 18.36 mm (14 days) and; 17.32 mm (7 days). And perimeter: 92.88 mm, 91.80 mm, 91.15 mm and 92.66 mm (0, 21, 14 and 7 days), respectively. The results corroborate with the values obtained in this study, showing that the growth of primary cladodes were not affected, even with different irrigation depths contributing to different frequencies.

3.2.2 Secondary cladodes

The summary of variance analysis using F test at 1 and 5% probability for length, width, area, perimeter and thickness of secondary cladode at 334 days after treatment applications are shown in Table 10.

Source of variation	DF	Quadratic Average (QA)
		SCL	SCW	SCA	SCP	SCT1
Depth	3	3.68ns	2.09ns	2573.55ns	15.42ns	4.63ns
Salinity	3	7.18ns	2.79ns	4025.10ns	36.87ns	5.53*
Depth x Salinity	9	4.00ns	4.11ns	3657.90ns	34.93ns	1.85ns
Block	3	7.11ns	4.02ns	5040.10ns	37.58ns	1.22ns
Error	45	2.39	2.04	1946.91	21.93	1.76
CV (%)	—	7.26	7.65	15.32	8.02	6.53
General average	—	21.32	18.66	287.93	58.38	9.39

Table 10.

Summary of variance analysis of secondary cladode length (SCL), secondary cladode width (SCW), secondary cladode area (SCA), secondary cladode perimeter (SCP) and secondary cladode thickness (SCT) of forage palm Orelha de Elefante at 442 days of cycle.

*

Significant (p ≤ 0.05).

^ns

Non-significant (p > 0.05) by F test.

¹

Data transformed by quadratic root.

According to the results of variance analyses by F test (Table 10), as for primary cladodes, treatments did not present effects (P > 0.05) on the variables evaluated: length, width, area and perimeter.

The variables of secondary cladode present average values that did not depend on the applied treatment: 21.32 cm of length, 18.66 cm of width, 287.93 cm² of area and 58.38 cm of cladode perimeter. These results were inferior to the results of primary cladodes due to plant morphometric characteristics.

Sarmento et al. [23] obtained secondary cladode length in different frequencies of irrigation: 27.77 cm (without irrigation) 27.28 cm (21 days); 28.23 cm (14 days) and; 29.26 cm (7 days). Regarding cladode width, from the lowest frequencies to the highest frequencies, the average values were the following: 21.84 cm; 24.33 cm; 24.69 cm and; 25.52 cm. And the perimeters: 87.73 cm, 83.24 cm, 85.38 cm and 86.96 cm from the lowest to the highest. In spite of the absence of significant effects, frequent irrigation contributed to the growth of forage palm Orelha de Elefante Mexicana. The results observed by the authors for the species O. stricta (Haworth) Haworth were superior to the ones obtained in the present study. The variables evaluated in this work may have been influenced by saline water reducing the growth in relation to the conditions of plant cultivation using non-saline water, even without statistical significance for treatments.

Borges et al. [15] studied three different forage palm (Orelha de Elefante Mexicana, Miúda and Baiana) submitted to nitrogen fertilization via fertigation and observed average cladode length varying between 25.40 to 27.52 cm in relation to the error pattern for Orelha de Elefante Mexicana. Thus, this difference between length obtained in the present study and the values verified by the authors is due to the average values between primary and secondary cladodes.

Sales et al. [24] evaluated the vegetative growth of forage palm Gigante under different densities of cultivation in Curimataú (river located in the states of Paraiba and Rio Grande do Norte) in the state of Paraiba. It was verified that the average value between densities of cultivation for cladode width was 18.98 cm and cladode length 33.89 cm at 710 days after sowing. On the other hand, the variable cladode area was affected by treatments showing values of 440.12 cm², 397.95 cm² and 383.05 cm². The authors did not perform regular intervals of irrigation during the experiment; the water depth applied was made through precipitation. Thus, the present study as well as the research carried out by the authors did not obtain significant statistical differences in relation to the application of treatments on the growth variables analyzed, except for plant height.

Pereira et al. [22] observed average values for secondary cladodes: 10.40 cm of length, 12.60 of width, 27.30 cm of perimeter and 74.60 cm² of area. The results for these variables were inferior to the ones of the present study. Statistically, in this research, the results were not significant (P > 0.05) in relation to the treatments applied. However, the salt in water may have influenced plant growth as salinity may have contributed to elevate forage palm evapotranspiration due to sodium (Na⁺) and to stomatal adjustment that fomented plant development. Moreover, according to Campos [25], water availability through irrigation increases real evapotranspiration of forage palm plant in comparison to rainfed cultivations. In this context, the plant increases its transpiration due to greater water availability.

