1. Introduction
Belonging to the family
Anacardiceae, cashew (
Anacardium occidentale L.) is a widely cultivated species in the semi-arid region of northeastern Brazil due to its high commercial and socioeconomic value, serving as an important source of employment and income for the population [
1]. Cashews are rich in vitamin C, calcium, phosphorus, and iron, and play a key role in the fruit-processing industry, giving origin to products such as sweets and juices, cashew nuts, and cashew bagasse, which is used to produce flours, animal food, and medications [
2,
3].
Brazil is one of the principal cashew nut producers in the world, with a production of 111,103 tons in 427,144 hectares. In 2021, the Northeast region of Brazil produced approximately 106,675 tons per hectare of cashew nuts, with the State of Ceará ranking as the largest cashew-producing state with 62,977 tons. Piauí and Rio Grande do Norte stand out as second and third place, producing 19,020 and 16,920 tons per hectare, respectively [
4]. In the same year, the State of Paraíba produced approximately 670 tons of cashew nuts per hectare [
4].
However, despite the potential of cashew production in the Northeast region, the irregularity and poor distribution of rainfall in the region, associated with high temperatures, results in high evapotranspiration rates, complicating the cultivation of this crop [
5]. In this scenario, the employment of irrigation emerges as a viable solution for continuous cultivation, allowing the expansion of productive regions over the year [
6]. However, most water sources (surface and subsurface) show high concentrations of dissolved salts, which constitutes a limiting factor for the cultivation of species sensitive to salt stress, e.g., the precocious-dwarf cashew. Excess salt in the water and/or soil can affect water availability for crops due to osmotic and ionic effects [
7].
The use of saline water for irrigation negatively affects the metabolic and biochemical functions of plants, inhibiting their growth and capacity to perform photosynthesis and protein synthesis, interfering with enzymatic activity and chlorophyll synthesis. Furthermore, saline water also influences the flow of electrons, altering the functioning of photosystem II [
8,
9]. The high concentrations of ions such as Na
+ and Cl
− in the roots cause a series of morphophysiological disorders due to the osmotic and ionic effects of stress caused by salinity, decreasing water and nutrient uptake [
10,
11].
The search for strategies capable of alleviating salt stress effects on plants and enabling irrigation with saline water in irrigated fruit farming is essential to obtain high crop yields with economic advantage. In this scenario, the foliar application of salicylic acid (SA) stands out as a viable strategy [
12]. SA is a natural phenolic compound present in some physiological processes of plants, e.g., floral induction, stomatal opening and closing, ion absorption, photosynthesis, transpiration, and in the activation and catalysis of antioxidative enzymes and biosynthetic proteins, degrading reactive oxygen species (ROS) [
13,
14]. However, the effects of SA depend on the species and stage of crop development, in addition to the concentration and application method employed [
14,
15].
Studies conducted by Ekbic et al. [
16] to evaluate the effects of salicylic acid application at concentrations of 6 and 9 mM in American grapevines irrigated with water of electrical conductivity up to 8.0 dS m
−1 highlighted a reduction in the deleterious effects caused by salt stress on plant growth and development. In another study, Samadi et al. [
17] observed that the foliar application of salicylic acid at a concentration of 100 µM mitigated the harmful effects of salinity in strawberry plants irrigated with 50 mM NaCl solution.
Silva et al. [
14] studied a
soursop crop irrigated with water of electrical conductivity levels ranging from 0.8 to 4.0 dS m
−1, and observed that the foliar application of salicylic acid at the concentration of 2.75 mM increased the relative growth rate in stem diameter.
The present study is based on the hypothesis that the foliar application of salicylic acid at adequate concentrations mitigates the deleterious effects caused by irrigation with saline water on the growth and physiology of the precocious-dwarf cashew, inducing plant tolerance to salt stress caused by the increase in the biosynthesis of photosynthetic pigments and the photochemical efficiency, reflecting on higher plant growth. From this perspective, this research aimed to evaluate the effects of the foliar application of salicylic acid on the chlorophyll fluorescence, photosynthetic pigments, and growth of precocious-dwarf cashew as a function of irrigation with saline water and the foliar application of salicylic acid during the post-grafting phase.
2. Results and Discussion
There was a significant effect of the interaction between irrigation water salinity levels and salicylic acid concentrations on the relative water content (RWC) and electrolyte leakage (% EL) in the leaf blade of precocious-dwarf cashew plants (
Table 1).
