The experiment was conducted in rain protected wirehouse under open-air conditions at the Experimental Research Farm, College of Agriculture, South China Agricultural University, Guangzhou city (23°14′ N, 113°37′ E, 20 M altitude) during the 2017–2018 cropping years. Paddy soil from the research farm was collected to a depth of 20 cm and then air-dried, sieved through a 4 mm sieve, mixed thoroughly, and filled in plastic pots (30 cm height) with 10 kg in each pot. The experimental soil was comprised of 4.96 mg/kg Cd, 18.73 g/kg organic matter, 0.81, 0.9, and 16.79 g/kg total NPK, and 69.15, 10.15, and 109.62 mg/kg, available NPK, respectively, with 5.92 soil pH.

The seeds of two fragrant rice cultivars, i.e., “Xiangyaxiangzhan” (Xiangsimiao126 × Xiangyaruanzhan, an indica type of fragrant rice cultivar with a maturity period of about 112–114 days), and “Meixiangzhan-2” (Lemont × Fengaozhan, indica type of fragrant rice cultivar with expected maturity period of 145–147 days), were collected from the Department of Crop Science and Technology, College of Agriculture, South China Agricultural University, Guangzhou, China. The seeds were soaked in a container with deionized water and kept at 25–30°C for 48 h to complete the germination process. Sprouted seeds were then sown in trays containing soil parachute. The trays were placed on leveled paddy field and protected with a plastic sheet in early March to prevent the chilling effect. The 21-day old seedlings were then transplanted to soil containing plastic pots with 3–4 seedlings per hill and 5 hills per pot. Then, 1 week after transplanting, the damaged and affected seedlings were replaced with the healthy ones to get a good stand establishment. Fertilizer was applied with 2.25 g N (urea 46%), 3.33 g P (superphosphate 12% P₂O₅), and 1.35 g K (potassium chloride 60% K₂O) in each pot. Further, 2 weeks before nursery transplantation, Cd in the form of CdCl₂2.5 H₂O was added to the soil to obtain the required Cd levels for each treatment in solution form. The experimental treatments were comprised of: Ck, control (normal conditions); Cd: 100 mg Cd kg^–1 of soil; MeJa: exogenous application of MeJa at 20 mM; and Cd + MeJa: 100 mg Cd kg^–1 of soil + exogenous MeJa application at 20 mM. The MeJa was uniformly applied two times, i.e., at vegetative and the reproductive stages. The pots were kept in submerged conditions by maintaining a water level of 2–3 cm above the soil surface to create fully anaerobic conditions within the pots.

The crop was observed throughout the growth period and plant samples were collected at panicle heading stages and stored at –80°C for physio-biochemical assays while yield components and quality were recorded at the maturity stage after harvesting.

Photosynthetic pigments were determined according to Arnon (1949). The fresh leaf samples (0.3 g) at the panicle heading stage were extracted with 15 ml of 95% ethanol and placed in a dark place overnight. The absorbance was read at 665, 649, and 470 nm.

