Effects of soil cadmium exposure on physio-ecological characteristics of Bletilla striata

Bletilla striata (Thunb.) Reichb.f. has shown rather extraordinary medicinal and economic value in recent years. However, the artificial cultivation of B. striata faces quite a lot of obstacles, especially the quality degradation and heavy metal pollution problems. Cadmium (Cd) in particular, is reported to generally exceed the standard in the artificial cultivation of B. striata. So far, little attention has been paid to analyze the effects of heavy metals on the growth and the medicinal treatment efficacy of B. striata. Herein, we investigate the physio-ecological response of B. striata under gradient Cd concentration treatment: Control (0.285 mg kg^-1); Tr-1 (0.655 mg kg^-1); Tr-2 (1.285 mg kg^-1); Tr-3 (7.675 mg kg^-1); Tr-4 (54.885 mg kg^-1), so as to provide a reference for the study of the Cd response regulation on B. striata. In this work, we examined the biomass, total carbon, total nitrogen, and content of B. striata polysaccharides (BSP, the functional component of B. striata), as well as the absorption proportion of Cd in B. striata. Based on the preliminary research, ecological risk assessment and human health risk assessment were allocated. Experiments were conducted in two batches (2018 and 2019, sampling in the same season) with the following findings: (1) The biomass showed no pronounced differences between the treatments or the sampling dates, only reached significant decrease at 54.885 mg kg^-1 (Tr-4) soil Cd concentration in 2019; (2) The total carbon of B. striata under Cd treatment was in line with the Hormesis effect and reached a peak at 0.655 mg kg^-1 (Tr-1) soil Cd concentration; (3) The total nitrogen content was generally promoted under Cd treatment with the highest content at 1.225 mg kg^-1 (Tr-2); (4) The total BSP content in two sampling years are both sorted in decreasing order: Tr-4 < Tr-3 < Tr-2 < Tr-1 < Control; (5) The Cd content in Bletillae Rhizoma (tuber of B. striata) under 0.655 mg kg^-1 soil Cd treatment was within the threshold stipulated in Chinese Pharmacopoeia in 2018 batch and in 2019 batch, only the control group was qualified with a safe plantation limit of Cd. The results of ecological risk assessment showed moderate toxic risk under Tr-1 (0.655 mg Cd kg^-1) and the human health risk assessment indicated negligible toxic effects on human health. Overall, the soil Cd concentration should be lower than 0.655 mg kg^-1, if safe cultivation, medicinal effect of B. striata and human health risk are taken into consideration.