For the relative water content (
Figure 1A), the SA concentration of 1.1 mM mitigated the effects of salt stress up to the electrical conductivity of irrigation water (ECw) of 1.0 dS m
−1, promoting an increase of 3.53% compared to plants subjected to the salinity of 0.4 dS m
−1 without application of salicylic acid (0 mM). On the other hand, as the irrigation water salinity increased and the salicylic acid concentrations decreased, the RWC was reduced, achieving the minimum value (38.63%) in plants subjected to the ECw of 3.6 dS m
−1 and the SA concentration of 3 mM. The RWC reduction as a function of the increase in water salinity can be explained by the osmotic effect, which restricts water uptake by plants and affects their water potential [
18]. In a study evaluating the morphophysiology of the
soursop cv. Morada Nova irrigated with saline water (ECw ranging from 0.8 to 4.0 dS m
−1), Silva et al. [
19] also observed a 13% reduction in the RWC as the salinity levels increased in the irrigation water.
For electrolyte leakage in the leaf blade (
Figure 1B), the concentration of 1.1 mM of salicylic acid mitigated the effects of salt stress, achieving the lowest value of 38.63% in plants irrigated with water of 1.0 dS m
−1. In plants subjected to the SA concentration of 0 mM and the ECw of 0.4 dS m
−1, the % EL was 3.52% lower compared to plants grown under the concentration of SA 1.1 mM. The reduction in electrolyte leakage in the leaf blade of cashew plants can be explained by the protection of the cell membrane and the photosynthetic activity, as SA interacts with the signaling of ROS, reducing oxidative stress [
20,
21,
22]. It should be noted that the electrolyte leakage observed in this study did not cause injuries to the leaf tissues since, according to Sullivan [
23], the cell is considered injured when the damage percentage surpasses 50%. However, the increase in the ECw associated with salicylic acid concentrations higher than 1.1 mM intensified the effects of salt stress on cashew plants, increasing electrolyte leakage in the cell membrane, with the highest estimated value of 38.63% in plants irrigated with water of the electrical conductivity of 3.6 dS m
−1 and foliar application of 3.0 mM of SA.
There was a significant effect of the irrigation water salinity on the contents of chlorophyll a (Chl
a), b (Chl
b), total chlorophyll (Chl
total), and carotenoids (Car) of precocious-dwarf cashew plants at 280 DAT. The SA (0, 1, 2, and 3 mM) concentrations significantly influenced the contents of chlorophyll a and b (
Table 2). The interaction between factors (S × SA) did not affect traits analyzed 280 days after transplanting.
The salinity levels of irrigation water influenced the contents of chlorophyll a (
Figure 2A), achieving the maximum estimated value of 190.47 µg mL
−1 in plants subjected to the ECw of 1.1 dS m
−1. On the other hand, the minimum estimated value of 141.43 µg mL
−1 was observed in plants grown under the ECw of 3.6 dS m
−1. The reduction in the contents of chlorophyll a is a result of the increase in the water salinity levels, increasing the activity of the chlorophyllase enzyme and being related to the reduction in the number of chloroplasts, affecting the thylakoid membranes and constituting a recurrent symptom of oxidative stress [
24,
25].
In a study conducted by Lima et al. [
26] using the precocious-dwarf cashew cv. Embrapa 51 irrigated with saline water (ECw ranging from 0.4 to 3.6 dS m
−1), the authors observed a reduction of 17.10% in the Chl
a content per unit increase in the ECw. The reduction in the chlorophyll contents is related to lipid peroxidation and the increase in the generation of ROS [
27].
The concentrations of salicylic acid increased the chlorophyll
a content (
Figure 2B) up to the estimated concentration of 1.7 mM, whose maximum estimated value was 194.03 µg mL
−1. Salicylic acid plays an important antioxidant role, increasing the activity of the peroxidase, superoxide dismutase, and catalase enzymes to eliminate reactive oxygen species, thus increasing chlorophyll synthesis [
28,
29]. Similar results were found by Silva et al. [
30] in a study with
soursop under salt stress (ECw ranging from 0.8 to 4.0 dS m
−1), as the application of 1.4 mM of SA mitigated the effects of salt stress up to the ECw of 1.5 dS m
−1.
