ABSTRACT

Soil salinity harmfully effect bell pepper production being salt sensitive crop. The aim of this study was to compare capsicum (bell-pepper) cultivars for their ability to tolerate salt stress. Electrical conductivity of different levels i.e., control (no sodium chloride), 1.5, 3.0, 4.5, and 6.0 dS m^-1, was developed by using NaCl, after 30 days of seedling emergence of six cultivars (Yolo Wonder, California Wonder, Cop-amber Hybrid, F1 Pangs No. 206, Sweet Pepper F1, Kaka F1). These cultivars were evaluated based on morphological (shoot and root length, plant fresh and dry biomass), physiological (chlorophyll content) and biochemical attributes (nitrogen concentration, and protein contents). Minimum chlorophyll contents (SPAD units) in Kaka F1 (16.77 SPAD units) at 1.5 dSm^-1. However, maximum value of chlorophyll contents was observed in Sweet Pepper F1 (49.23 SPAD units) at control followed by “Yolo Wonder” (41.53 SPAD units) at control. Maximum dry biomass was found in California Wonder (0.468 g) at 1.5 dSm^-1, while minimum value of dry biomass (g) was observed in Kaka F1 (0.095 g) at 4.5 dSm^-1. Six cultivars of capsicum were categorized into three groups. Salt tolerant cultivars included “California Wonder” followed by “Yolo Wonder”. “Kaka F1” was proved most salt sensitive and all others were moderately salt tolerant.

INTRODUCTION

Bell-pepper, Capsicum annuum, also known as” Sweet Pepper”, locally known as “Shimla ki Mirch” is probably of American origin. Bell pepper can be grown best in sandy loam and silt loam soils. It is much sensitive to salt stress (Hussein et al., 2012). Total world production of world bell pepper is 38.01MMT. China (18.98 MMT), Mexico (3.24 MMT), Turkey (2.63 MMT), Indonesia (2.59 MMT), Spain (1.40 MMT), and Egypt 764.29 KMT are its leading producing countries. However, Pakistan has very less per acre production of bell-pepper (Statista, 2019).

Salinity in major problem which is growing day by day. It is of immense important because of its significant impact on yield and quality. Pakistan is an agriculture-based country so this issue must be addressed. Various types of salts are present in soils. In one of previous study plant growth and development was significantly decreased by salinity in bell pepper (Rubio et al., 2011). Various biochemical and physiological process are affected by salinity which lead to damaging carbohydrate manufacturing and its consumption (Arif et al., 2020). This is due to reactive oxygen species production and their detrimental effects on metabolism (Kamran et al., 2020). Sodium accumulation alters photosynthetic rate which can lead to lessening plant biomass production (Azuma et al., 2010).

Sodium content in soil significantly reduce yield in various crops. Due to increase in salinity, millions of acres’ agricultural productive lands are becoming baron each year (Velmurugan et al., 2020). Marketable fruit yield and quality was highly reduced by the use of saline water in capsicum (Semiz et al., 2014). This might be due to the fact that water use efficiency was affected (Patil et al., 2014). Plant adopt mechanism of defensive against these detrimental changes by developing various enzymatic and non-enzymatic antioxidant (Ramanjulu and Sudhakar, 2001).

Most of the vegetables are drastically affected by soil salinity. So, management and different techniques are required to cope with this issue (Machado and Serralheiro, 2017; Selim et al., 2017). Screening of vegetables against salt stress is of vital important and feasible method to cope with this situation (Shaheen et al., 2013). The aim of study was to evaluate bell-pepper cultivars for their salt tolerance potential and to find cultivar suitable for cultivation in saline conditions.

