[Key Growth Stage of Pb Accumulation in Rice Through a Hydroponic Experiment with Pb Stress].

Pb accumulation in rice varies significantly at different growth stages. In this study, a hydroponic experiment was conducted to study the effects of exogenous Pb stress on Pb accumulation and transportation in rice plants and determine the key rice growth stages of Pb accumulation and their contribution to the Pb content in brown rice. For the hydroponic experiment, 0.5 mg ·L-1 of exogenous Pb was added to rice at different growth stages, including the tillering stage, jointing stage, booting stage, filling stage, dough stage, maturing stage, and whole growth stage (103 d) of the rice plant. Another treatment of the whole growth stage without Pb stress was established as the control. The results showed that: ① There were no significant effects of Pb stress in any single stage on the plant height, tiller number, and biomass, but the rice plant height and biomass significantly decreased under the Pb stress for the whole growth period. ② Pb contents in different tissues at the maturing stage of the rice plant were in the order of root > stem node 1 > other stem nodes > root rhizome > sti > leaf > ear > husk > brown rice. Pb contents in brown rice ranged from 0.1 to 1.2 mg ·kg-1 for all exogenous Pb treatments, and ranked in the growth stages as booting stage > jointing stage > tillering stage > filling stage > maturing stage > dough stage. ③ The relative contribution rates of Pb accumulation in the whole rice plants were relatively high during the reproductive growth period (filling stage, dough stage, and maturing stage), while those in the above-ground parts of the rice plants were relatively high during the vegetative growth stage (tillering stage, jointing stage, booting stage). ④ The booting stage was the key growth period for Pb accumulation in brown rice. Pb stress at this stage contributed 43.3% of Pb content in brown rice, followed by Pb stress at the jointing stage and the tillering stage, with contribution rates of 24.4% and 21.3%, respectively. ⑤ Water management regimes, application of amendments, or leaf resistance control techniques should be applied appropriately at the booting stage of the rice plants to reduce Pb accumulation in brown rice and to realize the safe use of rice paddies polluted with Pb.

[Key Stage of As Accumulation in Rice Under As Stress at Different Growth Stages].

The absorption and accumulation of As at different stages of rice growth are significantly different. To study the key growth stages of As accumulation in brown rice and to determine the contribution of As accumulation at different growth stages to As contribution in brown rice, a rice hydroponics experiment was carried out by adding external As during the different rice growth stages: tillering stage (30 d), jointing stage (16 d), booting stage (13 d), filling stage (17 d), dough stage (15 d), maturity stage (13 d), and full growth period (104 d). The results showed that: ① As stress at different growth stages had a significant effect on the biomass of rice plants. In comparison with the control CK, the treatments of As stress with five single-stages decreased the rice biomass, except during the tillering stage. Among these treatments, although the As stress at booting stage had the lowest rice biomass, the biomass of all the rice plants were higher than that for As stress in the full growth period. ② The content of As in brown rice for all the treatments of As stress at six single-stages, ranging from 0.08-0.24 mg ·kg-1, was higher than that of CK. Among these six treatments, the As stress at booting stage had the highest As content in brown rice, accounting for 64.9% of the As content in the full growth period. ③ The accumulation of As in brown rice for all the treatments of As stress at six single-stages was higher than that of CK, and ranged from 1.4-1.5 µg ·plant-1. Among these six treatments, the As stress at booting stage had the highest As accumulation, followed by filling stage. The accumulation of As in brown rice treated with As stress was highest in the full growth period with a value of 5.7 µg ·plant-1. ④ The relative contribution of As stress at the booting stage was the highest for the As accumulation in brown rice and reached to 40.3%, followed by filling stage with 26.0%. Therefore, the rice growth stages of the booting stage and filling stage were the key stages of As accumulation in brown rice.

[Effects of Exogenous Phosphorus on Rice Growth and Cadmium Accumulation and Transportation Under Cadmium Stress].

In this study, we investigated the effects of exogenous phosphorus on the accumulation and transportation of cadmium in rice plants through a hydroponic experiment. In the experiment, the rice variety was Huanghuazhan, P solution concentrations were 10.0-45.0 mg ·L-1 that was made using NaH2PO4, and Cd solution concentrations were 0.1 mg ·L-1 and 0.2 mg ·L-1. The results showed that: ① the biomass in all parts of rice plants and contents of photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoid) increased gradually with an increase in exogenous P. ② Content of Cd in rice stems, leaves, husk, and brown rice increased gradually with an increase in the amounts of exogenous P. The content of Cd in brown rice increased by 2.8%-22.8% and 40.9%-61.8% when treated with Cd concentrations of 0.1 mg ·L-1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ③ Cd accumulation in rice plants was accelerated due to the application of exogenous P. Cd accumulating amounts increased from 395.1 µg ·plant-1 to 542.6 µg ·plant-1 and 639.6 µg ·plant-1 to 1082.0 µg ·plant-1 when treated with Cd concentrations of 0.1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ④ With an increase in the applied amounts of exogenous P, the P/Cd quality ratio in rice roots increased, while those in rice stems, leaves, husks, and brown rice decreased; meanwhile, the Cd transfer coefficients from root to stem (TFroot-stem) and stem to leaf (TFstem-leaf) increased. This showed that there was a certain synergistic effect between P and Cd in the rice parts. Finally, the application of exogenous P promoted the transfer of Cd from the rice root to other rice tissues, resulting in a synergistic effect on Cd accumulation and transportation in various rice tissues and increased Cd contents in brown rice.