This hypothesis can be affirmed by Fonseca et al. [26] that concluded that higher average values of morphometric characteristics of forage palm under conditions of hydric availability indicates that even with use of saline water, irrigation provides better conditions of crop development due to the increment of photosynthetic taxes.

There were only significant statistical differences (P ≤ 0.05) for secondary cladode thickness in relation to salinity levels in the irrigation water applied. Effects of water depths and its interaction with salinity were not observed in forage palm thickness. Data showed low dispersion in the coefficient of variation (CV) of 6.53%. The other variables did not show significant effect (P > 0.05) for any factor evaluated.

There was an increment for cladode thickness up to 9.79 mm under a salinity level of 1.22 dS m⁻¹, verified by the graphic adjustment equation. On the other hand, with a higher salinity level, cladode thickness decreased. The value of R² in the adjustment equation was 0.70 (Figure 2).

Figure 2.

Secondary cladode thickness (SCT) of forage palm (Orelha de Elefante) as a function of salt in irrigation water (EC_w) at 442 days of the cycle.

Forage palm thickness is related to the accumulation of water in its cladodes. This stored water, according to Nobel [27], may favor palm gas exchange. This content may be an indicative of stress tolerance caused by saline water.

The effects of treatments (P < 0.05) on secondary cladode thickness may denote the development of tolerance mechanisms to salinity through juiciness. According to Willadino and Camara [28], sodium tends to be transported via xylem and accumulate in plant shoot system. Thus, plant may have developed juiciness in order to mitigate the effects of accumulated salts on secondary cladodes or even due to the osmotic adjustment, the responsible for causing a potential difference and induce water movement into guard-cell.

Although, Freire [29] concluded that higher salinity levels of 3.60 dS m⁻¹ or more, result in the decrease of forage palm juiciness. This maximum salinity level on thickness was 60% lower than the one verified by the author.

It can be inferred that osmotic adjustment contributed to prevent restrictions to plant stomatal opening, promoting the increase of transpiration and the reduction of water content in cell and, consequently, its thickness. This justifies the reduction of thickness of secondary cladode due to the increase of salt in irrigation water.

Cladode thickness is one of the species characteristics directly correlated to plant turgidity. Thus, the higher the thickness, the greater water quantity in cells, which is one of the main attributes of CAM plants [22].

Pereira et al. [22] obtained average values of cladode thickness in OEM, IPA and Miúda cultivars: 11.80; 18.50 and 14.10 mm, respectively. The thickness observed by the authors was approximately 17% superior.

Sarmento et al. [23] verified a significant effect of cladode thickness as a function of irrigation frequencies, which presented a decreasing quadratic effect, showing values of 12.31 (0 days); 13.58 (21 days); 11.44 (14 days); 11.30 (7 days). On the other hand, it did not occur for primary cladode.

When comparing the results of the thickness of primary cladode to the results of Pereira et al. [22], Sales et al. [24] and Sarmento et al. [23], it was verified that thickness was lower than the ones verified by the other authors. However, the aforementioned works studied the species Opuntia fícus-indica Mill while the present research analyzed the species O. stricta (Haworth) Haworth.

According to Rocha, Voltolini, and Gava [30], cladode thickness is of great importance for the photosynthetic capacity and for water storage in plant.

Orelha de Elefante clone has a great potential to adapt to conditions of low water availability in soil, presenting a greater capacity of water storage in its cladodes [22]. Thus, this capacity also contributes to plant tolerance in relation to salts.

3.3 Production

3.3.1 Number of primary and secondary cladodes

The results of variance analysis by F test at 1 and 5% probability showed a significant effect (P ≤ 0.05) only for the number of primary cladodes in relation to water depths. Regarding the other variables, the number of secondary cladodes and the total number of cladodes did not present significant statistical differences (Table 11).

Source of variation	DF	Quadratic Average (QA)
		NPC1	NSC	TNC
Depth	3	4.89*	3.43ns	1.64ns
Salinity	3	2.89ns	12.56ns	9.18ns
Depth x Salinity	9	1.78ns	14.25ns	14.52ns
Block	3	1.02ns	20.31ns	15.10ns
Error	45	1.73	19.35	16.66
CV (%)	—	11.79	21.19	12.06
General average	—	5.66	11.48	17.20

Table 11.

Summary of variance analysis for the number of primary cladodes (NPC), number of secondary cladodes (NSC) and total number of cladodes (TNC) of forage palm Orelha de Elefante Mexicana at 442 days of the cycle.

*

Significant (p ≤ 0.05).

^ns

Non-significant by F test.

¹

Data transformed by quadratic root.