MATERIAL AND METHODS

The pot study was conducted at Horticultural Research Area, University of Agriculture Faisalabad. Four salinity levels having EC i.e., 1.5, 3, 4.5, and 6 dS m^-1 NaCl) and six cultivars namely (“Yolo Wonder”, “California Wonder”, “Cop-amber Hybrid”, “F1 Pangs No. 206”, “Sweet Pepper F1”, “Kaka F1”) was arranged from reliable research stations and tested for screening and results compared with control (1.0 dS m^-1 NaCl) (without additional NaCl application). Each treatment was replicated thrice. The experiment was layout under completely randomized design (CRD). EC of the medium was noted daily. Hoagland’s solution was used as nutrient medium. Seeds were sown in plastic pots containing 7 Kg sand. Pots were kept in the open field. The number of seedlings per pot was adjusted to four and the seeds were watered according to the need of plant by observing the moisture of sand. Nutrient composition of the solution having pH of 6-6.5 kept maintained (Model Genway, 3510) USA. After every week Hoagland solution (Table 1) was applied for nourishment of seedlings.

Table 1: Composition of Hoagland nutrient solutions.

After two weeks of salt application data were collected. To evaluate the impact of different EC levels, various attributes were studied i.e., mortality (%), shoot length (cm), root length (cm), total length (cm), total fresh mass (g), total dry mass (g) were measured. Chlorophyll contents (CCI) were found out by chlorophyll meter (CCM-200plus Bio- Scientific USA).

Measurement of leaf nitrogen content

Leaf nitrogen contents and protein were measured by following procedure already reported by Ataulkarim et al. (2016). Fresh leaf samples were washed with detergent for surface disinfection then rinsed, dried, and then ground into fine powder by an electric grinder for further process. So, determination of the leaf N concentration (LNC) using the micro-Kjeldahl method and was expressed as mg g^-1 DW.

Analysis of leaf protein content

The total protein contents (%) were determined by multiplying leaf nitrogen amount by factor depending on the protein types. This value is called the ‘crude protein’ content. The leaf protein determination formula (AOAC, 2000) was as follow:

Crude protein= N x 6.25

Statistical analysis

Means were compared by Least Significant Difference (LSD) test and analysis of variance was performed by using Statistix 8.1 software package.

RESULTS

It was revealed that there was significant difference among selected bell-pepper genotypes against different selected levels of salinity. Complete mortality (100 %) was observed in “Pangs No. 206” and Kaka F1 at highest level of salt application (6 dS m^-1). At control minimum mortality (5.3 %) was found in sweet pepper F1 at par with Yolo Wonder (5.6 %) (Table 2).

Table 2: Response of different cultivars of bell-pepper for mortality percentage (%) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

Data regarding shoot length showed that there was significant difference among cultivars against salt stress. Maximum shoot length was found in Yolo Wonder (6.19 cm) at control followed by Yolo Wonder (6.08 cm) at 1.5 dSm^-1 of salt application was observed. At control minimum value regarding shoot length was observed in Kaka F1 (2.69 cm) at 4.5 dSm^-1 followed by Cop-amber hybrid (2.82 cm) at 4.5 dSm^-1 (Table 3).

Table 3: Response of different cultivars of bell-pepper for shoot length (cm) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤ 0.05 according to LSD test.

Data regarding root length showed that there was significant difference among cultivars against salt stress. Maximum root length was found in California Wonder (11.46 cm) at 6.0 dSm^-1 followed by Cope-amber F1 (9.21 cm) at 6.0 dSm^-1. While minimum (5.64 cm) root length was seen in California Wonder at 4.5 dSm^-1 of salt application at par with Kaka F1 (5.72 cm) at control (Table 4).

Table 4: Response of different cultivars of bell-pepper for root length (cm) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤ 0.05 according to LSD test.

It was observed that total plant length showed that there was significant difference among cultivars against salt stress. Maximum total plant length in California Wonder (14.42 cm) at 6.0 dSm^-1 followed by was observed in Yolo Wonder (13.28 cm) at 1.5 dSm^-1. Minimum value of total plant length was observed in Cop-amber hybrid F1 (7.87 cm) at 3.0 dSm^-1 followed by Sweet Pepper F1 (8.53 cm) 6.0 dSm^-1 (Table 5).

