Initial Development and Tolerance of Bell Pepper ( Capsicum annuum ) Cultivars under Salt Stress

The objective was to evaluate growth and tolerance of bell pepper cultivars under salt stress in the seedling stage. The experiment was carried out in protected environment (greenhouse) at the Center of Sciences and Agrifood Technology (CCTA) of the Federal University of Campina Grande (UFCG), located in the municipality of Pombal, Paraíba, from August to September 2014. Five bell pepper cultivars (C1 Dulce All Big; C2 All Big; C3 Italiano Amarelo Gigante; C4 Rubi Gigante; C5 Casca Dura Ikeda) were evaluated at five levels of irrigation water salinity (0.6 (control); 1.2; 1.8, 2.4 and 3.0 dS m), arranged in 5 × 5 factorial scheme, in a completely randomized design, with four replicates. Bell pepper cultivars were grown on trays for 20 days after sowing, period in which they were evaluated for emergence, growth, phytomass accumulation and tolerance index. The increase in irrigation water salinity reduced bell pepper emergence, growth and dry matter accumulation, and the cultivars C2 All Big and C4 Rubi Gigante were the most tolerant to salinity in the initial growth stage. Tolerance to salinity occurred in the following order: C2 All Big > C4 Rubi Gigante > C1 Dulce All Big = C5 Casca Dura Ikeda > C3 Italiano Amarelo Gigante.


Introduction
Bell pepper (Capsicum annuum L.) belongs to the Solanaceae family and is widely spread from the economic point of view, especially in Brazil, with an annually planted area of approximately 13 thousand hectares and production close to 290 thousand tons of fruits, which puts this vegetable among the five leguminous species with largest cultivated area in Brazil and worldwide (Moreira et al., 2008;Marouelli & Silva, 2012).The sources in bell pepper cultivation to solve the scarcity of water resources (Hespanhol, 2008).Savvas et al. (2007) also highlights that the availability of adequate water resources is a problem that occurs in agriculture at a global level, forcing many farmers to use water with relatively high saline concentration to irrigate crops.Hence, the use of saline waters in irrigation for plant production is a challenge that has been successfully overcome worldwide, due to the utilization of tolerant species and adoption of adequate practices of crop, soil and irrigation water managements (Munns & Tester, 2008).
Nevertheless, the excess of salts may compromise root system development, growth and production of the crop.High saline concentrations in the soil increase the osmotic pressure of the medium, harming root development and the absorption of water and nutrients by plants (Sá et al., 2013;Mesquita et al., 2015).Evaluating the physiology of bell pepper plants under saline conditions, Sá et al. (2016) observed that the increase in salinity directly affects their photosynthetic efficiency, due to damages of stomatal and non-stomatal origins.Leonardo et al. (2007) demonstrated that there is an inverse correlation between soil EC (electrical conductivity) and mean fruit weight, because an increment of 1 dS m -1 in the soil contributes, on average, to a decrement of 15% in the mean weight of each bell pepper fruit.
However, studies on C. annuum L. cultivars regarding its tolerance to salinity are either scarce or nonexistent, thus the present study will contribute to the expansion of the crop in semi-arid areas with qualitative scarcity of water resources.Hence, this study aimed to evaluate growth and tolerance of bell pepper cultivars under salt stress in the seedling stage.