For determining the antioxidant enzymes, the fresh leaf samples (0.3 g) were ground in liquid-N₂ and homogenized in 6 ml containing 50 mM sodium phosphate buffer (pH 7.8) using mortar and pestle in an ice bath and homogenate were centrifuged at 10,000 g for 20 min at 4°C and the supernatant was used to record the enzymatic activities. The activities of superoxide dismutase (SOD) were assayed according to Zhang et al. (2008) following the inhibition of photochemical reduction due to nitro blue tetrazolium (NBT). The SOD activity per unit was defined as the amount of enzyme required to inhibit NBT photochemical reduction to 50% as an activity unit (U). The peroxidase (POD) activity was measured by using the guaiacol method with minor modifications in Lück (1965). The reactions mixture containing 1 ml of sodium phosphate buffer (pH 7.8), 0.2% of 0.95 ml guaiacol, 1 ml of 0.3% H₂O₂, and 0.05 ml aliquot of enzyme extract was used and the absorbance was read at 470 nm. Then, 1 unit of POD activity was recorded as the amount of enzyme that caused the decomposition of 1 μg substrate at 470 nm. The catalase activity (CAT) was assayed according to Aebi (1984) whereas 1 unit of enzyme activity (U) was the decomposition of 1 M H₂O₂ at A₂₄₀ nm within 1 min in 1 g of fresh leaves samples. The ascorbate peroxidase (APX) activity was estimated by using the “APX determination kit” obtained from Nanjing Jiancheng Bioengineering Institute, China. The manufacturer’s procedure was followed carefully and the absorbance was read at 290 nm. The contents of malondialdehyde (MDA) were determined by following the methods devised by Hodges et al. (1999). Hydrogen peroxide (H₂O₂) contents were determined following Velikova et al. (2000). To determine electrical conductivity (EC), the fresh leaf samples were washed with distilled water and then cut into smaller pieces. Leaf disks (0.3 g) were placed in 10 ml of deionized water and incubated at 25°C for 6 h and the (EC1) was recorded with an EC meter (SX650, Sansin, China). Samples were further incubated at 90°C for 2 h, then cool down to 25°C for recording the second EC (EC2). The EC of leaves samples was calculated as: EC (%) = (EC1/EC2) × 100. The protein contents in fresh leaf samples were determined according to Bradford (1976) at panicle heading stage using G-250. The proline contents in fresh leaves samples were determined according to Bates et al. (1973) using ninhydrin. The soluble sugar contents were determined at panicle heading stage following the methods of Zhang et al. (2008). For the reduced glutathione (GSH) and oxidized glutathione (GSSG), estimation was done by using the enzyme-built kits (A006-1 for GSH and A061-2 for GSSG), purchased from the Nanjing Jiancheng Bioengineering Institute, China¹. The instructions were strictly followed and the absorbance was read at 420 and 412 nm, respectively. Total glutathione was the addition of both GSH and GSSG (GSH + GSSG) contents.

At maturity, three pots from each treatment were randomly selected and rice plants from each pot were manually harvested using a sickle and then threshed, the paddy was sun-dried to the required moisture content and grain yield per pot was determined and expressed in grams per pot (g pot^–1). Total panicles per pot were counted for each treatment and averaged. Filled and unfilled grains were separated and counted manually from each panicle to obtain a total number of filled and unfilled grains per panicle. The 1,000-grain weight was recorded by weighing a random sample of 1,000 filled grains and averaged.

Rice quality attributes were determined from the harvested grains stored at an appropriate moisture content. Brown rice rate was calculated as: (brown rice weight/paddy weight) × 100. The brown rice was then milled using a Jingmi testing rice miller (Zhejiang, China) and the milling recovery was recorded as: (milled rice weight/original weight) × 100. Milling degree was calculated as: (milled rice weight/brown rice weight) × 100. Head rice rate was calculated by separating whole milled grains from 100 grains and head rice rate was calculated on a percentage basis. Grain chalkiness percentage and chalkiness degree were assessed using an SDE-A lightbox (Guangzhou, China). Grain chalkiness is the grains with opaque spots either on the dorsal and or in the center of the grain. Grain moisture, amylose, and protein contents were determined by using an Infratec 1241 (FOSS-TECATOR) grain analyzer.

To determine the grain mineral and Cd contents in different plant parts, the plant samples were ground into powder form and digested with HNO₃ and HClO₄ in 4:1 (V/V) and the contents were estimated by atomic absorption spectrometer (AAS 6300C, Shimadzu, Japan).

The research was a two-factor factorial and pots were arranged in a randomized complete block design (RCBD), placing them so as to allow enough space to avoid shading, each treatment had 15 pots for sampling, yield, and quality analysis. The data were analyzed by using the statistical software “Statistix 8” (Analytical Software, Tallahassee, Florida, Unied Satse) while difference among treatments was separated using the least significant difference (LSD) test at p < 0.05.

