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.

Arbuscular mycorrhizal fungi (AMF) affect cadmium (Cd) accumulation and tolerance in host plants. However, the effects of AMF on Cd accumulation and phytotoxicity and their underlying mechanism in apples remain uncharacterized. In this study, the comprehensive physiological and molecular responses of uninoculated and Rhizophagus intraradices-inoculated Malus hupehensis Rehd. rootstocks exposed to 0 or 300 μM Cd were investigated. AMF inoculation mitigated Cd-induced growth and photosynthesis inhibition and nutrient ion disorders. It also lowered the concentrations of Cd in all tissues and reduced Cd transport to the shoots. Compared to uninoculated apple plants, those inoculated with mycorrhizal fungi reduced the mobility and toxicity of Cd by altering its form and binding it to the cell walls of the roots and leaves. AMF inoculation ameliorated Cd stress by altering endogenous phytohormone levels and triggering enzymatic and non-enzymatic antioxidant systems. Transcriptome analysis revealed that the differentially expressed genes (DEGs) associated with AMF under Cd stress regulated carbohydrate and amino acid biosynthesis and metabolism, as well as phytohormone biosynthesis and signal transduction. Furthermore, AMF inoculation downregulated certain genes involved in Cd uptake and transport while upregulating other genes involved in detoxification. These results suggest that AMF alleviate Cd phytotoxicity by orchestrated physiological and transcriptomic regulation in M. hupehensis Rehd., providing valuable insights into the efficacy of AMF inoculation in improving the heavy metal resistance of fruit trees.

Cadmium (Cd) is a nonessential element and highly toxic to apple tree. However, Cd accumulation, translocation and tolerance in apple trees planted in different soils remain unknown. To investigate soil Cd bioavailability, plant Cd accumulation, physiological changes as well as gene expression patterns in apple trees grown in five different soils, ‘Hanfu’ apple seedlings were planted in orchard soils collected from Maliangou village (ML), Desheng village (DS), Xishan village (XS), Kaoshantun village (KS) and Qianertaizi village (QT), and subjected to 500 μM CdCl₂ for 70 d. Results showed that soils of ML and XS had higher content of organic matter (OM), clay and silt, and cation exchange capacity (CEC) but lower sand content than the other soils, thereby reduced Cd bioavailability, which could be reflected by lower concentrations and proportions of acid-soluble Cd but higher concentrations and proportions of reducible and oxidizable Cd. The plants grown in soils of ML and XS had relatively lower Cd accumulation levels and bio-concentration factors than those grown in the other soils. Excess Cd reduced plant biomass, root architecture, and chlorophyll content in all plants but to relatively lesser degree in those grown in soils of ML and XS. The plants grown in soils of ML, XS and QT had comparatively lower reactive oxygen species (ROS) content, less membrane lipid peroxidation, and higher antioxidant content and enzyme activity than those grown in soils of DS and KS. Transcript levels of genes regulating Cd uptake, transport and detoxification such as HA11, VHA4, ZIP6, IRT1, NAS1, MT2, MHX, MTP1, ABCC1, HMA4 and PCR2 displayed significant differences in roots of plants grown in different soils. These results indicate that soil types affect Cd accumulation and tolerance in apple plants, and plants grown in soils with higher OM content, CEC, clay and silt content and lower sand content suffer less Cd toxicity.

The effects of organic waste coverage on the quality of soil carbon (C) pools in orchards remain unclear. In this study, we performed a 3-year experiment in an 8-year-old Fuji apple orchard to investigate the effects of inter-row weed growing/inner-row pruning residue coverage (PRC), weed coverage (WC), and inter-row weed growing/inner-row non-coverage (used as a control) on the contents of soil water, available nitrogen, phosphorus, and potassium, organic C components, glomalin-related soil protein (GRSP), soil microbial biomass C, and the number of microorganisms at different soil depths. PRC increased the percentage of readily oxidizable C (ROC), whereas WC improved the percentage of soil humin (HM). Thus, PRC enhanced SOC activity, whereas WC promoted SOC stability. Moreover, soil water, available nitrogen, phosphorus, and potassium contents increased in WC in 0–20-cm soil depths as compared to that in PRC, and the correlation between total GRSP (T-GRSP) and HM (r = 0.92) was higher than that between T-GRSP and ROC (r = 0.79). Therefore, T-GRSP is important in HM formation and maintaining stability of C pools. Overall, these results improve our understanding of the positive effect of organic waste coverage on soil internal C cycling and energy flow within an orchard ecosystem.

To unravel the physiological and molecular regulation underlying the variation in zinc (Zn) tolerance between two contrasting apple rootstocks, namely, Malus baccata Borkh. (Mb) and Malus hupehensis Rehd. (Mh), seedling were exposed to either 1 or 100 μM Zn under hydroponic conditions. Growth inhibition and impairments in leaf anatomical structure were weaker in Mh than in Mb. The Zn concentrations were 14.2 % and 50.25 % lower in the roots and stems of Mh than in those of Mb, respectively. The translocation factor was reduced by 67.89 % and 44.64 % in Mh and Mb, respectively, in response to excess Zn. The Mh roots presented higher proportions of water-insoluble Zn than the Mb roots. The subcellular distribution of Zn revealed that cell walls (CWs) played an important role in Zn detoxification in both rootstocks. Fourier transform infrared spectroscopy analysis revealed that CWs of Mh had a stronger binding capacity for Zn than did those of Mb. The disturbance of the redox balance induced by excess Zn was weaker in Mh than in Mb. Excess Zn induced a greater reduction in the expression of genes involved in Zn uptake and translocation in the Mh roots than in the Mb roots. However, the expression of genes related to Zn detoxification increased more in Mh roots than in Mb roots. Our results suggest that Mh is more tolerant than Mb to excess Zn, which is ascribed largely to the greater inhibition of Zn mobility and activation of physiological responses and the stricter regulation of the expression of key genes involved in Zn uptake, translocation, remobilization, and detoxification.