Table 5: Response of different cultivars of bell-pepper for plant length (cm) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

Fresh biomass (g) showed that there was significant difference among cultivars against salt stress. Maximum fresh biomass in California Wonder (1.07 g) at 3.0 dSm^-1 followed by (0.73 g) at control and by Cope-amber F1 (0.66 g) at 1.5 dSm^-1 was observed. Minimum value of fresh biomass (g) was observed in Kaka F1 (0.23 g) at 4.5 dSm^-1 followed by Sweet Pepper F1 (0.23 g) 6.0 dSm^-1 (Table 6).

Table 6: Response of different cultivars of bell-pepper for fresh biomass (g) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

Dry biomass showed that there was significant difference among cultivars against salt stress. It was seen that maximum in California Wonder (0.468 g) at 1.5 dSm^-1 followed by Pangs No. 206 (0.365 g) was observed at 4.5 dSm^-1. Minimum value of dry biomass was observed in Kaka F1 (0.095 g) at 4.5 dSm^-1 followed by Sweet Pepper F1 (0.110 g) at 6.0 dSm^-1 (Table 7).

Table 7: Response of different cultivars of bell-pepper for plant dry biomass (g) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

Number of leaves showed that there was significant difference among cultivars against salt stress. Maximum number of leaves were observed in California Wonder (5.89) at 3.0 dSm^-1 followed same cultivar (5.33) at 1.5 dSm^-1. At control minimum value of number of leaves was observed in Sweet Pepper F1 (3.89) at both 3.5 dSm^-1 and 6.0 dSm^-1 (Table 8).

Table 8: Response of different cultivars of bell-pepper for number of leaves to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤ 0.05 according to LSD test.

It was depicted that chlorophyll contents showed that there was significant difference among cultivars against salt stress. It was seen that minimum chlorophyll contents (SPAD units) in Kaka F1 (16.77 SPAD units) at 1.5 dSm^-1 followed by California Wonder (17.23 SPAD units) at the same level of salinity. At control maximum value of chlorophyll contents was observed in Sweet Pepper F1 (49.23 SPAD units) at control followed by Yolo Wonder (41.53 SPAD units) at control (Table 9).

Table 9: Response of different cultivars of bell-pepper for chlorophyll contents (SPAD units) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

Data regarding nitrogen content showed that there was significant difference among cultivars against salt stress. Maximum nitrogen content in Pangs No. 206 (0.517 mg g^-1 DW) at control followed by (0.450 mg g^-1 DW) was observed in Sweet Pepper F1 at control and at 1.5 dSm^-1 of salt application at par with Pangs No. 206 (0.450 mg g^-1 DW) at 1.5 dSm^-1 and Cop-amber hybrid F1 (0.450 mg g^-1 DW) at 3.0 and 4.5 dSm^-1. At control minimum value regarding nitrogen content was observed in Kaka F1 (2.50 mg g^-1 DW) at 1.5 dSm^-1 and 3.0 dSm^-1, similarly same value was observed in Yolo Wonder 2.50 mg g^-1 DW) and California Wonder 2.50 mg g^-1 DW) at 6.0 dSm^-1 (Table 10).

Table 10: Response of different cultivars of bell-pepper for leaf nitrogen contents (mg g^-1 DW) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤ 0.05 according to LSD test.

Data regarding leaf crude protein content showed that there was significant difference among cultivars against salt stress. Maximum leaf protein was found in Pangs No. 206 (3.23 %) at control followed by (2.81 %) in Sweet Pepper F1 at control and at 1.5 dSm^-1 of salt application at par with Pangs No. 206 (2.81 %) at 1.5 dSm^-1 and Cop-amber hybrid F1 (2.81 %) at 3.0 and 4.5 dSm^-1. At control minimum value was observed in Sweet Pepper F1 (1.56 %) at 4.5 dSm^-1 and 6.0 dSm^-1 at par with Yolo Wonder and California Wonder (1.56 cm) at 6.0 dSm^-1, similarly in Kaka F1 (1.5 %) showed same value at 1.5 dSm^-1 and 3.0 dSm^-1 (Table 11).

