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The yield-reduction effect of abiotic stressors such as salinity and heat stresses with the growing world population threatens food security. Although adverse effects of salinity and heat stress on plant growth and production parameters have been documented, in nature, abiotic stresses occur sequentially or simultaneously. In this study, the stress tolerance and yield capacity of Yukinkomai, YNU31-2-4, and YNU SL rice genotypes tested under control (26 °C, 0 mM NaCl), salinity (26 °C, 75 mM NaCl), heat (31 °C, 0 mM NaCl), and heat and salinity (31 °C, 75 mM NaCl) stress combinations at vegetative and reproductive stages with six different scenarios. The results show that salinity and the heat and salinity combination stresses highly reduce plant growth performance and yield capacity. Heat stress during reproduction does not affect the yield but reduces the grain quality. The YNU31-2-4 genotype performs better under heavy salt and heat and salinity stress then the Yukinkomai and YNU SL genotypes. YNU31-2-4 genotypes accumulate less Na+ and more K+ under salt and multiple stresses. In the YNU31-2-4 genotype, low Na+ ion accumulation increases photosynthetic activity and pigment deposition, boosting the yield. Stress lowers the glucose accumulation in dry seeds, but the YNU31-2-4 genotype has a higher glucose accumulation.

Salinity is an abiotic stress factor that reduces yield and threatens food security in the world’s arid and semi-arid regions. The development of salt-tolerant genotypes is critical for mitigating yield losses, and this journey begins with the identification of sensitive and tolerant plants. Numerous physiologic and molecular markers for detecting salt-tolerant wheat genotypes have been developed. One of them is proline, which has been used for a long time but has received little information about proline-related genes in wheat genotypes. In this study, proline content and the expression levels of proline-related genes (TaPTF1, TaDHN, TaSRG, TaSC, TaPIMP1, TaMIP, TaHKT1;4, TaGSK, TaP5CS, and TaMYB) were examined in sensitive, moderate, and tolerant genotypes under salt stress (0, 50, 150, and 250 mM NaCl) for 0, 12, and 24 h. Our results show that salt stress increased the proline content in all genotypes, but it was found higher in salt-tolerant genotypes than in moderate and sensitive genotypes. The salinity stress increased gene expression levels in salt-tolerant and moderate genotypes. While salt-stress exposure for 12 and 24 h had a substantial effect on gene expression in wheat, TaPTF1, TaPIMP1, TaMIP, TaHKT1;4, and TaMYB genes were considerably upregulated in 24 h. The salt-tolerant genotypes showed a higher positive interaction than a negative interaction. The TaPTF1, TaP5CS, TaGSK1, and TaSRG genes were found to be more selective than the other analyzed genes under salt-stress conditions. Despite each gene’s specific function, increasing proline biosynthesis functioned as a common mechanism for separating salt tolerance from sensitivity.

Here, for the first time, we aimed to identify in rice the key mechanisms and processes underlying tolerance to high-temperature (HT) or salt stress (SS) alone, the co-occurrence of both stresses, and recovery using physiological and biochemical measurements and gene expression analysis. We also investigated whether recovery from the two stressors depended on the relative intensities/relief of each stressor. Wild type (‘Yukinkomai’) rice plants were found to be more susceptible to salinity or heat applied individually. SS leads to a depletion of cellular water content, higher accumulation of Na+, and alterations in photosynthetic pigments. The stress-tolerant cultivar ‘YNU31-2-4’ (YNU) displayed a lower Na+/K+ ratio, higher water content in cells and improved photosynthetic traits, antioxidant system, and expression of defence genes. Strikingly, the SS + HT combination provided a significant level of protection to rice plants from the effects of SS alone. The expression pattern of a selected set of genes showed a specific response and dedicated pathways in plants subjected to each of the different stresses, while other genes were explicitly activated when the stresses were combined. Aquaporin genes were activated by SS, while stress-related (P5CS, MSD1, HSPs, and ions transporters) genes were shaped by HT. Hierarchical clustering and principal component analyses showed that several traits exhibited a gradually aggravating effect as plants were exposed to the combined stresses and identified heat as a mitigating factor, clearly separating heat + salt-stressed from salt-non-heat-stressed plants. Furthermore, seedling recovery was far more dependent on the relative intensities of stressors and cultivars, demonstrating the influence of one stressor over another upon stress-release. Taken together, our data show the uniqueness and complexity of the physiological and molecular network modules used by rice plants to respond to single and combined stresses and recovery.