Does the sunblock alleviate abiotic stress in mango trees grown in the tropical semiarid?

Ten-year-old “Palmer” mango trees (Mangifera indica L.), with uniform size and vigour, were used in this study. The experiment was conducted between January and July in 2019 simultaneously in two experimental orchards located in Aracê farm (9°19′S and 40º 41′W; at an altitude of 423 m above sea level), Petrolina, Pernambuco State, Brazil. The climate of this region is classified as BSh (Köeppen), which corresponds to a semi-arid region. During the experiment, the meteorological data were monitored in an automatic meteorological station for air temperature (ºC), air humidity (%) and rainfall (mm) (Figure 1).

Figure 1

Table 1 contains the physical and chemical characteristics of the soil before the executions of the experiments.

The tree's nutritional status was determined through leaf analysis before the experiment, as can be seen in Table 2. Leaf samples were collected in the middle part of the canopy in the last vegetative flush of branches to perform the characterisation of plant nutritional status. Leaves were chemically analysed after they were washed and rinsed with distilled water and dried at 65°C until reaching constant mass, following the methodology described by Bataglia et al. (1983).

The plants, spaced by 6.0 m between the rows and 3.0 m between the plants, were daily irrigated (drip) with eight emitters per tree, for a flow of nearly 2 L · h⁻¹ each emitter. All management practices such as pruning, control of weeds, pests and diseases, plant growth regulators for gibberellin inhibition [paclobutrazol (PBZ), Cultar^®] and dormancy break (calcium nitrate and potassium nitrate) were performed following the instructions of Genú and Pinto (2002). From 60 days to 90 days after PBZ, the water blade reduction was performed, irrigating 50% of crop evapotranspiration (Etc). Nutrient management was performed through a fertigation system, according to plant demand (Genú and Pinto, 2002; Barbosa et al., 2016; Carneiro et al., 2017). The production pruning was mechanically carried out, and manual tip pruning was performed to synchronise vegetative flush events in the canopy.

The experiment followed a randomised block design with six treatments, four repetitions per treatment and three plants per replication. The treatments were defined considering the plant demands and physiological changes that occur during the mango phenological stages according to the production system conducted in the São Francisco Valley, properly described by Genú and Pinto (2002) and Cavalcante et al. (2018).

The treatments consisted of different strategies of sunblocks as follows: (T1) control (no sunblock); (T2) calcium carbonate (50 g · L⁻¹); (T3) sunblock (5 mL · L⁻¹); (T4) sunblock (5 mL · L⁻¹) + calcium carbonate (50 g · L⁻¹); (T5) sunblock (5 mL · L⁻¹) + sunblock silicon concentrated (20 mL · L⁻¹) and (T6) sunblock (20 mL · L⁻¹). All treatments were applied once, immediately after the flowering induction, which was performed with potassium nitrate (2.5%) foliar sprays (Cavalcante et al., 2018) on 21^st February 2019, without adjuvants and using a Jacto Arbus^® sprayer to completely wet the canopy.

The sunblock used was Humigel Plus A^®, which is a foliar fertiliser that acts as a protective barrier by forming a film (biodegradable film) and also contains N (2%), CaO (4%), Zn (2.9%), SiO₂ (15%) and fulvic acids (12%). The sunblock silicon concentrator used was Humigel Plus Silício^®, which also is a foliar fertiliser that acts as a protective barrier by forming a film (biodegradable film) and also contains N (2%), CaO (4%), Zn (2.9%), SiO₂ (27%) and fulvic acids (14%).

Data were submitted to analysis of variance (ANOVA). Statistical analyses were performed using software R (R CORE TEAM, 2018) at each evaluation date using combined data of both experiments simultaneously carried out, and the averages were compared by Tukey's test at p < 0.05.

There was a significant effect of the treatments on the net photosynthesis of mango trees with different effects depending on the time elapsed after the treatments were applied (Figure 2). A significant increase on net photosynthesis was registered for all treatments at 3 DAT, except for the control, which remained practically stable (Figure 2A). After that, there was a decrease in all treatments, probably due to precipitation which occurred in the experiment area. After that, it is observed, especially for T6 maintenance and even an increase in photosynthetic rate over time, even after the rain that has elapsed after the application of the treatments. It is noteworthy that the biotic and abiotic stresses in which plants are submitted during their cycle cause several changes in their physiological functioning, as in net photosynthesis, for example, which reduces with increasing stress (Hayat et al., 2012). Thus, it appears that the application of treatments, especially T6, was able to provide protection against the effects of light and thermal stress, while allowing transmissions of sufficient sunlight for the net photosynthesis of mango trees.