The chlorophyll
b contents of precocious-dwarf cashew plants were reduced linearly with the increase in the irrigation water salinity (
Figure 2C), amounting to an 11.5% per unit increase in the ECw. When comparing the Chl
b contents of plants irrigated with water with the highest salinity (3.6 dS m
−1) to those with the lowest salinity (0.4 dS m
−1), there was a reduction of 38.77%. The inhibition in chlorophyll synthesis is related to the increase in the synthesis of 5-aminolevulinic acid, a molecule responsible for chlorophyll production and that acts in the degradation of photosynthetic pigment molecules [
31].
The concentrations of salicylic acid also influenced the chlorophyll
b contents of the precocious-dwarf cashew plants (
Figure 2D), achieving the maximum estimated value of 94.61 µg mL
−1 in plants without its application (0 mM). In turn, the minimum value of 62.11 µg mL
−1 was achieved in plants subjected to the foliar application of SA 1 mM. The benefits of salicylic acid are related to the increase in enzymatic and photosynthetic activity, in addition to maintaining the balance between the degradation and production of reactive oxygen species [
20,
32].
The results found for chlorophyll
b agree with the study conducted by Lima et al. [
33], in which the authors analyzed three clones of precocious-dwarf cashew (Faga 11, Embrapa 51, CCP 76) irrigated with water of different electrical conductivity (0.4, 1.2, 2.0, 2.8, and 3.6 dS m
−1), obtaining reductions of, respectively, 16.86, 14.86, and 16.92% per unit increase in the ECw.
The carotenoid and total chlorophyll contents (
Figure 3A,B) of the precocious-dwarf cashew plants decreased linearly with the increase in the electrical conductivity of irrigation water, with reductions of 7.55 and 8.42% per unit increase in the ECw. When comparing the carotenoid and total chlorophyll contents of plants irrigated with the highest salinity (ECw = 3.6 dS m
−1) with those that received the ECw of 0.4 dS m
−1, there was a reduction of 24.94 and 27.89%, respectively.
The reduction in the carotenoid contents of plants grown employing high salinity waters highlights the damage that occurred to the photosynthetic apparatus since carotenoids are involved in the transfer of captured light to chlorophyll, thus influencing this transfer and the photosynthetic relations [
34]. Conversely, Lima et al. [
26] evaluated precocious-dwarf cashew plants irrigated with ECw levels ranging from 0.4 to 3.6 dS m
−1 and verified a 60.03% increase in the carotenoid contents of these plants.
There was a significant effect (
p ≤ 0.05 and 0.01) of the electrical conductivity levels of irrigation water on the initial (F
0), maximum (Fm), and variable fluorescence (Fv) and on the quantum efficiency of photosystem II (Fv/Fm) of precocious-dwarf cashew plants (
Table 3), 280 days after transplanting.
Water salinity linearly increased the initial fluorescence of precocious-dwarf cashew plants (
Figure 4A), with an increase of 8.29% per unit increase in the ECw. When comparing the F
0 of plants irrigated with 3.6 dS m
−1 to those under the ECw of 0.4 dS m
−1, there was an increase of 21.19%. The F
0 increase leads to lower utilization of available energy, which explains the damage caused by salt stress in the capture of light energy by photosynthetic pigments [
35,
36]. This behavior was already observed in cashew plants by Lima et al. [
37], in a study where the authors observed a 19.68% increase in the F
0 of plants irrigated with water of electrical conductivity up to 3.6 dS m
−1 in relation to plants under the lowest water salinity (0.4 dS m
−1). Silva et al. [
30] evaluated the photosynthetic efficiency of the
soursop (
Annona muricata L.) cv. Morada Nova under salt stress (ECw ranging from 0.8 to 4.0 dS m
−1) and observed that the increase in the ECw of irrigation water increased the initial fluorescence by 2.27% per unit increase in the ECw.
Conversely to what was observed in the F
0 (
Figure 4A), the maximum fluorescence decreased with the increase in the ECw (
Figure 4B), reducing by 6.16% per unit increase in the ECw. The plants irrigated with the ECw of 3.6 dS m
−1 showed an Fm reduction of 20.21% (263.57) compared to those cultivated with the lowest salinity (0.4 dS m
−1). Salinity reduces the capture of energy in the reaction centers, probably because the excessive accumulation of specific ions causes an imbalance in the plant’s metabolic activity, leading to the formation of reactive oxygen species, which limits the energy activity of photosynthetic pigments [
38].