Both Cd toxicity and MeJa affected the activities of SOD, POD, CAT, and APX in both rice cultivars. The SOD activity was substantially reduced under Cd toxic conditions for both rice cultivars, however, maximum toxicity was observed in the Cd treatment and SOD activities were reduced when compared with the control treatment for both varieties. In Xiangyaxiangzhan, comparing the control treatment with exogenous MeJa application, 4.85 and 5.92% increase in SOD activities were observed for MeJa and MeJa + Cd treatments, respectively, whereas, 1.1% decrease in SOD activities was observed in the Cd treatment when compared with the control treatment. In Meixiangzhan-2 cultivar, exogenous MeJa application increased SOD activities by 3.85, 30.12, and 37.35% for Cd, MeJa, and MeJa + Cd treatments, respectively. Both Xiangyaxiangzhan and Meixiangzhan-2 cultivars experienced a 2.95 and 14.14% decrease, respectively, in the POD activities in the Cd treatments. For MeJa and MeJa + Cd treatments, 15.38 and 20.07% increase were observed for Xiangyaxiangzhan, respectively, while 3.74 and 38.84% increase were observed for Meixiangzhan-2 cultivar, respectively, when compared with the control treatment. The activities of CAT were decreased by 13.72 and 12.9% for the Cd treatments in both Xiangyaxiangzhan and Meixiangzhan-2 cultivars, respectively, while maximum increased CAT activity (28.41%) was observed in the MeJa + Cd treatment for Xiangyaxiangzhan cultivar. The APX activities saw significant reduction (60.37 and 34.58%) in the Cd treatment for Xiangyaxiangzhan and Meixiangzhan-2 cultivars, respectively; while MeJa addition augmented APX activities in the MeJa + Cd treatment for both cultivars (Table 1).

In the Cd treatments, the levels of GSH, i.e., 20.16 and 15.3% were higher in Xiangyaxiangzhan and Meixiangzhan-2, respectively. Maximum GSSG content (41.7%) was observed in the MeJa + Cd treatment for Meixiangzhan-2 cultivar. Total glutathione (GSH + GSSG) were recorded for the two cultivars studied and the increased levels were higher in Xiangyaxiangzhan than in Meixiangzhan-2 cultivar. The exogenous supplementation of MeJa reduced contents of GSH, GSSG, and total glutathione (GSH + GSSG) in Cd-affected plants.

The MDA contents in the leaves of both rice cultivars were substantially increased under Cd stress conditions. Exogenous MeJa application and MeJa + Cd reduced the amount of MDA by about 31.8 and 50% for Xiangyaxiangzhan and Meixiangzhan-2 cultivars, respectively. In addition, H₂O₂ was found to increase by 45.32 and 35.12% in the Cd treatments for Xiangyaxiangzhan and Meixiangzhan-2 cultivars, respectively. Plants treated with only Cd showed increased H₂O₂ content, but decreased significantly with MeJa application. In comparison with the control treatment, a significant reduction in H₂O₂ content, i.e., 34.14 and 52.87% was recorded for MeJa + Cd treatments for both Xiangyaxiangzhan and Meixiangzhan-2, respectively. Furthermore, significant reduction of electrolyte leakage (EL) was recorded in plants under MeJa + Cd treatment.

The proline content increased by 36 and 27% for Xiangyaxiangzhan and Meixiangzhan-2 cultivars, respectively, under Cd stress conditions whereas the proline contents were lowered by 28 and 49% in the MeJa + Cd treatments for Xiangyaxiangzhan and Meixiangzhan-2, respectively. Significant reduction in protein contents, i.e., 14.52 and 38.71% was observed in the MeJa + Cd treatments for Xiangyaxiangzhan and Meixiangzhan-2, respectively. Soluble sugar contents were found to increase almost 36% in the MeJa + Cd for both rice cultivars.

