3.1. Physiological Signs
An important physiological mechanism of drought resistance is osmotic adaptation. Swelling is the most important period in the plant development cycle from sowing to germination and a condition for seed germination. It begins when the seeds reach a moisture content above the critical value, the physical state of the grains changes, and conditions are good for the start of biochemical metabolism and the development of biological processes [
25]. The further vegetative and productive development of plants depends on the swelling conditions. This method has been tested on many agricultural crops and reflects the adaptive capacity of the plant organism to soil drought, and it actualizes through the processes of growth and development [
26].
Considering the intensity of seed swelling throughout the experiment, it is worth noting that in the initial and final periods of the experiment, a high intensity was noted, while in 4–6 h there was a significant slowdown in the increase in the mass of caryopses. A similar dynamic of the swelling process was noted for other agricultural crops, including sorghum [
27].
The highest water absorption by the seeds of varieties and lines of grain sorghum on average over the test period is mainly characteristic of the control variant (distilled water): from 51.9 to 61.7% (2021) and from 71.3 to 83.2% (2022). Seed swelling in solutions of sucrose and potassium nitrate was lower: in sucrose 42.2–52.1% and 63.1–75.7%, accordingly; in potassium nitrate 35.3–44.9% and 58.0–73.5%, accordingly. (
Table 3). The research results indicate that climatic conditions during the period of seed formation affect the degree of their swelling. On average, over 2 years of research, the greatest swelling of seeds during the experiment period was found in varieties RSK Korall and RSK Kakholong, the smallest in the varieties Magistr and line L-65/14.
The study of the dynamics of swelling of the seeds of the samples revealed some features. Thus, in the Magistr variety and line L-65/14, during the first 24 h, more intense seed swelling occurred in a sucrose solution, and only during the second day of the experiment did it decrease compared to the process occurring in distilled water (
Figure 1a,d). It is worth mentioning that in line L-65/14, a higher degree of seed swelling was found in the experiment with potassium nitrate in the first 1–6 h of the experiment, however, the intensity decreased in the subsequent hours and increased in sucrose. On average, over 48 h, the swelling in the experiment with potassium nitrate (55.2–56.5%) and sucrose (56.7–58.9%) in these samples was at the level of the control variant (61.6–63.7%), which indicates their drought resistance in the initial period of plant development.
Considering the intensity of the swelling of the samples separately by years of research, it should be noted that in the conditions of 2022, the indicators of the RSK Kakholong variety in the experiment with potassium nitrate did not significantly differ from the indicators in the control variant—72.1 and 78.4%, accordingly; in line L-50/14, the values of seed swelling in the experiment with sucrose were at the control level—75.2 and 81.4%.
Researchers noted that sorghum is sensitive to drought and high air temperatures before and after flowering [
12,
15,
16]. Prolonged drought during the flowering period affects the productivity of sorghum; in the period after flowering, the graininess of the inflorescence and the frailty of the seeds, which also lead to a loss of yield [
17,
28]. Despite this, sorghum tolerates drought more easily than other agricultural crops due to the peculiarities of osmotic adaptation and stomatal regulation [
16]. Drought-tolerant genotypes and varieties have been reported to maintain high relative water content even under arid stress [
29].
In addition, drought resistance was determined by the parameters of the water regime of the leaves and in the critical-for-sorghum period—flowering. In samples, the values of total tissue water content varied in the range of 74.20–78.85%, with water deficiency—14.05–18.49%, water-holding capacity—83.18–86.49%, and average moisture loss per 1 h per day—2.86–3.09% (
Table 4).
Significant changes in the values of water deficit depending on the hydrothermal conditions during the given growing season were revealed. Thus, in the conditions of 2022, the water deficit in the varieties RSK Korall and RSK Kakholong is significantly lower than in the conditions of 2021: 9.09–10.49% and 17.61–19.46%, respectively. The lowest variability of the indicator for two years was in the variety Magistr (13.07–15.67%) and line L-65/14 (18.38–18.61%). The influence of hydrothermal conditions on the parameters of the water regime of leaves was previously revealed in the CMS lines of grain sorghum [
3].
The variety Magistr was distinguished by lower values of tissue hydration and moisture loss for 1 h/day: on average for 2 years, they amounted to 74.20% and 2.86%, accordingly. In line L-65/14, indicators of water deficiency, water content of leaf tissues, water-holding capacity, and average moisture loss per 1 h per day remained stable throughout the study period.
The study of moisture loss by leaves in dynamics made it possible to reveal the genotypic specificity of the samples. Thus, in the first 30–90 min of the experiment, the varieties Magistr and RSK Kakholong were characterized by the lowest intensity of moisture loss: 5.51–5.60% after 30 min; 9.86–9.94% after 60 min; and 13.51–14.44% after 90 min of wilting on average for the study period, which indicates a high ability of plants to retain moisture under stress conditions (
Table 5). Low values of moisture loss were noted in the varieties Magistr (38.55%) and L-50/14 (70.77%) after 24 h. This indicator turned out to be the highest—74.28%—in the RSK Kakholong variety.
Overall, the study of the characteristics of the water regime of the leaves made it possible to characterize the source material as drought-resistant. According to the classification of relative drought resistance, the presented samples should be classified as medium-drought-resistant only in terms of water deficit.
3.2. Morphometric Traits and Yield
The manifestation of drought (both soil and air) during the flowering period of plants negatively affects their growth and yield formation [
30]. The study of breeding traits, such as plant height, leaf area, grain weight per inflorescence, grain yield, and biomass dry matter, and many others, are widely used in diagnosing genotypes for drought resistance [
13,
15]. The literature marks that leaf surface area can contribute to resistance to water stress [
31]. During the evaluation of morphometric characteristics, attention was paid to the main indicators that are elements of plant productivity—plant height, inflorescence length, and area of the largest leaf.
