1. Introduction
Rice is ranked the second important cereal crop after wheat and it is the most important food crop in the world and in Egypt [
1]. Rice production in the world has exceeded 513 million metric tons, with 166.47 million hectares under cultivation [
2]. It is grown in irrigated lowland in flooded conditions with a constant water depth of 5 to 10 cm [
3]. Lowland rice is primarily directly seeded or transplanted on puddled soils by ploughing under saturated water conditions, then by harrowing and levelling management. In many parts of the world, the supply of irrigation water for agriculture particularly in rice production is challenged, not only by a global lack of water resources [
4], but also by rising urban and industrial demand [
5]. Rice farming consumes a lot more water than other crops around the world and it is estimated that irrigated rice uses roughly 40% of the global water utilized for irrigation [
6]. To ensure food security and develop an acceptable yield in water shortage conditions in Egypt, drought-tolerant and water-saving rice varieties are becoming increasingly important. [
1].
With the anticipation of future climate change, it is necessarily the time for exploring the possibilities of drought-tolerant crops for all crop species [
7]. Drought stress is a major problem that limits the adoption of high-yielding rice genotypes in drought-prone rainfed rice environments [
8], where moderate drought stress can be broadly characterized by a 31–64% loss in rice grain yield compared to normal irrigation conditions [
9,
10]. Hall [
11] defines drought tolerance as the relative yield of a genotype compared to others subjected to the same drought stress. Drought resistance is a complex phenomenon, which is the manifestation of both drought tolerance (tissue tolerance, maintenance of photosystem, etc.) and drought avoidance (deep root, leaf rolling, etc.), traits that are governed by multiple genes [
12]. Blum and Jordan [
13] showed that drought resistance is obstructed by the low heritability and deficiency of successful selection methods. Therefore, the selection of rice genotypes should be adapted to drought stress conditions [
1].
Salicylic acid (SA) is a promising phenolic compound and oxidative plant growth regulator. SA is associated with stress tolerance in plants through the regulation of multiple physiological processes under drought stress conditions, such as the photosynthesis rate, antioxidant defense system, transpiration rates, proline metabolisms, stomatal closure reversal, signal transduction inhibition, seed germination promotion, the induction of flowering, and nutrients uptake [
14,
15,
16]. Several researchers have investigated the impact of exogenously foliar-applied substances, such as SA or nutrients, on the morpho-physiological traits and yield of field crops, like rice under abiotic stress, including drought stress [
17,
18,
19,
20].
Some researchers believe in selection under favorable conditions [
21] and some believe in selection under typical drought conditions [
22]. Nevertheless, there exist numerous researchers that chose the midway and believe in selection under both favorable and stressed conditions [
23,
24,
25]. To determine drought-tolerant genotypes, several drought indices have been suggested on the basis on a mathematical relationship between yield under drought and non-stressed conditions. These indices are based on either the drought resistance or drought susceptibility of genotypes [
26]. The stress susceptibility index (SSI) was suggested by Fischer and Maurer [
27], whilst the tolerance index (TOL) and mean productivity index (MP) were suggested by Rosielle and Hamblin [
28]. The geometric mean productivity (GMP) and stress tolerance index (STI) were defined by Fernandez [
25]. The yield index (YI) was suggested by Gavuzzi et al. [
29], the yield stability index (YSI) was suggested by Bouslama and Schapaugh [
30], drought resistance index (DI) was proposed by Lan [
31], the yield reduction ratio (YR) was proposed by Golestani–Araghi and Assad [
32], the harmonic mean (HM) was proposed by Hossain et al. [
33], and the golden mean (GOL) was proposed by Moradi et al. [
34] in order to evaluate the stability of genotypes under both stress and non-stress conditions. The abiotic tolerance index (ATI) and stress susceptibility percentage index (SSPI) were introduced by Moosavi et al. [
35] for screening drought-tolerant genotypes under stress and non-stress conditions.
There is a need to use principle competent analysis (PCA) to show the results of rice experiments and to select based on a combination of correlations and drought tolerance indices. Thus, many researchers such as [
1,
36,
37,
38,
39] have used PCA to assess the relationship and diversity between several rice germplasms, in addition to knowing the relationships between yield and other quantitative traits of rice. The current study hypothesized that the exogenous application of SA may positively affect rice performance, drought tolerance indices, water productivity, and leaf photosynthetic pigments. Therefore, our main objective was to study the response of two lowland rice cultivars grown under normal and drought stress conditions.
4. Discussion
Drought stress is a principal constraint on rice production worldwide and in Egypt. Rice production is being ravaged by drought in the arid and semi-arid ecosystems of the world, as drought affects grain yield and other important traits of rice [
20]. In the present work, the two rice cultivars under normal and drought irrigation were subjected to different concentrations of SA to investigate their effects on the grain yield and studied traits, and to find the relationship between these studied traits.
