Ratoon rice is a cropping pattern that occurs after harvesting the main crop of rice, and suitable cultivation measures are adopted to promote the germination of dormant buds, the growth of ratoon tillers, and the booting, flowering, grain filling, and harvest of the ratoon season.
The promotion area of ratoon rice in Southern China has been expanding in recent years. Cadmium (Cd) is a toxic heavy metal element that is difficult to dissolve and has a high migration rate. Rice is one of the crops that efficiently uptake and accumulate Cd, particularly in ratoon rice. In order to identify the effect and mechanism of Cd accumulation in ratoon rice, the experiment selected two pairs of OsNramp5 gene knockout line materials, namely Shaoxiang100(S100) and its wild-type 44-5(44-5) and Lianliangyou1(L21) and its wild-type Zhenliangyou8612(Z8612), under two Cd treatments to evaluate the differences in Cd accumulation between the main and ratoon season. The results demonstrated that the Cd concentration of mutant materials was significantly lower than the WTs in all organisms of ratoon rice, indicating that knocking out the osNramp5 gene could significantly reduce Cd concentration to the main and ratoon seasons of rice, especially in brown grains. Moreover, the return of straw also affects Cd accumulation during the ratoon season, with Cd accumulation significantly increasing when the straw is returned. In the ratoon season, the Cd concentration and accumulation of the ratoon shoots and grains are positively correlated with the position of the nodes. The photosynthetic and transpiration rates of sword leaves were also positively correlated with the position of the nodes. The higher transpiration rate of the ratoon leaves at higher nodes leads to greater Cd accumulation. It is suggested that ratoon buds at higher nodes more easily accumulate Cd, leading to a risk of excessive Cd concentration in ratoon rice grains.