An assessment of the morphometric parameters of sorghum and yield showed that in the 2021 season, under drier conditions, the value of breeding traits was lower than in the 2022 season. In line L-50/14, only the area of the largest leaf did not change significantly: in the conditions of 2021, it was 176.5 cm
2, and in 2022 it was 173.0 cm
2 (
Table 6). Line L-50/14 formed the yield of biomass and grain under the conditions of 2022 higher than in 2021: 23.25 and 15.28 t/ha, and likewise 5.24 and 3.37 t/ha, respectively. Moreover, an increase in plant height (123.0 cm) and panicle length (27.0 cm) was also noted. In terms of plant height, the most stable indicators were in the RSK Kakholong variety—115.6–117.2 cm—and significant differences in the area of the largest leaf—184.9–313.7 cm
2.
In 2022, RSC Korall had higher plant height, the largest leaf area, grain yield, and biomass.
A slight reaction to the variability of climatic conditions was in line L-65/14: the length of the inflorescence varied in the range of 19.8–21.1 cm, the biomass yield was 12.88–14.78 t/ha, and the grain yield was 3.78–3.96 t/ha. Apparently, this line turned out to be more stress-resistant.
The analysis of valuable breeding traits showed a specific reaction of grain sorghum genotypes to changes in hydrothermal conditions during the growing season of plants, which made it possible to identify the most stress-resistant samples.
3.3. Biochemical Indicators
The biochemical composition of grain, which determines its quality, forms as a result of complex metabolic processes occurring in plants under the influence of various factors (biotic and abiotic), alongside a result of the implementation of information embedded in the genotype. Apparently, high-quality grain forms only with the optimal physiological and biochemical state of plants. The results of the experiment on the content of major substances in sorghum grain of various genotypes are shown in
Figure 2.
The conducted studies showed that the level of protein in sorghum grain varied from 8.8 to 9.6%. Sorghum sample L-65/14 contained the largest amount of protein, which significantly differed from other samples within 1.3–9.0%. In terms of fat content, on the contrary, this sample had the lowest value of all (3.5%). The maximum fat index (3.8%) was in the samples of RSK Coral and L-50/14. Mineral substances in the grain of the studied samples were in the range of 1.29–1.67%. On this basis, the Magister variety stood out, in which the excess of the indicator turned out to be at the level of 22%. Fiber and starch were determined from polysaccharides in laboratory conditions. It is worth mentioning that the smallest amount of fiber was present in the Magistr and L-65/14 samples (1.2%). These samples had the lowest starch content in the grain, at 75%. Obviously, a decrease in the level of some components occurs at the expense of an increase in others. The results of the analysis of the main components of the sorghum grain of the experimental samples have confirmations in similar studies by other scientists [
32,
33,
34,
35,
36].
The proteins of agricultural crops are unequal in amino acid composition, solubility, and digestibility; therefore, the quality of crop products is assessed not only by the content but also by the usefulness of proteins based on the study of their fractional composition. Based on such studies, it is possible to obtain the amino acid profile of the protein in the grain. The study of the quality of the grain protein complex makes it possible to identify genotypes with the most valuable properties and conduct the selection in this direction. According to our data, the quantitative values of protein fractions significantly depended on the sorghum genotype (
Figure 3).
Given that the albumin fraction is the most complete protein fraction, which contains all the essential amino acids, line L-65/14 stands out from all samples. The amount of albumin in the protein of this sample exceeded the smallest value by almost two times in comparison with the RSK Kakholong variety. The globulin fraction also characterizes a significant amount of essential amino acids: in the grain protein of the studied sorghum samples, it was 8.01–9.87%. Significant differences were within 20%, and the indicators of globulins in sorghum Magistr, L-65/14, and RSK Korall did not differ statistically. It is important to note that along with a high level of complete protein in grain L-65/14, this sample has the largest number of defective protein fractions—prolamin and glutelin. In addition, the L-65/14 line was distinguished by the minimum value of the insoluble protein residue compared to RSC Coral, which has the highest value of the insoluble residue; the difference was 15.6%.
The main proteins of sorghum are prolamins and glutelins, which are characterized by low digestibility and inferiority due to the incomplete composition of essential amino acids [
37]. According to other data, sorghum protein contains quite a lot of albumins and a high amount of globulins [
38], which was also noted in our studies. At the same time, the protein digestibility of sorghum conformingly depends on many factors: the organizational structure of the grain, the number of phenolic compounds, cell wall components, and starch, which can vary depending on the genotype of the sample [
39].
Storage proteins are mainly in the endosperm of the grain. It is common knowledge that the prolamine fraction of sorghum protein consists of kafirin, which has anti-nutritional properties. Kafirins make up 48–70% of whole-grain proteins. They are rich in proline, asparagine, and glutamine and, conversely, contain very low levels of lysine [
40]. According to our data, the prolamin fraction of the protein in the experimental samples of sorghum was in the range of 5.1–11.5 g/100 g of protein. The second-largest protein fraction in the studied samples was glutelin. These values are well consistent with the results of studies by a number of researchers [
41].
Therefore, in the studied samples of sorghum, the content of complete proteins in the grain was at a high level (11.89–22.75 g/100 g of protein) and has wide variability depending on the genotype. On average, the rest of the protein fractions were allocated glutelin, then globulin, and the lowest amount of prolamins.