Figure 2

The good gas exchange results of T6 in many evaluation dates could be caused by its higher silicon composition (27% of SiO₂) in relation to the other treatments. Silicon provides the mechanical strength to plants from various biotic and abiotic stresses (Diniz et al., 2020). According to Rajput et al. (2021), Si reduces the negative impacts of different stresses in plants, and its accumulation in plant tissue surface is primarily responsible for these positive influences in plants, such as increasing antioxidant activity.

Some treatments presented a pattern of distribution for leaf transpiration (Figure 2B) similar to that found for net photosynthesis, which occurred because, when exposed to stresses, the tendency of stomata to close and consequently to reduce sweating, preventing a decrease in the water plant content (Kerbauy, 2004). On the other hand, it is observed that T3 plants promoted high transpiration and net photosynthesis levels, which can be attributed to the adequate functioning of the photosynthetic apparatus, even under abiotic stress conditions.

As can be seen in Figure 3, carbohydrate concentrations recorded on the different days after treatments (DAT) showed a difference only at 5 DAT and 46 DAT. At 4 DAT, all treatments presented a peak of leaf carbohydrate concentration, when the highest value was promoted by T3, corresponding to 197.54 μmol · g⁻¹ FM. After reaching the peak, at 5 DAT, all treatments recorded a considerable reduction for carbohydrate concentration, except for T5, which in the previous evaluation had 136.66 μmol · g⁻¹ FM, showed the smallest data decay, only −4.64 μmol · g⁻¹ FM. At 6 DAT, T5 showed the strongest reduction (lowest value recorded); in the subsequent evaluations at 7, 13, 20 and 34 DAT, there were small variations in the carbohydrate concentration of the different treatments. On the last evaluation date (46 DAT), T6 presented the highest carbohydrate concentration of leaf (102.54 μmol · g⁻¹ FM), while T2 represented the lower average (60.67 μmol · g⁻¹ FM).

Figure 3

According to Da Cunha et al. (2022), the higher carbohydrate concentration during the initial flowering stage, the period in which panicles are formed, may be due to the higher activities of hydrolytic enzymes and also the mobilisation of leaf metabolites for the panicle development. Davenport (2007) infers that those carbohydrates when accumulated in ideal amounts in leaves can provide the necessary energy for reproductive development, especially for panicle formation. Thus, as can be seen in Figure 3, the carbohydrate concentration peak in the early stages (0–5 DAT) indicates that the treatments enable the plants to obtain more uniform and vigorous flowering. In addition, carbohydrate metabolism provides energy to ATP synthesis, in addition to reducing agents and intermediate compounds that assist in NO₃₋ assimilation (Phavaphutanon and Krisanapook, 2000) which justifies the reduction of its contents during the initial panicle development (5–13 DAT), as it constitutes a strong drain.

For leaf starch contents (Figure 4A), unlike what happened for the total soluble carbohydrate concentration, the treatments peaked on different evaluation dates, i.e., the peak for T1, T2, T3 and T4 was recorded at 3 DAT, while T5 and T6 showed a low increase on that date when compared with the control. The evaluation at 4 DAT was the only one in which the treatments were affected by treatments, where T6 was superior to the other treatments, with an increase of 323.14% in relation to its content in the previous evaluation date; however, its value at 5 DAT returned to decrease. The variations recorded at the later dates did not promote significant changes in the comparisons between treatments. Regardless of when they reached their highest values, all treatments showed an increase in the initial phase, indicating that even after the end of induction, starch continued to be accumulated. The concomitant reduction at the beginning of panicle development was reported by Urban et al. (2004) who evaluated the starch concentration of those leaves close to the floral buds and verified a reduction of 74% at 5 days after the flowering beginning; according to Ruiz et al. (2001), the starch content during pre-flower stage is one of the most important factors for flowering and fruit development. Thus, T6, by promoting the highest starch content, enabled better conditions for flowering.

Figure 4

The results of α-amylase were not affected by treatments on the evaluated dates (Figure 4B); it is clear that for all treatments there was an increase of up to 4 DAT, with different data distributions in the subsequent evaluations; the control treatment however continued increasing to 5 DAT; the second peak for all treatments occurred at 34 DAT, followed by a sharp reduction at 46 DAT, the stage in which the second physiological fruit fall occurred.