In this study, a significant mean square due to the main effects of irrigation conditions, cultivars, SA, as well as their interactions on grain yield and most studied traits was observed. The significant effects of cultivars, irrigation conditions, SA, and their interactions on rice quantitative traits were previously reported by [
18,
20,
21,
38,
46,
47]. The irrigation conditions, followed by cultivars and SA concentrations, determined a large proportion of the total variation in the grain yield and most studied traits. Garg et al. [
48] reported that variations are expected to increase under drought stress conditions and various genotypes respond differentially. The genetic variation between rice cultivars is fundamental to the development of drought tolerance cultivars because they react reversibly to drought stress [
49]. Under higher osmotic stress levels, the variation of SA concentrations shows more pronounced effects [
21]. These indicate that there was sufficient desirable variability in the two rice cultivars’ responses to SA concentrations under normal and drought irrigation conditions, which may be utilized in improving the rice grain yield under drought regions in Egypt.
Drought stress significantly increased the carotenoids, IGP, and WP, and significantly decreased the grain yield and other studied traits as compared to the normal conditions. These results are in accordance with the findings of [
36,
50,
51,
52,
53]. Significant differences in the averages between drought-stressed and well-watered conditions lead to variations in rice grain yield [
37]. The detrimental effect of drought stress on the growth and yield traits might be related to the role of water in physiological processes resulting in a reduction in the photosynthetic rate, cell division, and nucleic acid synthesis [
54,
55], due to the decrease in the number of leaves and plant growth [
56].
The Giza179 cultivar showed remarkable superiority in the grain yield and all studied traits over the Giza177 cultivar under both irrigation conditions, except the 100-GW trait. Similar results were also obtained by [
36,
57]. Under drought conditions, the rice grain yield reduced by 24% and 13%, while WP increased by 19% and 29% in Giza177 and Giza179, respectively, compared to normal irrigation conditions. Giza179 showed relatively higher morpho-physiological traits along with high WP, whereas Hatfield and Dold [
58] found that the high photosynthetic rate and water use efficiency are important traits for an effective drought-tolerant genotype. This indicates that the Giza179 cultivar may have drought tolerance in its genetic background and be a good source of drought-tolerance genes; thus, it may be used in the development of drought tolerant cultivars. Drought-tolerant genotypes can develop a set of mechanisms that are more effective in protecting their structure and membrane functions compared to drought-sensitive genotypes [
59]. The cultivars that exhibit the highest drought tolerance are often used to investigate drought tolerance [
49].
Compared with the control, the grain yield and all studied traits were significantly increased by applying 400 µM of SA, reached a maximum with 700 µM of SA, and then decreased with the increasing rate of 700–1000 µM. Applying 700 µM of SA led to a desirable significant decrease in HD and IGP traits. Applying 700 µM of SA increased the rice grain yield and WP by 8% more in Giza179 than in Giza177 under drought conditions. The rice yield contributed morpho-physiological traits and were positively and significantly affected by the application of different concentrations of SA [
18,
20,
60]; therefore, SA significantly increases the rice grain yield.
Many aspects of physiological and biochemical processes are affected by SA; thus, SA is a promoted growth regulator to increase plant tolerance under drought stress conditions [
61]. Khalvandi et al. [
61], Hayat et al. [
62], Mutlu et al. [
63], Pirasteh-Anosheh et al. [
64], and Wang et al. [
65] reported that SA may play a main role in promoting drought tolerance in plants through increased elements uptake, increasing the photosynthetic rate, improving the enzymatic and nonenzymatic antioxidant activity, decreasing oxidative stress, concealing the reactive oxygen species (ROS), reserving water in plant cells, improving cell membrane stability, and providing protection for cell structure. SA could be used as a potential protectant to regulate the drought response of plants, thus improving plant growth and increasing yield traits under drought stress conditions [
18].
In many other studies, the application of SA led to increased osmotic potential under drought conditions, by increasing morpho-physiological traits, improving yield traits, and inducing changes in the protein expression in rice under drought conditions, for example [
19,
60,
66]. According to our results, the application of SA seems to be beneficial in coping with drought stress conditions, through ameliorating the negative effects of drought stress and improving plant growth and the sustainable productivity of rice and other crops under drought stress.
The I × C × SA interaction had significant effects on the carotenoids content, but not on grain yield and all studied traits. The rice grain yield and its components are greatly affected by the combined influence of drought stresses and SA application [
18]. The cultivar Giza179 fertilized with 700 µM of SA was the most tolerant cultivar to drought stress, which severely increased its grain yield and all the other studied traits, as a result of which this cultivar became the most tolerant under drought irrigation conditions compared to cultivar Giza177. A drought tolerance of 100-GW was observed in cultivar Giza177 fertilized with 700 µM of SA. Thus, the performance of Giza177 and Giza179 might depend upon the application of SA, apart from their genetic architecture under drought stress conditions.