Material and Methods
The experiment was carried out from August to September 2014 in protected environment (greenhouse) at the Center of Sciences and Agrifood Technology (CCTA) of the Federal University of Campina Grande (UFCG), located in the municipality of Pombal, Paraíba, at geographic coordinates 6º47′20″ S and 37º48′01″ W, at altitude of 194 m.
Five bell pepper cultivars (C 1 -Dulce All Big; C 2 -All Big; C 3 -Italiano Amarelo Gigante; C 4 -Rubi Gigante; C 5 -Casca Dura Ikeda) were evaluated at five levels of irrigation water salinity (S1 -0.6 (control); S2 -1.2;S3 -1.8; S4 -2.4 and S5 -3.0 dS m -1 ), of which two are below the crop salinity threshold of 1.5 dS m -1 (Maas & Hoffman, 1977) and three above it, in order to identify genotypes with higher tolerance or sensitivity to water salinity.Treatments were arranged in 5 × 5 factorial scheme, in a completely randomized design, with four replicates.
Bell pepper plants were cultivated on trays of 30 cells with capacity for 0.1 dm 3 of substrate, until 20 days after sowing (DAS).The substrate for seedling production was composed of soil and commercial substrate, mixed at 1:1 proportion, and its chemical characterization is presented in Table 1 (Santos et al., 2013).For sowing, 30 cells were allocated to each treatment, so that each cell received one seed, totaling 30 seeds per treatment.After emergence, seedlings were thinned to leave only the most vigorous plant per cell.The seeds of both cultivars were purchased in a commercial establishment, with 99% purity and 95% germination.
Table 1.Chemical characteristics of the components of the substrate used for bell pepper cultivation Note.SB = sum of bases; EC = electrical conductivity; T = total cation exchange capacity; OM = organic matter; A = Soil; B= commercial substrate.
Irrigations were daily applied to maintain soil moisture close to the maximum retention capacity, based on the drainage lysimetry method; applied volume plus a leaching fraction (LF) of 20%.The volume applied per container (V a ) was obtained by the difference between the previous volume (V prev ) minus the mean drainage (d), divided by the number of containers (n), as shown in Equation 1: (1) Irrigation water with various salinity levels was prepared considering the relationship between EC w and concentration of salts (10•meq L -1 = 1 dSm -1 of EC w ) according to Rhoades et al. (1992), valid for EC w interval from 0.1 to 5.0 dS m -1 , which encompasses the studied levels.Public-supply water available in the area (EC w = 0.3 dS m -1 ) was used, mixed with salts (NaCl) as necessary (Table 2).Note.EC = electrical conductivity; SAR = Sodium adsorption ratio.
After prepared, the saline solutions were stored in 30-L plastic containers, one for each EC w level, properly protected to avoid evaporation, entry of rainwater and contamination by material that could compromise quality.
To prepare the solutions with the respective EC levels, salts (NaCl) were weighed according to the treatment and water was added until reaching the desired EC level.EC values were verified using a portable conductivity meter, with conductivity adjusted to temperature of 25 ºC.
During the experiment, bell pepper emergence was monitored by counting the number of emerged seedlings, i.e., with cotyledons above the soil level, daily performed without discarding the plants, thus obtaining a cumulative value.Hence, the number of emerged plants referring to each count was obtained by subtracting the reading of the previous day from the current reading.Thus, the number of emerged seedlings relative to each reading, obtained in greenhouse, was used to calculate the mean time of emergence (MTE) through the formula described by Araújo et al. (2016): Where, MTE = mean time of emergence (dias); G = number of emerged seedlings observed in each count; N = number of days from sowing to each count.
After emergence stabilization, emergence percentage (EP) (%) was determined by the relationship between the number of emerged seedlings and number of seeds sown.
To monitor the morphological aspect of the crop, plant growth was evaluated at 20 DAS: number of leaves (NL), based on the count of mature leaves; plant height (PH), measured with a ruler; and stem diameter (SD), measured using a caliper.After growth analysis, plants were collected, separated into shoots and roots and dried in a forced-air oven at 65 ºC, until constant weight.The dried material was then weighed on analytical scale to determine shoot dry matter (SDM) (g) and root dry matter (RDM) (g).These data were used to determine total dry matter (TDM) (g), through the sum of SDM and RDM, and root/shoot ratio (R/S), through the division RDM/SDM.
Total dry matter production data were used to calculate the percentages partitioned among vegetative organs and the salinity tolerance index, by comparing the data of the saline treatments with those of the control (EC w = 0.6 dS.m -1 ), according to the methodology of Fageria et al. (2010), based on four levels of classification: T (tolerant; 0-20%), MT (moderately tolerant; 21-40%), MS (moderately sensitive; 41-60%) and S (Sensitive; > 60%), as demonstrated in Equation 3: (3) The calculations of these indices used the total dry matter production of the cultivars as the main parameter to determine their tolerance to salt stress.
The obtained data were subjected to analysis of variance by F test and, in case of significance, regression analyses were applied for the factor irrigation water salinity levels, while Tukey test was applied for the factor cultivars, both at 0.05 probability level, using the statistical software SISVAR® (Ferreira, 2011).The data were subjected to standardization to show null mean value (X = 0) and variance equal to one (σ = 1).Subsequently, cluster analysis was performed through the hierarchical method, Ward's minimum variance, using the Euclidean Distance as the measure of dissimilarity (Hair et al., 2009).

Table 2 .
Chemical analysis of the public-supply water used to prepare the solutions