The number of primary cladodes showed a quadratic tendency with R² equal to 0.8643 (Figure 3). The greatest average was presented by the number of primary cladodes, based on the graphical adjustment equation, for the water depth of 376.00 mm with 4.03 cladodes. The same depth is the closest to L2 (370.66 mm), which corresponds to 50% of water retention capacity of soil – WRC. The other depths presented average values of 3.43 cladodes (L3–75% of WRC equal to 321.04 mm), 3.76 cladodes (L4–100% of WRC equal to 338.87 mm) and 2.72 cladodes (L1–25% of WRC equal to 457.12 mm), also obtained using the adjustment equation.

Figure 3.

Number of primary cladodes of forage palm Orelha de Elefante Mexicana as a function of water depths at 442 days of the cycle.

Thus, it was verified that the number of primary cladodes increased due to the water depth increase up to 376.00 mm and, for values higher than that, it decreased.

The other variables did not present significant effect (P > 0.05) with average values of 11.48 of secondary cladodes (NSC) and 17.20 of total number of cladodes (TNC).

Cavalcante, Leite, Pereira and Lucena [31] evaluated forage palm Orelha de Elefante Mexicana with and without cure of cladodes and did not verify statistical differences (P > 0.05) in the number of primary cladodes, number of secondary cladodes and total number of cladodes. But, the average values were 2.35 for primary cladodes, 5.37 for secondary cladodes and 12.31 for total number of cladodes. However, tertiary cladodes were considered by the authors when counting the number of total cladodes, and, in the present research, tertiary cladodes were not observed in forage palm. The values obtained in this study were superior to the ones by Cavalcante, Leite, Pereira and Lucena [31] probably due to the application of water depths, promoting the production of the cultivar.

Lima et al. [21] did not verify significant interaction (P > 0.05) in the number of primary cladodes, but obtained significant effect in the total number of cladodes with a maximum value of 20.60 of cladodes per plant. The authors also used saline water on forage palm, but productive characteristics were evaluated in forage palm Gigante. Lima et al. [21] concluded that the great number of cladodes tent to increase CO₂ capture, which results in a higher photosynthetic tax, contributing to maximize the production.

Sarmento et al. [23] obtained values that corroborate with the present research. The authors verified that Orelha de Elefante Mexicana, under different irrigation frequencies (0, 7, 14 and 21 days), presented number of cladodes equal to: 6.06 (0 days), 4.92 (21 days), 4.85 (14 days) and 4.82 (7 days) of primary cladodes; 12.40 (0 days), 11.91 (21 days), 11.91 (14 days) and 10.50 (7 days) of secondary cladodes; 18.60 (0 days), 17.20 (21 days), 16.94 (14 days) and 15.44 (7 days) of total number of cladodes.

According to Borges et al. [15], the greatest number of cladodes in forage palm plants reflects in a greater production. The authors verified that Orelha de Elefante Mexicana under fertigation presented 13.00 cladodes per plant, which is equivalent to the total number of cladodes.

Pereira et al. [22] verified that hydric availability conditions did not affect the number of primary cladodes and the total number of cladodes of forage palm O. stricta (Haworth) Haworth. However, there was significant effect for the number of secondary cladodes. The authors observed the following average values: 8.11 of primary cladodes and 13.50 of total number of cladodes. The number of secondary cladodes showed the following values, respectively: 7.67 units (7 days frequency with a total water depth of 558.00 mm); 1.56 units (14 days frequency with water depth of 475.00 mm) and; 4.22 units (frequency of 28 days with water depth of 438.00 mm).

In the present study, the number of secondary cladodes presented average values of 11.48 units for water depths that varied between 321.04 mm and 457.12 mm. Comparing these results to the ones obtained by Pereira et al. [22], there was a better use of water irrigation by plant in the production of cladodes, since palm produced a quantity of 33% higher under water depths inferior than the ones applied by the authors.

4. Conclusions

The research analyzed the effect of irrigation with different water depths and levels of salinity on Orelha de Elefante Mexicana cultivar. This cultivar can produce even under saline conditions and irrigation with saline water is an alternative strategie for forage production in semiarid regions.

The growth of forage palm Orelha de Elefante Mexiacana was not affected by water depths and by the salt in water irrigation, except thickness of secondary cladodes, which was positively affected by water electrical conductivity.

The production of primary cladodes was affected by water availability for forage palm Orelha de Elefante Mexicana, which was maximized by water depth corresponding to 50% of water retention capacity of soil presenting 4.03 cladodes.

Regarding the conditions to carry out the research, saline water did not affect the total production of Orelha de Elefante Mexicana cultivar.