The combination of drought tolerance indices under the different concentrations of SA may provide a more useful criterion to evaluate the drought tolerance of the two cultivars studied. The highest values of Yp, Ys, MP, GMP, STI, YI, YSI, DI, HM, and GOL indices, as well as the lowest values of SSI, TOL, YR, ATI, and SSPI indices were observed in cultivar Giza179 fertilized with 700 µM of SA. Hence, these indices were useful in identifying cultivar Giza179 as more drought-tolerant compared to cultivar Giza177, also indicating the higher importance of applying 700 µM of SA in the drought tolerance of wheat compared to other applications of SA concentrations. The PCA of drought tolerance indices exhibited that the highest indices of PC1 and the lowest indices of PC2 which can be referred to as the drought-tolerant high-yield component. The relationship between grain yield (Yp and Ys) and drought tolerance indices is a useful criterion for screening the best indices and identifying superior genotypes under normal and drought conditions. Based on the biplot diagram and according to Fernandez [
25], indices of MP, GMP, STI, YI, YSI, DI, HM, and GOL had the best indices of drought tolerance, due to their high correlations with rice grain yield under both normal and drought irrigation conditions.
Additionally, the Yp, Ys, MP, GMP, STI, YI, YSI, DI, HM, and GOL indices were in the opposite direction to SSI, TOL, YR, ATI, and SSPI indices, indicating their adverse correlation with each other. These findings agree with those obtained by [
1,
36,
53,
67,
68]. Generally, the PCA of drought tolerance indices exhibited the highest indices of PC1 (Yp, Ys, MP, GMP, STI, YI, YSI, DI, HM, and GOL) and the lowest indices of PC2 (SSI, TOL, YR, SSPI, and ATI), and can be referred to as the drought-tolerant high-yield components in relation to Giza179 fertilized with 700 µM of SA.
Positive correlations between the two traits indicated that the selection for the increased value of one trait will result in an increase in the value of the other [
69]. Strong positive correlations among most studied traits were observed under normal and drought irrigation conditions. These previous results were reported in several studies [
52,
53]. The highest positive correlations were found among the studied traits under drought conditions and under normal irrigation conditions and were compared to determine the response to drought stress. A statistically significant correlation was found between the rice grain yield and all studied traits under drought stress conditions, except
Chl. A, IGP, and 100-GW, indicating that the rice grain yield can be improved and increased by increasing these traits. Falconer and Mackay [
70] reported that the correlations of these traits indicated that their drought tolerance abilities are controlled by genes in linkage disequilibrium and/or with pleiotropic effects.
In the current study, the statistical PCA was used to identify the drought tolerance in two rice cultivars under SA concentrations and both normal and drought irrigation conditions, and to estimate the relationships between the studied traits across these variables. The first two extracted PCs had eigenvalues higher than one and contributed 92.42% of the total diversity for combined data during normal and drought irrigation conditions. These findings were consistent with [
37,
39,
71]. The PC1 accounted for 77.92% of the total variance of all analyzed variables, followed by PC2 and PC3. Thus, PC1 can be the basis in the weighting of the selection of variables such as genotypes and SA concentrations under both conditions. In other studies of rice, PC1 contributed the highest variance proportion with 51.10%, 57.65%, 58.83%, and 96.46% of the total variability [
37,
38,
39,
71], respectively.
According to the PCA plot, the Giza179 cultivar and the application of 700 µM SA had the maximum and positive weight on PC1, which are strongly and positively correlated with the grain yield and all analyzed variables, except IGP and WP measures. Therefore, the PC1 can be referred to as the drought-tolerant high-yield component and is important to increase the rice grain yield under drought stress conditions. As for PC2, the IGP and WP measures have the same eigenvector direction and variance as the Giza179 cultivar and the application of SA at 700 µM. PCA confirmed that a positive correlation was observed among all studied traits except IGP and WP under the normal and drought irrigation conditions.
Generally, all analyzed variables by PCA indicated that the cultivar Giza179 was positively correlated with grain yield traits and with the morpho-physiological traits of rice under the application of 700 µM of SA and drought irrigation conditions. The variables analyzed by PCA which contributed the highest for of the total variance could be manipulated during yield improvement programs in rice as suggested by [
39,
72,
73]. Based on our results, the cultivar Giza179 fertilized with 700 µM under drought conditions has the potential to improve plant growth and increase the sustainable productivity of rice in Egypt.