Table 11: Response of different cultivars of bell-pepper for leaf crude protein contents (%) to different levels of salinity.

Figures sharing same letters do not differ from each other at P ≤0.05 according to LSD test.

DISCUSSION

Gradual increase in salt stress level in the growing medium enhanced damaging impacts on all bell pepper cultivars. Various cultivars under study showed varied response to different salinity levels. Some cultivars tolerated to highest salinity levels, but a few couldn’t as high salt content in soil hinder uptake of nutrients in plants (Tanaka et al., 2018). Ai et al. (2021) reported about four genotypes of chili behaved differently against different salinity levels produced by NaCl or CaCl₂. Some cultivars adopt defensive mechanisms by improving enzymatic and non-enzymatic antioxidants during salt stress (Bano et al., 2014). Various types of cultivars of vegetables could be screened out based on their physiological and biochemical characteristics (Tahira et al., 2014). Present study depicted that some cultivars completely died at its highest level of salt stress. This fact was previously observed when salinity induced harmful changes which lead to complete death of plants with increase in its level and with the passage of time (Meng at el., 2011) .

Bell pepper plants were severely affected in this study due to hindrance in nutrient uptake and low chlorophyll pigment production under high salt contents in soil. Such changes were previously reported by Ashraf and Harris (2004). Such results were seen when shoot length was deceased due to salt stress because of accumulation and sodium ions (Janagard et al., 2008). Hamayun et al. (2010) found out the reason of low height was due to gibberellic acid production.

Taïbi et al. (2016) confirmed the correlation of chlorophyll content with height. A similar effect was observed when “Cop-amber Hybrid” F₁ had gained more height and chlorophyll content (SPAD value) and hence photosynthetic apparatus is significantly impaired (Pessarakli et al., 2004; Ashraf et al., 2005; Brown et al., 2006). Root length was also prominently reduced by increasing salt stress doses; this was initially reported by Maiti et al. (2010). Plant biomass decreased due to injury of sodium and chloride ions (Amirjani, 2010). Potassium has very important role in stomatal opening and closing but due to high Na⁺ replaces or disturbs its proper functioning ultimately transpiration effects and nutrient absorption hinders (Pakniyat and Armion, 2007; Yasar et al., 2008). Protein was higher in amount in salt tolerant cultivars as proteins play an imminent role in plant stress response since, they are directly involved in the acquisition of an enhanced stress tolerance (Kosová et al., 2011).

CONCLUSION

It was concluded that plant damage was increased with enhancement of salt concentration in nutrient medium. Outcomes showed that some cultivars of bell-pepper were more tolerant to salt stress than others. Salt tolerant cultivars included “California” Wonder followed by “Yolo Wonder”. “Kaka F1” was salt sensitive and “Cop-amber Hybrid”, “F1 Pangs No. 206” along with “Sweet Pepper F1”, were moderately salt tolerant.

DECLARATION OF COMPETING INTERESTS

All authors declare no conflict of interest for this publication.

AUTHOR CONTRIBUTION STATEMENT

Muhammad Zohaib Nasir: Conceptualized idea and conducted this research work and collected data. Mujahid Ali: Conceptualized idea and designed this research work and collected data. Saqib Ayyub and Saqib Ayyub: Assisted in methodology, validation, editing, reviewing. Rashid Hussain: Assisted in laboratory analysis; Zaid Mustafa: Editing, reviewing, software analysis and improving of the manuscript. Hafiz Muhammad Tayyab Khan: Assisted in methodology, validation, editing and reviewing.

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