Cadmium stress reduced the rice yield and its components while an application of MeJa improved yield and its related attributes. In Xiangyaxiangzhan, compared with CK, the maximum number of panicles per pot was recorded in the MeJa + Cd treatment, while the minimum number was recorded in the Cd treatment. Similarly, spikelet number and seed setting rate were reduced under Cd treatment while increased under MeJa and MeJa + Cd treatments. Significant differences were not observed in the 1,000 grain weight in all treatments except for the MeJa + Cd treatment which had a maximum 1,000 grain weight (23.19 g). Furthermore, grain yield was 53.47% higher in MeJa + Cd than Cd treatment. For Meixiangzhan, the highest number of panicles per pot was recorded in MeJa treatment, followed by MeJa + Cd treatment whereas, the least number of panicles per pot were recorded in Cd treatment. In the terms of spikelet number per panicle and 1,000 grains weight, no significant differences were noted except in the MeJa + Cd treatment. Moreover, grain yield in MeJa + Cd treatment was 51.88% higher than Cd treatment. Overall, Cd has negative impacts on grain yield while exogenous MeJa application enhanced the grain yield under Cd stress conditions (Table 2).

Cadmium toxicity was found to affect grain quality traits while significant improvements were observed in quality-related attributes for both rice cultivars augmented with MeJa. For Xiangyaxiangzhan cultivar, brown rice was reduced under Cd treatment by (7.36%) but increased by (5.28%) in MeJa + Cd treatments. Maximum reduction in amylose contents, i.e., 22.71% was recorded for MeJa + Cd treatment, whereas the highest grain chalkiness, i.e., 43.49% was recorded in Cd treatment. Moreover, the maximum reduction in milled and head rice rate, i.e., 20.85 and 21.15%, respectively, was recorded under Cd treatment. Reduced protein content was recorded in the Cd-induced treatment while increased in the treatment with supplemented MeJa.

In Meixiangzhan 2, the Cd was found to have obstructed grain quality while MeJa improved grain quality attributes. For example, protein contents, brown rice rate, and milled rice rate were remained higher under MeJa + Cd than Cd treatment. Moreover, the maximum grain chalkiness (43.49%) and chalkiness degree (27.32%) and minimum brown rice rate (41.54%), head rice rate (52.13%), protein contents (29.39%), and milled rice rate (10.23%) were observed in the Cd treatment. Overall, all grain quality attributes were relatively affected by Cd toxicity and the application of MeJa improved the grain quality of both Xiangyaxiangzhan and Meixiangzhan 2 cultivars (Table 3).

Significant differences were observed in grain mineral contents among treatments. For instance, in Xiangyaxiangzhan, the mineral elements were decreased in the Cd treatment while the application of MeJa enhanced the grain mineral content. Compared with CK, there were 52.32, 40.94, 95.28, 67.45, 67.13, 20.92, 35.98, 35.74, and 62.85% decrease in N, P, K, Mg, Ca, Mn, Zn, Fe, and Cr contents, respectively, in the Cd treatment, while in the MeJa + Cd treatment, the contents of N, P, K, Mg, Ca, Mn, Zn, Fe, and Cr increased by 11.76, 4.74, 11.73, 24.12, 8.53, 8.84,14.53, 16.47, and 41.83%, respectively. Similar trends were observed in Meixiangzhan 2 cultivar, in which Cd induced treatment severely affected grain mineral contents but with MeJa application, grain mineral contents were augmented (Table 4).

Cadmium accumulation in the various organs, i.e., roots, stems, leaves, and grains were substantially higher in Cd treatment, while lower in MeJa applied treatment (Table 2). Exogenous application of MeJa reduced Cd uptake and accumulation in the rice roots and shoots. In the combined treatment (MeJa + Cd), Cd uptake was substantially reduced in the roots, stems, leaves, and grains of Xiangyaxiangzhan by 72.41, 157, 109, and 70%, respectively, while in Meixiangzhan 2, there were 32.75, 119.68, 162.21, and 601.50% decrease in Cd contents of roots, stems, leaves, and grains, respectively. On average, Cd accumulation was in the descending order: root > stem > leaves > grains for all treatments. In summary, Cd uptake and translocation were higher in the roots than stems, leaves, and grains for all the cultivars. However, the grain Cd contents in Xiangyaxiangzhan were substantially lower in Cd + MeJa than Cd treatment whereas found statistically similar (p > 0.05) for Meixiangzhan 2.