Exploring the physiological mechanisms of different types of cucurbits rootstocks in response to cadmium stress

Abstract:【Objective】Cadmium metal has an inhibitory effect on plant growth. When Cd2 + in the soil enters the plant body and accumulates, the plant will experience cellular oxidative stress response, producing a large amount of reactive oxygen species (ROS) and accumulating, causing membrane lipid peroxidation, increasing cell membrane permeability, and affecting plant physiological balance. Membrane lipid peroxidation can serve as a marker of plant oxidative damage, and the level of MDA content reflects the degree of plant membrane lipid peroxidation, ROS content and the degree of stress damage suffered. Under cadmium stress, plants produce a large amount of ROS and accumulate it, leading to an increase in ROS content in the body and inducing an increase in antioxidant enzyme activity. POD and SOD are the main antioxidant enzymes for scavenging ROS in plants. Oxidative stress in plants can inhibit photosynthesis, leading to damage to the chloroplast membrane system and a decrease in photosynthetic rate, and resulting in delayed plant development and even wilting and death. The chlorophyll fluorescence parameters of plants reflect the characteristics of PSⅡ and the efficiency of light energy utilization, thereby reflecting the harm of external stress on PSⅡ. Therefore, the study on the physiological conditions, such as MDA content, SOD and POD activity, and chlorophyll fluorescence parameters of different types of rootstocks under cadmium stress, can provide reference for rootstock breeding andsafe production of melon grafting cultivation in cadmium contaminated soil.【Methods】Wild watermelon type rootstock YZ1, Chinese pumpkin type rootstock SZ111, Indian pumpkin and Chinese pumpkin hybrid rootstock SZ7, and gourd type rootstock LZS were used as materials for hydroponic treatment with 0.6 mg ·L^-1 CdCl2 solution. The seedlings cultured without cadmium were used as the control. The control and treatment were designed to repeat for three times, with 10 plants in each replicate for 15 days. Before measurement, move the plants to a dark place and wait for 20 minutes. Then, use a JuniorPAM modulated chlorophyll fluorescence meter to detect the photosynthetic parameters on the fourth true leaf. The minimum fluorescence Fo, maximum fluorescence Fm, photosynthetic electron transfer rate ETR, maximum photochemical efficiency Fv/Fm of PhotosystemII (PS II), and the actual photochemical efficiency Y(II) = (Fm' - F')/Fm' calculated by the formula, as well as the potential photochemical efficiency Fv/Fo of PS II = (Fm Fo)/Fo were examined. After measuring the ground fresh weight, the roots and leaves were sampled, the leaves and root samples were extracted using the kit of Suzhou Grith Biotechnology Co., Ltd., and the MDA content, superoxide dismutase activity and the influence degree were calculated by the formula [(treatment- control) × 100 / control]. Add 20 mL of solution to the seed germination bag for cultivation, conduct experiments on plant root length, and measure the length of the main root. Data analysis, processing and plotting were carried out by using SPSS 26.0 and WPS tables. The significant differences between groups were tested by one-way ANOVA and Duncan method.【Results】Compared with the control, the degree of influence of YZ1, SZ111 and SZ7 was 26.75%, 59.35% and 45.72%, respectively, while LZS was only 9.06%, and the difference was not significant. The MDA content of YZ1 significantly increased by 8.96 nmol · g-1 , SZ111 and LZS by 0.68 and 2.39 nmol· g^-1 , respectively, and SZ7 decreased by 1.12 nmol· g^-1 . The SOD activity in YZ1 leaves significantly decreased by 309.07 U · g^{- 1}, while the root significantly increased by 974.73 U · g^{- 1} . The SOD activity in LZS leaves and SZ7 roots significantly decreased by 385.1 U · g-1 and 786.5 U · g-1 , respectively. The SOD activity in LZS roots significantly decreased by 1 056.03 U· g^-1 . The SOD activity in SZ111 leaves, roots, and SZ7 leaves was 88.53, 54.71 and 175.49 U· g-1 , respectively, and there was no significant difference compared with the control. The POD activity in YZ1 leaves significantly increased by 1 690.79 △OD470 ·min^-1 ·g^-1 , while in SZ111 leaves and SZ7 roots, both decreased by 442.58 and 1 126.89 △OD470 · min^-1 · g^-1 . The roots of SZ111 and LZS were significantly reduced by 1 891.83 and 872.51 △OD470 ·min^-1 ·g^-1 , respectively. There was no significant difference between YZ1 roots and LZS leaves and the control group. The Y(Ⅱ) values of SZ111 and SZ7 were significantly reduced by 0.17 and 0.21, while YZ1 was not significantly reduced by 0.02 and LZS did not significantly increase by 0.08. The ETR values of SZ111 and SZ7 significantly decreased by 13.62 and 16.90 μmol· m-2 ·s -1 , YZ1 did not significantly reduce 1.62 μmol · m^{- 2} · s^{- 1} , and LZS did not not significantly increase by 6.25 μmol·m^-2 ·s^-1 . The Fv/Fm values of YZ1 and SZ7 were significantly reduced by 0.10 and 0.19, while SZ111 and LZS were not significantly reduced by 0.10 and 0.01. The Fv/Fo values of YZ1 and SZ7 were significantly reduced by 1.86 and 2.16, while SZ111 and LZS were not significantly reduced by 1.03 and 0.32.【Conclusion】The response of SZ111 and LZS rootstocks to cadmium stress was relatively gentle, with a relatively small impact. YZ1 was more severely affected in response to cadmium stress, and various physiological indicators may have been damaged. SZ7’s response to cadmium stress was average, so SZ111 and LZS can be recommended as cucumber rootstocks applied in cadmium polluted areas.