As can be seen in Figure 5A, the activity of the CAT enzyme was significantly affected by the treatments evaluated on the different evaluation dates. When comparing the treatments specifically on the dates when there was a significant effect (4, 6, 7 and 46 DAT), it is impossible to identify different responses depending on the time of assessment. Initially, the CAT activity was significantly reduced by T2, characterising this effect as a physical barrier with a short-lasting effect, since CAT remains relatively high in this treatment and presents activity quite high at 46 days DAT. At the same time, the T6 treatment, in addition to providing a significant increase in CAT activity 4 days after application, keeps it among the lowest values on the other dates, even with the lowest activity at 46 DAT. In general, when comparing the values of CAT (Figure 5A) with net photosynthesis (Figure 2A), especially for T1 and T6, an inversely proportional correction is observed, that is, on the dates when the enzymatic activity of CAT was high net photosynthesis was lower, which can be characterized as indicative of abiotic stress.

Figure 5

It is pertinent to infer that CAT is the most active enzyme produced by nature and converts H₂O₂ into H₂O and O₂. In plants, catalases are present in various isoforms and are the main H₂O₂ detoxification enzymes able directly dismute H₂O₂ or oxidize substrates, such as methanol, ethanol, formaldehyde and formic acid (Nunes Júnior et al., 2017). Thus, higher plant CAT activity values indicate higher H₂O₂ concentration and, consequently, greater plant stress.

CAT and APX are very important enzymes among the H₂O₂ detoxification components (Bhatt and Tripathi, 2011). The action of CAT and peroxidases highlights the basic difference between the two main metabolic pathways of cell H₂O₂. The H₂O₂ removal by peroxidases requires a small reducing molecule (or proteins such as cytochrome and/or thioredoxin) to act as a regeneration cofactor and does not lead to the evolution of O₂ because water is the product of the reaction (Mhamdi et al., 2012).

The APX activity was significantly affected by the application of treatments only on two evaluation dates (3 and 5 days after the application of treatments), as shown in Figure 5B. When comparing the CAT and APX results in Figure 5, it is possible to infer that CAT presented results contrary to those presented for APX, especially T1 and T6. According to Herzog and Fahimi (1976), the activity conditions for these enzymes are different; therefore, in a medium that is conducive to the activity of one of the enzymes, it is not conducive to another, which explains the opposition of the results presented. This is because while APX has a high affinity with H₂O₂, with a Michaelis–Menten (KM) constant in the order of μM, allowing the elimination of H₂O₂ even at low concentrations, CAT has a high KM for H₂O₂; thus, it only acts when this molecule is found in high concentrations. This may explain, for example, the fact that APX activity (Figure 5A) was higher than CAT activity (Figure 5B).

When compared with the results of Cunha et al. (2022), also in a study with mango, the averages contained in Figure 5 are much lower since the referred author recorded values between 60 and 120 (μM H₂O₂ · min⁻¹ · g⁻¹ FM). On the other hand, it should be noted that the study by Cunha et al. (2022) was carried out during the “branch maturation” phase, when characteristically there is a reduction in water depth in the mango crop. On the other hand, the lowest data quoted in Figure 5A are compatible to results of Silva et al. (2020) who evaluated the use of a plant biostimulant containing yeast extract and amino acids to alleviate abiotic stress in mango cv. Tommy Atkins, grown in semiarid environment.

In relation to the visual aspects of treatment effects on mango leaves, as can be seen in Figure 6, the sunblock treatments promoted different visual effects on leaf surface coverage, highlighting the dispersion of particles in T4, the lack of particles in T2 and the uniformity of coverage in T3, T5 and T6, composed only of protective films, without adding carbonates. It is important to note that even with the leaf coverage, the plant photosynthetic activity was not affected, as well as the silicon contained in T5 promotes a different leaf coverage pattern in relation to the other treatments, but that did not reflect in additional protection to the leaf tissue. In the field, there was no visual symptoms of plant oxidative stress (data not shown) to any treatment, instead of the differences shown in Figure 6.

Figure 6

Fruit yield (t · ha⁻¹) of mango was affected by sunblock treatments (Figure 7). Although the lack of significant difference among T1, T2, T3, T4 and T6, the fruit yield promoted by T6 (42.6 t · ha⁻¹) is 4.2 t · ha⁻¹ higher than that of T1 (control treatment), while that of T5 had the worst performance, even lower than the control treatment.

Figure 7

Differentially, T6 has the highest dose of a protective film (20 mL · L⁻¹), four times higher than that applied in T3, T4 and T5 (5 mL · L⁻¹); given the results, it can be seen that the addition of the protective film containing Si (T5) was not beneficial for fruit yield. The fruit yield of T6 is compatible to the best results of Cavalcante et al. (2020) who studied the metconazole on inhibition of gibberellin biosynthesis and flowering management in “Palmer” mango; but the fruit yield of T6 is lower than that of the results of Silva et al. (2021) who evaluated the PBZ interaction with fulvic acids and free amino acids for mango, but it is important to detach that this last manuscript was performed with “Keitt” mango cultivar, which traditionally is a more productive cultivar in comparison with “Palmer”.