*3.2. Priming Effect of SPM on Photosynthetic Pigments, Photosystem II and Gas Exchange Parameters under Cr Stress*

Compared to control plants, seeds primed with SPM treatments marginally enhanced photosynthetic pigments such as chlorophyll-a (*Chl* a), chlorophyll-b (*Chl* b), chlorophyll- (a+b) (*Chl* (a + b)) and carotenoids in both rice cultivars, while treatments with Cr alone significantly reduced the *Chl* a (68.6% and 34.4%), *Chl* b (47.5% and 29.13%), *Chl* (a + b) (43.5% and 26.2%), and carotenoids (40.9% and 29.2%) in CY927 and YLY689, respectively (Figure 1). Seed priming with SPM enhanced the *Chl* a, *Chl* b, *Chl* (a + b), and carotenoids by 30.1%, 28.4%, 25.2%, 33.6% in CY927, and in 36.3%, 31.7%, 29.6%, 38.9% YLY689, respectively, under Cr stress (Figure 1a–d). Current investigations displayed that seed priming with SPM increased the *Chl* a, *Chl* b, *Chl* (a + b) and carotenoids in both rice cultivars as compared to respective controls in both treatments, with and without Cr toxicity. Likewise, the SPM application enhanced the *Fv*/*Fm* values as compared to relative controls, and the increment was 11.4% more in YLY689 than CY927 under Cr exposure (Table S2). In addition, this was further verified by taking visual images of *Fv*/*Fm*, and *Fm*. The rice leaves were exposed to 100 μM Cr stress more than untreated plants, showing as light blue/green, correspondingly, by decreasing the *Fv*/*Fm*, and *Fm* ratio (Figure 1e–h).

The exposure to Cr alone notably declined the values of gas exchange parameters including Pn, Tr, Gs, and Ci by 57.1%, 46.8%, 32.0%, 49.3% in CY927, and 38.7%, 33.1%, 23.9%, and 42.11% in YLY689 rice cultivars, correspondingly, but more decline was noticed in the values of the CY927 cultivar than YLY689 (Table S2). Nevertheless, seeds primed with SPM significantly improved the values of gas exchange indices under Cr toxicity in both CY927 and YLY689 rice cultivars. In control conditions, non-significant differences in the values of Pn, Tr, gs, Ci were noted between seeds primed with water and SPM of both cultivars. These findings revealed that seeds primed with SPM mitigated the toxic effects of Cr on photosynthetic pigments and gas exchange parameters in both cultivars, but this effect was more prominent in YLY689 relative to the CY927 cultivar.
