Objectives Foliar application of silicon (Si) has emerged as a widely adopted practice to ensure the safe production of rice in cadmium (Cd)-contaminated farm lands. However, its efficacy varies significantly across different planting environments and rice varieties. We investigated the underlying reasons for the variations observed among rice varieties in terms of their leaf physiology and molecular responses, which contribute to differences in their performance following foliar application of nano-silicon.

Methods Based on previous research, a pot experiment was conducted using the rice cultivars Yahui 2816 and Yixiangyou 2115, which exhibited significant reductions in brown rice Cd content following foliar spraying of nano-Si, and Y liangyou 1, which showed no significant decrease in brown rice Cd content with foliar Si application, as the test materials. CdCl₂·2.5H₂O was added to the soil, and water was then poured to a depth of 3−4 cm above the soil surface. This mixture was allowed to equilibrate for 30 days, resulting in a soil with an equilibrium Cd concentration of 1.07 mg/kg. The nano-Si preparation contained 2.5 mmol/L of NaSiO₃·9H₂O. For each rice cultivar, two treatments were established: one with Si application and one without. In the Si treatment, nano-Si preparation was sprayed onto the rice leaves at the heading and filling stages, with each pot receiving 50 mL of the solution. During the grain filling stage of rice, samples of the first, second, and third leaves from the top were collected for physiological analysis. During the mature stage, samples of stems, the first to third leaves from the top, and seeds were collected to analyze the distribution of Cd accumulation, the subcellular distribution of Cd in leaves, the Cd retention capacity of cell walls, and the expression levels of related genes. The adsorption characteristics of Cd on cell walls were analyzed based on isothermal adsorption and adsorption kinetics experiments.

Results The distribution ratio of Cd in various organs of the three rice cultivars was all in order of stem>grain>leaf. Foliar spraying nano-Si had no significant effect on the total accumulation of Cd in rice shoot, but affected the distribution of Cd in rice stems, leaves, and grains. Compared with control, spraying Si decreased the distribution ratio of Cd in the leaves of Y liangyou 1, while did not change the Cd content in brown rice. However, spraying Si increased the distribution ratio of Cd in the leaves of Yahui 2816 and Yixiangyou 2115, especially in the second leaf from the top (2LFT), with the increase in Cd content and accumulation being the most significant, reaching 1.3−2.5 times that of the control. The Cd content in brown rice also decreased significantly, with decreases of 21% and 32%, respectively. Foliar application of nano-Si significantly increased the Cd content and distribution ratio in the cell walls of the 2LFT of Yahui 2816 and Yixiangyou 2115. Kinetic analysis showed that the saturated adsorption capacity of Cd in the cell walls increased by 13.72% and 42.81%, and the relative peak areas of hydroxyl functional groups (−OH) in the cell walls increased by 43.99% and 74.21%, respectively. However, for Y liangyou 1, there was no significant change in the above physiological characteristics after foliar spraying of nano-Si. Pectin is the main component of the leaf cell walls that binds to Cd, and the lower the degree of methylation of pectin, the more favorable it is for Cd binding. After foliar spraying of nano-Si, the Cd content in the cell wall pectin of Yahui 2816 and Yixiangyou 2115 significantly increased by 43% and 19%, respectively. The relative expression levels of the pectin synthesis genes OsGAUT1 and OsGAUT4 in both varieties also significantly increased, but there was no significant change in the relative expression levels of the pectin demethylation genes. However, for Y liangyou 1, the relative expression levels of the above genes did not significantly change between treatments.

Conclusions The response difference of rice cultivars to foliar Si application is mainly caused by the retention effect of Cd on leaves, especially the second leaf from the top. After foliar spraying of nano-Si preparation on Yahui 2816 and Yixiangyou 2115, the relative expression levels of the cell wall pectin synthesis genes OsGAUT1 and OsGAUT4 will be significantly up-regulated, and the relative peak area of the hydroxyl functional group (−OH) in the cell walls be significantly increased, thus greatly enhancing the retention amount and retention intensity of Cd by cell walls, ultimately reducing the transfer of Cd to rice grains.