Drought stress is a principal constraint to rice production worldwide and in Egypt. Rice production is being ravaged by drought in arid and semi-arid ecosystems of the world, which drought affects grain yield and other important traits in rice (Ali et al. 2021). In the present work, the two rice cultivars under normal and drought irrigation have been subjected to different concentrations of SA in order to investigate their effects on grain yield and studied traits, to estimate genetic parameters and to find the relationship between these studied traits.
In this study, 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, were observed. Significant effects of cultivars, irrigation conditions, SA and their interactions on rice quantitative traits have been previously reported by Yang et al. (2019); Hosain et al. (2020); Ahmad et al. (2021); Ali et al. (2021); Rafiq et al. (2021); El-Mouhamady et al. (2022). The irrigation conditions, followed by cultivars and SA concentrations were showed a large proportion of total variation for grain yield and most studied traits. Garg et al. (2017) reported that under drought stress conditions are expected to increase variations, where 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 (Rasheed et al. 2020). Under higher osmotic stress levels, the variation of SA concentrations showed more pronounced effects (Rafiq et al. 2021). These indicate that there was sufficient variability and desirable in the two rice cultivars responses with 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 content, infertile grain panicle− 1 and water productivity, and significantly decreased grain yield and other studied traits as compared to the normal conditions. These results are accordance with the findings of El-Hashash et al. (2018), Torres and Henry (2018), Sohag et al. (2020); El-Mowafi et al. (2021); Hussain et al. (2021). Significant differences in the averages between the drought-stressed and well-watered conditions lead to variations in rice grain yield (Bii et al. 2020). 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 (Abdalla et al. 2007; Abdelaal et al. 2020), due to the decrease in leaves numbers and plant growth (Boyer 1988).
Giza 179 cultivar showed remarkably superiority in the grain yield and all studied traits over the Giza 177 cultivar under both irrigation conditions, except 100-grain weight trait. Similar results were also obtained by El-Hashash et al. (2018); Gaballah et al. (2021). Under drought conditions, reduced rice grain yield by 24% and 13%, while increased water productivity by 19% and 29% in Giza 177 and Giza 179, respectively as compared to normal irrigation conditions. The cv Giza 179 showed relatively higher morpho-physiological traits along with high water productivity, whereas Hatfield and Dold (2019) cleared that high photosynthetic rate and water use efficiency are important traits for an effective drought-tolerant genotype. This indicates that tolerance to drought in Giza 179 cultivar has a common genetic background, and it may be a good source of drought tolerance genes, thus it can be used for the development of cultivars to drought tolerance. Drought-tolerant genotypes can evolve a set of mechanisms that are more effective in protecting their structure and membrane functions compared to drought-sensitive genotypes (Grzesiak et al. 2019). The cultivars that exhibited the highest drought tolerance are often used to investigate drought tolerance (Rasheed et al. 2020).
Compared with control, the grain yield and all studied traits were significantly increased with applying 400 µM of SA, then reached a maximum with applying 700 µM of SA, then decreased with increasing rate of 0 µM into 1000 µM. Applying 700 µM of SA led to a desirable significant decrease in heading date and infertile grain panicle− 1 traits. Applying 700 µM of SA increased rice grain yield and water productivity by 8% in Giza 179 than in Giza 177 during drought conditions. Rice yield contributing and morpho-physiological traits are positively and significantly affected by the application of different concentrations of SA (Issak et al. 2017; Hosain et al. 2020; Ali et al. 2021), thus SA significantly increased 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 to drought stress conditions (Khalvandi et al. 2021). Hayat and Ahmad (2007), Mutlu et al. (2016), Pirasteh-Anosheh (2015), Wang et al. (2019) and Khalvandi et al. (2021) reported that SA maybe plays the main role in promoting drought tolerance in plants through increased elements uptake, increased photosynthetic rate, improved enzymatic and nonenzymatic antioxidant activity, decreased oxidative stress or conceal the reactive oxygen species (ROS), reserve water in plant cells, improved cell membrane stability and provide protection for cell structure. SA could be used as a potential protectant to regulating the drought response of plants, thus, improving plant growth and increasing yield traits under drought stress conditions (Hosain et al. 2020). In many other studies, the application of SA led to increased osmotic potential under drought conditions, thus increased morpho-physiological traits, improved yield traits and changes in protein expression in rice under drought conditions (for example Wang et al. 2016; Issak et al. 2017; Kimbembe et al. 2020). 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 sustainable productivity of rice and other crops under drought stress.
