Abstract
Cadmium (Cd) is a nonessential and highly toxic element causing agricultural problems. However, little information is available about the variation in Cd tolerance among apple rootstocks and its underlying physiological regulation mechanisms. This study investigated Cd accumulation, subcellular distribution, and chemical forms as well as physiological changes among four apple rootstocks exposed to either 0 or 300 μM CdCl2. The results showed that variations in Cd tolerance existed among these rootstocks. Cd exposure caused decline in photosynthesis, chlorophyll and biomass in four apple rootstocks, which was less pronounced in M. baccata, indicating its higher Cd tolerance. This finding was corroborated with higher Cd tolerance indexes (TIs) of the whole plant in M. baccata than those in the other three apple rootstocks. Among the four apple rootstocks, M. baccata displayed the lowest Cd concentrations in roots, wood, and leaves, the smallest total Cd amounts as well as the lowest BCF. In apple rootstocks, it was found that to immobilize Cd in cell wall and soluble fraction (most likely in vacuole) and to convert it into pectate- or protein- integrated forms and undissolved Cd phosphate forms may be the primary strategies to reduce Cd mobility and toxicity. The physiological changes including ROS, carbohydrates and antioxidants were in line with the variations of Cd tolerance among four apple rootstocks. In comparison with the other three apple rootstocks, M. baccata had lower concentrations of ROS in roots and bark, H2O2 in roots and leaves and MDA in roots, wood and bark, but higher concentrations of soluble sugars in bark and starch in roots and leaves, and enhanced antioxidants. These results indicate that M. baccata are more tolerant to Cd stress than the other three apple rootstocks under the current experiment conditions, which is probably related to Cd accumulation, subcellular partitioning and chemical forms of Cd and well-coordinated antioxidant defense mechanisms.
Highlights
The application of urban composts, metal-based pesticides, ripening agents, and fertilizers and wastewater irrigation have resulted in increasing cadmium (Cd) accumulation in orchard soil of many parts of the world especially in developing countries (Andreu and Gimeno-García, 1996; Pinamonti et al, 1997; Li et al, 2014; Wang et al, 2015b; Duan et al, 2016)
Concentrations of chlorophyll a were markedly reduced by 6.7% in M. hupehensis, but were unaffected in other three apple rootstocks exposed to Cd compared to those without Cd exposure
The reduction in total biomass was smallest in M. baccata (7.0%), followed by M. micromalus “qingzhoulinqin” (13.7%), M. robusta (13.7%), and M. hupehensis (21.1%) (Table S2)
Summary
The application of urban composts, metal-based pesticides, ripening agents, and fertilizers and wastewater irrigation have resulted in increasing cadmium (Cd) accumulation in orchard soil of many parts of the world especially in developing countries (Andreu and Gimeno-García, 1996; Pinamonti et al, 1997; Li et al, 2014; Wang et al, 2015b; Duan et al, 2016). The huge variations in Cd uptake, accumulation and tolerance among species or cultivars make this choice possible and feasible (Shi and Cai, 2009; LópezCliment et al, 2011; Liu et al, 2014). This approach requires a mechanistic understanding of Cd toxicity and how it is mobilized within plants at the physiological and genetic levels (Mendoza-Cózatl et al, 2011). Previous studies have demonstrated the physiological mechanisms of Cd accumulation, transport, and tolerance in rootstocks of citrus (Podazza et al, 2016), cucumber (Savvas et al, 2013), and pepper (Morikawa, 2017).
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