*3.9. Priming Effect of SPM on Cellular Ultrastructural Changes under Cr Stress*

Within control group (seeds primed with water and SPM without Cr stress), the leaf ultrastructure of both cultivars CY927 and YLY689 displayed a well-shaped cell wall, vigorous chloroplast, mitochondria, vacuoles, peroxisomes and the usual organized granule thylakoids inside grana containing normal granum thylakoids, stroma thylakoids, starch grains and plastoglobuli (Figure 6A,B,E,F). However, the ultrastructural analysis of rice cultivar (CY92, seeds primed with water) compared with Cr toxicity indicated a ruptured nuclear membrane, expansion of double-layered nuclear membrane, swollen mitochondria and damaged chloroplast (Figure 6C). Relatively, seeds primed with SPM presented a developed nuclear membrane, normal structure of chloroplast, plastoglobuli, and less swollen mitochondria than seeds primed with water beneath Cr exposure in CY927 (Figure 6D). In YLY689, seeds priming water (with Cr disclosure) revealed structural abnormalities such as thylakoids disruption, slightly irregular structured starch grains, mitochondrial damages, and swollen chloroplast (Figure 6G) than their relative control groups (Figure 6E,F). Interestingly, the seeds primed with SPM under Cr treatments minimized the structural damages induced by Cr stress in the mesophyll cells of cultivar YLY689 as observed by the welldeveloped cell wall, peroxisomes, vacuoles, well-shaped stroma thylakoids, starch granules, chloroplast, granum thylakoids, plastoglobuli and matured mitochondria (Figure 6H).

**Figure 6.** Effect of seeds primed with SPM on cellular ultra-structural changes in the leaves of two different rice cultivars (CY927 and YLY689) against chromium (Cr). (**A**) Leaf mesophyll cell of CY927 (primed with water) at control level, (**B**) Leaf mesophyll cell of CY927 (primed with Spm) at control level, (**C**) Leaf mesophyll cell of CY927 (primed with water) under Cr stress, (**D**) Leaf mesophyll cell of CY927 (primed with Spm) under Cr toxicity, (**E**) Leaf mesophyll cell of YLY689 (primed with water) at control level, (**F**) Leaf mesophyll cell of YLY689 (primed with Spm) at control level, (**G**) Leaf mesophyll cell of YLY689 (primed with water) under Cr stress, (**H**) Leaf mesophyll cell of YLY689 (primed with Spm) under Cr toxicity. Abbreviations: N (nucleus), SG (starch grain), NM (nuclear membrane), PG (plastogloboli), CW (cell wall), Va (vacuole), Ch (chloroplast), CM (cell membrane), (chloroplast), GT (granule thylakoids); M (mitochondria), ST (stomata thylakoids).
