Accumulation characteristics of heavy metals in three wild rice species and adaptation of root morphology and anatomical structure to native soil heavy metals in Yunnan

Abstract

Wild rice is a natural gene pool for modern cultivated rice germplasm resources. In the long process of independent evolution, wild rice has developed an extremely rich genetic diversity through natural selection in a variety of adverse environments. Wild rice is belonging to grade II of national protect plant in China and rarity. Field surveys were conducted to collect samples of seven Oryza granulata Balli., three Oryza rufupogon Griff. and three Oryza officinalis Wall. plants and relative native habitat soils from different sites in Yunnan province, China. The physical and chemical properties of soil, heavy metal (Hg, Cr, Ni, Sn, Cu, Zn, Cd, Pb) contents of soil and plants, morphological and anatomical structural characteristics of wild rice roots were analyzed. The results showed that soil pH was predominantly acidic. The soil total nitrogen, total phosphorus, available potassium, available nitrogen of O. officinalis were all at low levels. Soil heavy metal contents (Hg, Cr, Ni, Cu, Zn, Sn and Pb) were below the standard values, except for the soil Cd contents of O. granulata, which reached the level of moderate to light contamination (>0.3 mg·g⁻¹, GB 15618–2018). O. granulata had the largest thickness of endodermis, exodermis and thick-walled tissues and the the biggest areas of stele and vessel. O. officinalis had the most developed root system which had the highest area of diameter and cross-sectional in root. Compared to the other wild rice, Cd contents in roots and shoot of O. granulata were the highest. The bio-accumulation factor (BCF) of Cd was all higher than 1 in O. granulata and translocation coefficient(TC) of six heavy metals (Hg, Cr, Ni, Cu, Si and Pb) was the largest, which was higher than 1. The BCF of four heavy metal (Cr, Zn, Cd and Si) in O. rufupogon and O. officinalis was higher than 1 and TC of three heavy metals (Hg, Cr and Zn) was higher than 1. In conclusion, O. granulata increased the thickness of endodermis, exodermis and thick-walled tissues to adapt the stress of soil Cd. O. rufupogon increased total root length to get more nutrition and O. officinalis increased diameter and cross-sectional area of root to adapt submerged environment.