Growth of “Biquinho” Pepper Plants under Salt Stress in a Hydroponic System ^†

Abstract

In the absence of matric potential, the hydroponic cultivation method of employing brackish water to prepare a nutrient solution permits satisfactory growth, even of non-leafy vegetables. The present study evaluated the growth variables, stem diameter, plant height, number of leaves, and the shoot dry mass in the different phenological stages of “Biquinho” pepper in response to the electrical conductivity of the nutrient solutions (ECsol), varying from 2.70 to 7.77 dS m⁻¹ in a Nutrient Film Technique (NFT) hydroponic system. For the number of leaves, linear reductions of 3.89 and 9.29% (dS m⁻¹)⁻¹ were observed at 10 and 30 days after transplanting (DAT), respectively. At 60 DAT, plants began to preserve their leaves up to ECsol of 4.60 dS m⁻¹ and presented a linear decrease of 23.32% per unit increment above the salinity threshold. At 10 DAT, the plant height and stem diameter were not affected and only at 30 DAT were there significant differences due to ECsol, with a linear reduction of 3.98 and 5.27% (dS m⁻¹)⁻¹, respectively. While at 60 DAT, the salinity response for these variables was represented by a plateau followed by a linear decrease, with salinity thresholds of 5.18 and 5.01 dS m⁻¹, and thereafter a relative decrease of 9.31 and 11.9% per unit increase in ECsol above the threshold, respectively, the values up to the salinity threshold being 0.87 m and 15.77 mm, respectively. The shoot dry mass up to 60 DAT reduced linearly with an increase in ECsol, but the plants under moderate salinity after acclimatization (90 DAT) surpassed the control treatment, and maximum dry mass accumulation was observed at an ECsol of 5.14 dS m⁻¹. The results reveal that in hydroponic cultivation of “Biquinho” pepper under salt stress, growth is attenuated in the reproductive phase.

1. Introduction

Appreciated throughout the world since colonial times, peppers of the genus Capsicum are present in everyday life, being used fresh in the preparation of meals, the composition of salads and preserves, dry as a condiment/seasoning, and processed in the preparation of jams, sauces, and antipasti [1]. Among the cultivated species, C. chinense Jacq. is explored in several Brazilian regions [2]. The species C. chinense has high genetic variability for agronomic characteristics and capsaicin content, which is responsible for the pungency of peppers [3]. Among the genotypes, “Biquinho” is known for the length of the fruit apex, being one of the most consumed and cultivated in Brazil [3]. However, this crop can be affected by several abiotic stresses, including salinity [4].

Several studies have shown good results with the use of brackish water up to a certain level of salinity in the preparation of nutrient solution in a hydroponic system, including non-leafy species such as those of the genus Capsicum [5,6], Abelmoschus esculentus [7]. Therefore, the objective of the present study was to evaluate the growth of “Biquinho” pepper cultivated in the hydroponic system using brackish waters in the preparation of the nutrient solution.

2. Material and Methods

The experiment was conducted in a single-arch greenhouse, located at the Federal University of Recôncavo da Bahia, in the municipality of Cruz das Almas, Bahia, Brazil (12°40′19″ S, 39°06′23″ W, 220 m). The greenhouse is 7 m wide and 33 m long, with a ceiling height of 4 m, installed in the East–West direction, being protected on the sides by a black net (50% shade) and on the top by a 150-micron anti-UV plastic film. During the experiment, the minimum, maximum, and mean values of air temperature and relative humidity of the air were, respectively: 17.20, 35.70, and 23.88 °C; 66.46, 93.29, and 78.79%.

The experimental design adopted was randomized blocks, with seven levels of electrical conductivity of the nutrient solution (ECsol-2.70 (control), 3.64, 4.58, 5.28, 6.09, 6.90, and 7.77 dS m⁻¹,), and 6 replicates. The nutrient solution (NS) adopted was recommended by Sonneveld and Straver [8]. The NS was prepared using municipal-supply water (ECw = 0.34 dS m⁻¹), and ECsol greater than 2.7 dS m⁻¹ was obtained by dissolving NaCl. The hydroponic system used was the Nutrient Film Technique (NFT), with 42 experimental plots. Each plot was composed of five plants arranged in a PVC channel (0.075 m diameter and 6.0 m length). Each cultivation channel was independent, in respect of its hydraulic system, NS reservoir, and supply water tank. The spacing used was 0.83 m between plants and 0.70 m between rows. The NS circulated for 15 min at intervals of 15 min.