Fernandes et al. [
39] studied the effects of irrigation with saline water (ECw of 1.3 and 4.0 dS m
−1) in custard apples and observed a reduction of 14.59% in the Fm of plants irrigated with the ECw of 4.0 dS m
−1 in relation to those that received 1.3 dS m
−1. Furthermore, according to these authors, the reduction in the maximum fluorescence can be explained by the action of the excess of salts in the photoreduction of quinone and in the thylakoid membranes, because of the entry of electrons into the photosystem.
Similar to the Fm, the variable fluorescence (Fv) decreased with the increase in irrigation water salinity (
Figure 4C), whose decrease was 10.77% per unit increase in the ECw. When comparing in relative terms, there was a 36.01% reduction in the Fv of plants irrigated with the ECw of 3.6 dS m
−1 compared to those subjected to the water salinity of 0.4 dS m
−1. Since it corresponds to the active potential energy in the photosystem, the Fv reduction demonstrates the limitation in the activation of the electron transport chain, which is responsible for the production of ATP and NDPH in the Calvin cycle, reducing the plant’s photosynthetic capacity [
2,
36].
Similar results were reported by Silva et al. [
30] when analyzing the photochemical efficiency of the
soursop (
Annona muricata L.) cv. Morada Nova plants irrigated with saline water (ECw of 0.8 to 4.0 dS m
−1), observed an Fv reduction of 17.41% in plants grown under the ECw of 4.0 dS m
−1 in comparison to those that received 0.8 dS m
−1.
The quantum efficiency of photosystem II was also affected by salt stress (
Figure 4D), showing a reduction of 5.81% per unit increase in the ECw, equivalent to a 19.05% reduction between plants cultivated under the water salinity levels of 0.4 and 3.6 dS m
−1. This situation indicates photochemical damage to cashew plants under salt stress, with part of the light energy available in the thylakoid membrane associated with the metabolic damage of salt stress, accelerating the production of ROS and degrading chlorophylls in the reaction center [
35,
38]. Similar responses were observed in studies developed by Diniz et al. [
40] in yellow passion fruit (
Passiflora edulis f;
flavicarpa) and by Xavier et al. [
41] in guava (
Psidium guajava L.).
There was a significant effect of the interaction between water salinity (S) and the concentrations of SA on the stem diameter below the grafting point (SD
rootstock) and the diameter above the grafting point (SD
scion) (
Table 4) of precocious-dwarf cashew plants. As an isolated factor, the salinity (S) significantly affected the diameter at the grafting point (SD
grafting point) and the vegetative vigor index (VVI). While the concentrations of salicylic acid significantly influenced plant height (PH) and the vegetative vigor index.
The increase in water salinity reduced the stem diameter below and above the grafting point of precocious-dwarf cashew plants (
Figure 5A,B), achieving the maximum values of 30.94 and 17.27 mm, respectively, at the ECw of 0.4 dS m
−1 and the SA concentration of 3 mM. On the other hand, the minimum values of 23.72 and 13.87 mm were observed in plants subjected to the ECw of 3.6 dS m
−1 and 0 mM of SA. The application of salicylic acid increased the growth in diameter below and above the grafting point under irrigation with saline water, highlighting the role of this plant hormone in the regulation of plant development processes, acting in root growth, meristem expansion, and in the gas exchange variables [
19,
42].
Water salinity reduced the growth in stem diameter at the grafting point (
Figure 6A), whose maximum estimated value of 26.53 mm was observed in plants irrigated with the ECw of 0.4 dS m
−1. On the other hand, a minimum value of 22.08 mm was observed in plants cultivated under the ECw of 3.6 dS m
−1. The inhibition in stem diameter growth could be associated with limitations in water and nutrient uptake and the accumulation of toxic ions (Na
+ and Cl
−), limiting pectin as a function of calcium deficiency (Ca
2+) and resulting in cellular disruption, decreasing the rigidity of the cell wall, and directly affecting cell expansion in the stem [
43]. Similar results were obtained by Lacerda et al. [
6] when studying the guava cv. Paluma under water salinity (ECw of 0.4 and 3.2 dS m
−1), observing a 13.25% decrease in the diameter of the rootstock with an increase in water salinity 390 days after transplanting.
The height of precocious-dwarf cashew plants (
Figure 6B) increased linearly with the increase in the salicylic acid concentrations, with a 3.78% per unit increase in the SA concentration. When comparing, in relative terms, the growth of plants under the SA concentration of 3.0 mM compared to those without application (0 mM), there was an increase of 9.79 cm in PH. Salicylic acid is an essential hormone for plant development as it plays multiple roles that promote plant growth. SA stimulates cell division, resulting in a significant increase in plant growth. Furthermore, this hormone also activates metabolic pathways involved in plant growth, thus boosting plant development more robustly and vigorously [
44].
