Selection of Salt-Stress-Tolerant Genotypes during Germination, Growth, and Development in Durum Wheat (Triticum turgidum subsp., durum Desf.)  ^†

Abstract

Salinity is a serious threat to agriculture, causing the inhibition of and alterations in germination and plant growth and development. Durum wheat is highly sensitive to salinity. Therefore, the main objective of this study was to establish a screening method of wheat genotypes, under saline conditions, at the germination and plant growth stages. Our results show a very significant effect of salt stress on the different parameters evaluated in durum wheat, for all treated genotypes. The tolerance screening test during growth and development was more effective than the germination test. The chlorophyll content allowed distinguishing tolerant from sensitive genotypes.

1. Introduction

Salinity is a major abiotic stress that affects and inhibits soil fertility and cereal growth and therefore reduces crop yield. The salt present in water irrigation or in the soil causes enormous damages that are manifested at several levels, in morphological aspects, and in physiological states [1]. Indeed, it causes a delay in germination and reduces the number of seedlings as well as the duration of their emergence, vegetative growth, and plant density, in addition to delaying the stages of growth and development (flowering, maturity, etc.). The effects of salinity vary according to the species, the intensity of the stress, and the duration of application [2]. Wheat is one of the most important cereal crops in the world, and it tends to be called the king of the grains. Wheat occupies a very important place in the market, whether at the international level, where the production reached almost 769.6 million tons in 2021 [3], or at the national level, with a total wheat production of 7.54 million tons (5.06 Mt of common wheat and 2.48 Mt of durum wheat) in 2020 [4]. However, wheat consumption in Morocco is still dependent on importation. Wheat crops are considered to be highly sensitive to salinity. It negatively affects the growth and development of this crop, leading to diminished grain yield and quality [5]. To improve breeding techniques, there is a need for an efficient screening method to select the correct and desired salt-tolerant varieties. Therefore, the main objective of this study was to compare the behavior of different durum wheat genotypes, under saline conditions, at both the germination and growth stages following [6] protocol. To achieve this objective, we (1) identified the median lethal concentration (LC50) of salt that can be effectively used to screen durum wheat plants during seed germination; (2) identified durum wheat genotypes that are tolerant to salinity during germination, growth, and development; and (3) evaluated the different biochemical and morphological changes in plants subjected to different salinity concentrations.

2. Materiel and Methods

2.1. Determination of the Median Lethal Salt Concentration (LC50) in Durum Wheat

To select an appropriate salt concentration for screening the tolerant genotypes of durum wheat during germination, we first investigated the effects of salt concentrations on germination rates. Nine saline solutions were created using 10 mL Hoagland solution in Petri dishes containing 0, 15, 30, 45, 60, 75, 90, or 105 mmol⋅L⁻¹ of NaCl. A germination percentage was calculated using the following formula: GP= (x/y) × 100, where x = number of seeds germinated after 7 days, and y = number of seeds per Petri dish (50).

2.2. Screen for Tolerance at the Growth and Development Stages

To assess salt tolerance in the growth and development stages, a total of 70 durum wheat genotypes were used for this experiment. The 70 genotypes including Simeto as a salt-tolerant genotype were planted separately in cone-tainers (Figure 1). When the third leaf was fully developed, 40 L of salt solution was added to the bottom of the container for the plants to take up. A total of 4 treatments with 3 replications were created including 3 treatments with 45, 90, and 135 mM of salt and 1 control without any salt. The plant height, number of dried leaves, and chlorophyll content (Chla and Chlb) were noted at the heading stage to determine the tolerant genotypes. The data from 70 durum wheat genotypes were analyzed using a GGE biplot.

3. Results

3.1. Determination of the Median Lethal Salt Concentration (LC50)

The number of germinated seeds decreased as the salinity concentrations increased (

Table 1. GP for four durum wheat genotypes under different concentrations of salt.

Concentration (mmol·L−1)	Karim	Kyperonda	Icamor	Nax1 DW	Mean
0	64.667	70.667	88.667	71.333	73.833
15	56.667	66.000	69.333	42.000	58.500
30	58.667	43.333	64.000	43.333	52.333
45	57.333	46.667	59.333	47.333	52.667
60	36.000	26.000	57.333	40.000	39.833
75	23.333	22.000	47.333	25.333	29.500
90	5.333	8.667	41.333	18.667	18.500
105	3.333	0.000	38.667	11.333	13.333
LSD	24.923	25.651	16.353	18.858	20.841

). For this reason, we used the concentrations at which 50% of seeds fail to germinate (LC50) to screen the tolerant genotype at the germination stage. The average LC50 value for the four genotypes was 45 mM (Figure 2). These results indicate that salt solutions used for screening germination tolerance should have concentrations higher than 45 mM. Furthermore, through this experiment, the screening of tolerant and sensitive genotypes can be determined at the germination stage. As shown in Figure 3, the genotype Icamor showed tolerance to salt at the germination stage. The germination percentage (GP) was still high even at the highest salt concentration, with 0.4 at 105 mM of salt.

3.2. Screen for Tolerance at the Growth and Development Stages

The plants behaved differently under different salt concentrations. When the salt concentration was high, the leaf area was reduced, the number of yellow and dry leaves increased, the plants shortened, and their density decreased (Figure 4a). The concentration of 135 mM was the most discriminative salinity treatment. Therefore, the analysis was conducted using the 135 mM data. Figure 4b shows the “Rank genotypes” view of the biplot. The genotypes are ranked along the line with a single arrow. The tolerant genotypes that had a high chlorophyll content value under 135 mM salt stress are placed near “Chlorophyll”, indicating that genotypes 46, 113, 103, 33, and 32 exhibited high chlorophyll content values with the lowest number of dried leaves. Similarly, for the dried leaves, genotype 158 registered the highest number of dried leaves with the lowest chlorophyll content value.

The multiple comparison test (Duncan test) of the chlorophyll rate averages obtained by the plants developed under the 135 mM salt concentration revealed that among the 70 genotypes, 4 genotypes (46: Bidi-17; 103: Bidi-17-xSyrica; 113: Simeto; 32: Ajili) had a significantly high average chlorophyll content, and it seemed that these genotypes showed a tolerance to salinity (

Table 2. The highest chlorophyll content (Chla + b) at a 135 mM salt concentration using the Duncan test.

Genotype	Chlorophyll Content
46	13.659 a
103	12.021 ba
113	11.325 ba
32	10.748 bac

Different superscript letters in the chlorophyll content column indicate a statistically significant difference (p < 0.05) with n = 3.

). Furthermore, our data reveal that the chlorophyll content in tolerant genotypes was higher than that in sensitive genotypes. The genotype Simeto was already reported by [7] as a tolerant genotype, which supports our results. This suggests that the chlorophyll content could be considered as an effective physiological trait for salt tolerance and could be used effectively in the breeding of tolerant durum wheat genotypes. However, the same phenomenon was not observed for the number of dried leaves and plant height.

4. Conclusions

Our results show a very significant effect of salt stress on the different parameters evaluated in durum wheat, for all treated genotypes. Indeed, salinity caused a decrease in germination, plant height, and chlorophyll content, and an increase in the number of dry leaves. The tolerance screening test during growth and development was more effective than the germination test. The chlorophyll content allowed distinguishing tolerant from sensitive genotypes; this result could be useful for salt tolerance screening tests. The results of this study would be a major asset in the selection and breeding improvement of durum wheat varieties. As future perspectives, it is important to apply this study to several levels of the wheat life cycle of stressed plants in the field, to use higher concentrations of salt, and to test several large populations of genotypes.