*2.11. Transmission Electron Microscopic Analysis*

The leaf ultrastructure changes were observed by the Sheteiwy et al. [47] procedure with minute modifications. The leaf samples without veins (7-10 per treatments) were randomly selected after treatment applications, then immersed in 2.5 percent (*v*/*v*) glutaraldehyde in 0.1 M PBS (sodium phosphate buffer, pH 7.4) and eroded thrice with the same PBS. Moreover, the 1% OsO4 (osmium (VIII) oxide) was used to postfix the leaves for nearly 1 h. Further, it was washed three times in 0.1 M PBS with a 15 min gap between each wash. In addition, leaf samples were dehydrated using various classified categories of ethanol, such as 50%, 60%, 70%, 80%, 90%, 95%, and 100%, correspondingly, and washed by using absolute acetone for 20 min. Later, samples were immersed overnight in Spurr's resin. Thus, ultra-thin segments (80 nm) of samples were amended, then retained inside copper nets for visualization via transmission electron microscope (JEOLTEM-1230EX) at 60.0 kV.

#### *2.12. Statistical Analysis*

The experimental results were investigated by applying analysis of variance (One-way ANOVA) with the least significant differences (LSD) at *p* < 0.05 and 0.01 levels between mean values using Statistix (8.1) software. Three replications were used for each experiment and standard errors (S.E) were represented in figures.

#### **3. Results**

#### *3.1. Priming Effect of SPM on Plant Phenotype, Seed Germination and Plant Growth Traits under Cr Exposure*

In this study, seeds priming with water were utilized as a control. Under control conditions, plant height was visibly enhanced with SPM treatment as compared to seed primed control plants (Figure S1). The Cr treatments alone significantly decreased the plant height/length and yellowing/burning of leaves compared to their controls, while severe toxic symptoms were observed in rice cultivar CY927 than YLY689. Under Cr stress, seed priming with SPM reversed the Cr toxicity as noticed by the improved overall plant height/length and greenish texture of leaves (Figure S1). In control treatments, no significant difference in the GE, PG, GI and VI of rice seeds were observed. Nevertheless, the Cr-exposure alone caused a significant reduction in seed germination parameters such as GE, GP, GI, and VI relative to controls (Table 1). This decrease was more pronounced in CY927 as compared to YLY689 under Cr-alone treatments. The mean germination time (MGT) was noticeably increased in CY927 compared to YLY689 under Cr applications, whereas seed priming with SPM significantly reduced the MGT under Cr stress. In contrast, seeds primed with SPM noticeably enhanced the GE, GP, GI and VI of rice seedlings under Cr exposure (Table 1). It was observed that these germinations indices were more improved in YLY689 when compared to CY927.

*Antioxidants* **2022**, *11*, 1704



 within a column designate that there was no significant difference at a 95% probability level at the *p* < 0.05 level according to LSD test, correspondingly.

Concerning the morphological changes, Cr-stress alone significantly reduced the plant height including shoot length (S/L), and root length (R/L), along with plant biomass including fresh weight (F/W) and dry weight (D/W). Compared with seeds primed with water (H2O) in the nutrient only solution, seeds primed with SPM enhanced plant growth as well as biomass, especially in YLY689 under Cr stress (Table 1). These outcomes designated that seed priming with SPM mitigated the toxic effects of Cr on rice seedlings and improved the seed germination, plant growth and biomass efficiently under Cr-stress conditions, with more pronounced improvement noticed in YLY689 than CY927.