The vegetative vigor index (VVI) (
Figure 7A) of precocious-dwarf cashew plants linearly decreased with the increase in the ECw, with a 2.86% reduction per unit increase in the ECw. When comparing the VVI of plants irrigated with the ECw of 3.6 dS m
−1 with those grown under the ECw of 0.4 dS m
−1, there is a reduction of 10.23%. The SA concentrations also influenced the vegetative vigor index linearly (
Figure 7B), with an increase of 2.66% per unit increase in the SA. Plants subjected to application with an SA of 3.0 mM showed an increase of 9.72% in the VVI compared to those under treatment without its application (0 mM).
The VVI reduction due to the increase in water salinity can be explained by the osmotic stress in plants, which results in an imbalance in plants cells, leading to the reduction in water availability as the consequence of a negative impact in nutrient uptake by the roots, causing nutrient deficiency and decreasing cell turgidity [
45]. These results agree with studies conducted by Lacerda et al. [
6] in the guava cv. Paluma under irrigation with saline water (ECw of 0.6 and 3.2 dS m
−1), in which the authors observed that the increase in the ECw levels reduced the VVI 390 days after transplanting.
The interaction between water salinity and salicylic acid concentrations significantly interfered in the relative growth rate of the rootstock (RGB
SDrootstock) and the relative growth rate of the scion (RGB
SDscion) of precocious-dwarf cashew plants during the period 220 to 280 DAT (
Table 5).
The cashew plants irrigated with the ECw of 0.8 dS m
−1 and subjected to the SA concentration of 1.0 mM obtained higher relative growth rates in both rootstock and scion diameter (
Figure 8A,B), with 0.0057 and 0.0041 mm mm
−1 day
−1 for the RGB
Sdrootstock and RGB
Sdscion, respectively. The cashew plants irrigated with the Ecw of 0.8 dS m
−1 and subjected to the SA concentration of 1.0 mM increased by 5.65% (0.00031 mm mm
−1 day
−1) and 7.9% (0.0003 mm mm
−1 day
−1) the RGB
Sdrootstock and RGB
Sdscion, respectively, in relation to those under the Ecw of 0.8 dS m
−1 and without SA application (0 mM). Furthermore, the foliar application of salicylic acid at concentrations higher than 1.0 mM intensified the effects of salt stress, with the lowest RGB
Sdrootstock (0.0025 mm mm
−1 day
−1) and RGB
Sdscion (0.0023 mm mm
−1 day
−1) values recorded in plants irrigated with the Ecw of 3.6 dS m
−1 and sprayed with SA at the concentration of 3.0 mM.
Salicylic acid is considered a signaling molecule and acts as a natural regulator of plant growth [
46]. As verified in the present study, the beneficial effect of salicylic acid on early dwarf cashew plants depended on the applied concentration. In addition to concentration, the effect of salicylic acid is related to the stage of crop development, method, and frequency of application [
15,
47,
48].
SA can act as a signaling molecule changing the expression of antioxidant genes and influencing the quantity and/or activity of proteins under salinity, enabling a greater accumulation of ions responsible for the osmoregulation and structuring of membranes, e.g., K
+ and Ca
2+, and reducing the concentration of toxic ions such as Na
+ and Cl
− [
11]. This fact could be related to the increase in the relative growth rate in diameter of the rootstock and scion observed in this study (
Figure 8).
In general, the results obtained in the present study indicate that the saline stress caused by the increase in the electrical conductivity of the irrigation water increased the percentage of electrolyte leakage, reduced the relative water content in the leaves of the early dwarf cashew plant, and limited the synthesis of photosynthetic pigments and the quantum efficiency of photosystem II. This is directly reflected in the reduction in growth, especially in the control (
Figure 9), i.e., in the plants that did not receive foliar application of salicylic acid (0 mM).
On the other hand, the foliar application of SA between concentrations of 1.0 and 1.1 mM attenuated the effects of salt stress on the relative water content of the leaf and electrolyte leakage. Furthermore, the 1.7 mM concentration of SA increased the synthesis of photosynthetic pigments. The growth variables of the dwarf-early cashew plant also increased as a function of the increase in salicylic acid concentrations (
Figure 10).