I × C × SA interaction had significant effects on carotenoids content, but not on grain yield and all studied traits. Rice grain yield and its components are greatly affected by the combined influence of drought stresses and SA application (Hosain et al. 2020). The cultivar Giza 179 fertilized with 700 µM of SA was the most tolerant to drought stress, leading to a severely increased grain yield and all studied traits, as a result of which this cultivar became the most tolerant under drought irrigation conditions compared to cv Giza 177. The drought tolerance of 100-grain weight in cultivar Giza 177 fertilized with 700 µM of SA was observed. So, the performance of Giza 177 and Giza 179 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 lowest values of SSI, TOL, YR, ATI and SSPI indices, were observed in cv Giza 179 fertilized with 700 µM of SA. Hence, these indices were useful in identifying cv Giza 179 as more drought tolerant as compared to cv Giza 177, also indicating the higher importance of applying 700 µM of SA in drought tolerance of wheat than other applications of SA concentrations. PCA of drought tolerance indices exhibited that the highest indices of PC1 and the lowest indices of PC2 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 (1992), indices of MP, GMP, STI, YI, YSI, DI, HM and GOL had the best indices of drought tolerance, due to that have a high correlation with rice grain yield under both normal and drought irrigation conditions. Also, 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 are in agreement with those obtained by Chaeikar et al. (2018); El-Hashash et al. (2018); El-Hashash and EL-Agoury (2019); Basavaraj et al. (2021); Hussain et al. (2021). Generally, PCA of drought tolerance indices exhibited that 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 related to cv Giza 179 with fertilized 700 µM of SA can be referred to as the drought-tolerant high-yield components.
Positive correlations between the two traits indicate that selection for the increased value of one trait will result in an increase in the value of the other (Yehia and El-Hashash 2021). Strong positive correlations among most studied traits were observed under normal and drought irrigation conditions. These previous results have been reported in several studies such as El-Mowafi et al. (2021); Hussain et al. (2021). The highest number of positive correlations among studied traits during the drought conditions were recorded compared to during the normal irrigation conditions, this may be a response to drought stress. A statistically significant correlation was found between rice grain yield and all studied traits under drought stress conditions, except chlorophyll a, infertile grain panicle− 1 and 100-grain weight, indicating that rice grain yield can be improved and increased by increasing these traits. Falconer and Mackay (1996) reported that correlations of these traits indicated that their drought tolerance abilities are controlled by genes in linkage disequilibrium and/or with pleiotropic effects.
In this study, statistical analysis PCA has been used to identify 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 PCs extracted 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 Bii et al. (2020); Laraswati et al. (2021); Khan et al. (2022). The PC1 accounted for 77.92% of the total variance of all analyzed variables, followed by PC2 and PC3. So, PC1 can be the basis in the weighting of selection variables such as genotypes and SA concentrations under both conditions. In other studies of rice, PC1 contributed the highest variance proportion with a value of 51.10%, 57.65, 58.83% and 96.46% of the total variability (Ahmad et al. 2021; Khan et al. 2022; Laraswati et al. 2021; Bii et al. 2020, respectively) According to the PCA plot, Giza 179 cultivar and the application of SA at 700 µM had the maximum and positive weight on PC1, which are strongly positively with grain yield and all analyzed variables, except infertile grain panicle− 1 and water productivity measures. Therefore, the PC1 can be referred to as the drought-tolerant high-yield component and is important to increase rice grain yield under drought stress conditions. As for PC2, infertile grain panicle− 1 and water productivity measures have the same eigenvector direction and variance with the Giza 179 cultivar and the application of SA at 700 µM. PCA confirmed a positive correlation was observed among all studied traits except infertile grain panicle-1 and water productivity under the normal and drought irrigation conditions. Generally, all analyzed variables by PCA indicate the cv Giza 179 positively correlated with grain yield traits and with morpho-physiological traits of rice under applying 700 µM of SA and drought irrigation conditions. Khan et al. (2017 and 2022) suggested that analyzed variables by PCA which contribute the highest for of the total variance could be manipulated during yield improvement programs in rice. Based on our results, the cv Giza 179 fertilized with 700 µM under drought conditions has the potential to improve plant growth and increase the sustainable productivity of rice in Egypt.