The plant material used in this study was pepper (Capsicum chinense Jacq.) of the varietal group “Biquinho”, without pungency, using seeds of the Horticeres, sown in previously washed cubical cells of phenolic foam (0.02 × 0.02 × 0.02 m). Transplanting to the hydroponic system occurred 43 days after sowing (DAS). At 20 days after transplanting (DAT), a 50% shade net was installed at 3.5 m height to reduce radiation and avoid thermal stress in plants. The experiment was completed at 163 DAS, i.e., 120 DAT. The replenishment of the water consumed was carried out daily with supply water (ECw = 0.34 dS m⁻¹). The NS was substituted completely in all treatments at 28, 61, and 90 DAT.

The growth of pepper plants was evaluated by stem diameter (SD), plant height (PH), and the number of leaves (NL) 10, 30, and 60 DAT. All evaluations were performed in two pre-identified plants in the central part of each experimental plot. The shoot dry mass (SHDM) was evaluated 30, 60, 90, and 120 DAT after drying at 65 °C.

The data were subjected to analysis of variance (ANOVA) using the statistical SAS Program. When the effect by the F test was significant, the factor ECsol was subjected to linear and quadratic regression analysis to obtain the most adequate model representing the data. The models of Maas and Hoffman [9] and Plateau followed by exponential decay were also used. The Solver Microsoft Excel tool was used to analyze the parameters, aiming to minimize the sum of square deviations.

3. Results

For the NL, linear reductions of 3.89 and 9.29% (dS m⁻¹)⁻¹ were observed at 10 and 30 DAT, respectively (Figure 1A,B). At 60 DAT, plants began to preserve their leaves up to ECsol of 4.60 dS m⁻¹, with a mean number of 1034.27 leaves per plant and a linear decrease of 23.32% per unit increment of ECsol above the salinity threshold (Figure 1C).

At 10 DAT, mean PH and stem diameter were, respectively, 0.195 m and 7.85 mm for all treatments, and only at 30 DAT were there significant differences due to ECsol, with a linear reduction of 3.98 and 5.27% (dS m⁻¹)⁻¹, respectively (Figure 2A,C). At 60 DAT, the variables PH and SD showed a plateau followed by a linear decrease, with salinity thresholds of 5.18 and 5.01 dS m⁻¹, and a relative decrease of 9.31 and 11.9% per unit increase in ECsol above the threshold, respectively. The values of these variables up to the salinity threshold were 0.87 m and 15.77 mm, respectively (Figure 2B,D).

The shoot dry mass (SHDM) accumulation of “Biquinho” pepper at 30 and 60 DAT decreased linearly by 10.00 and 10.71% per unit increase in ECsol (Figure 2E,F), respectively. Subsequently, at 90 and 120 DAT, the effect of ECsol on SHDM was represented by a quadratic equation, with maximum values no longer observed in the to control treatment (ECsol = 2.70 dS m⁻¹), but to intermediate salinity treatments: ECsol of 4.42 and 5.02 dS m⁻¹, with SHDM estimated at 355.12 and 498.67 g plant⁻¹, respectively (Figure 2G,H). Based on the equation of Figure 2H, SHDM values of the same magnitude (316.72 and 315.76 g plant⁻¹) were verified between the control treatment (ECsol = 2.70 dS m⁻¹) and ECsol of 7.34 dS m⁻¹ at 120 DAT.

4. Discussion

The increase in salts in the NS reduced the growth variables NL, PH, SD, and SHDM. In the literature, there are several studies with different crops, including bell pepper, in the hydroponic system under salt stress, with a reduction in these growth variables [5,6,10] due to the reduction in water absorption, accumulation of toxic ions, and physiological effects, such as reduction in photosynthesis and stomatal conductance [6,10].

For basil crops under similar treatments and experimental conditions, the PH was not considered a good indicator of salinity effects compared to dry mass accumulation [11]. For basil and pepper, both shrub plants, the reduction in height tends to be a little intense; after a certain time, the plants start to mobilize more energy for the growth of the lateral branches, to the detriment of vertical growth, which tends to result in plants with heights that vary little, even under different levels of ECsol. These results corroborate results observed by Furtado et al. [5] in hydroponic bell pepper plants (Capsicum annuum L. “All Big”) in which the absolute growth rate decreased from 0.096 to 0.03346 cm day⁻¹ with a unit increase in salinity in the periods 60–75 and 75–90 DAT, respectively. According to Horticeres Sementes [12], the cultivar “Biquinho” used in the present study produces very uniform compact plants with a mean height of 0.60 m. Therefore, the plants of the present study grown under hydroponic conditions, even with high-salinity NS (ECsol = 7.7 dS m⁻¹), reached this mean standard of height (Figure 2B).

5. Conclusions

“Biquinho” pepper plants subjected to salinity soon after transplanting had, as consequences of the osmotic effect, restricted development of areal parts and the number of leaves, especially in the first weeks. The results reveal that in hydroponic cultivation of “Biquinho” pepper under salt stress, growth is attenuated in the reproductive phase.