Principal component analysis of the alleviating effects of CaCl2 on NaCl stress in sour jujube (Ziziphus acidojujuba) seedlings

Abstract: 【Objective】Calcium plays an important role in the responses of plants to stresses. Some studies have indicated that exogenous CaCl₂ can reduce NaCl stress in jujube(Ziziphus acidojujuba) seedlings.The objective of this hydroponic experiment was to identify indexes that could be used to evaluate the use of CaCl₂ to alleviate NaCl stress in sour jujube seedlings.【Methods】A hydroponic experiment was conducted with three treatments:(i) no NaCl stress(control),(ii) NaCl stress, and(iii) NaCl stress plus CaCl₂.These treatments were chosen based on the results of our preliminary studies. Free water/bound water ratio, cell membrane permeability, superoxide dismutase(SOD) activity, peroxidase(POD) activity, catalase(CAT) activity, and malondialdehyde(MDA) concentration were measured in the shoots and roots of jujube seedlings. Principal component analysis was used to select physiological indexes for evaluating the alleviating effect of CaCl₂ on NaCl stress.【Results】Compared with the control, NaCl stress significantly reduced the ratio of free water/bound water in the shoots and roots of jujube seedlings on all sampling dates.In contrast, NaCl stress significantly increased cell membrane permeability. On day 5, the cell membrane permeability of shoots and roots in the NaCl stress treatment was 2.62 and 1.24 times higher than in the control, respectively. The NaCl stress significantly increased MDA concentrations in jujube shoots and roots. On day 5, shoot and root MDA concentrations were 1.81 times and 3.27 times higher than those in the control, respectively. Shoot SOD activities were significantly greater in the NaCl stress treatment than in the control on all sampling dates. In jujube roots, SOD activities were significantly greater in the NaClstress treatment than in the control on days 1, 2, 3, and 4. However, on day 5, NaCl stress significantly reduced root SOD activities. On day 3, SOD activities in shoots and roots were 1.53 and 1.43 times higher than those in the control, respectively. The NaCl stress significantly increased POD activities in both the shoots and roots. On day 3, shoot and root POD activities were 1.41 and 2.32 times higher than in the control, respectively. The NaCl stress treatment significantly increased CAT activity in jujube roots on all sampling dates. Shoot CAT activities were significantly greater in the NaCl stress treatment than in the control on days 1, 2, 3, and 4. However, there was no significant difference in shoot CAT activity between the control and NaCl stress treatments on day 5. On day 3, the CAT activities in the shoots and roots were1.96 and 4.10 times higher than those in the control, respectively. Compared with the NaCl stress treatment, the NaCl plus CaCl₂ treatment significantly increased the free water/bound water ratio on days 4 and5. On day 5, the ratio in the shoots and roots was 2.54 and 1.04 times higher than those in the NaCl stress treatment, respectively. NaCl stress plus CaCl₂ treatment significantly increased shoot SOD activities on days 1, 3, 4 and 5 and root SOD activities on days 1, 2 and 5. On day 5, shoot and root SOD activities were15.15% and 27.91% higher than those in NaCl stress treatment, respectively. Addition of CaCl₂ significantly increased shoot POD activities on all sampling dates and root POD activities on days 1, 2, 4, and 5.On day 5, shoot and root POD activities were 5.91% and 46.25% higher than those in NaCl stress treatment, respectively. The NaCl plus CaCl₂ treatment also significantly increased shoot CAT activities on days 1 to 4 and root activities on days 3 and 4. On day 3, shoot and root CAT activities were 80.63% and68.53% higher, respectively. The NaCl plus CaCl₂ treatment significantly reduced cell membrane permeability in shoots on days 3, 4, and 5 and in roots on days 4 and 5. On day 5, cell membrane permeability in roots and shoots were 26.62% and 16.62% lower than that in NaCl stress treatment, respectively. The NaCl plus CaCl₂ treatment also significantly reduced MDA concentrations in the shoots and roots on all sampling dates. On day 5, the MDA concentrations in shoots and roots were reduced by 55.22% and 55.00%respectively by addition of CaCl₂.【Conclusion】Principal component analysis indicated that cell membrane permeability, MDA concentration, POD activity, and CAT activity can be used to evaluate the alleviating effect of CaCl₂ on NaCl stress in sour jujube shoots. In sour jujube roots, cell membrane permeability,MDA concentration, and SOD activity are good indicators.