Effects And Mechanism Of Exterior Ascorbic Acid On The Growth Of Rape Seedlings Exposed To Cd And Acid Rain

Cadmium (Cd) and acid rain (AR) pollution injure rape (Brassica napus L.) seedlings, cause oxidative stress, disrupte their metabolism, lower seedlings' stress resistance, inhibit seedlings' growth, and ultimately affect the growth and development of plants. In order to probe into the mechanisms of Exterior Ascorbic acid (ExAsA) mitigation effects on Cd an AR damage in rape seedlings, and gain the best applying concentration and method of ExAsA, effects of ExAsA on Cd intimidation was reseached by solution culture experiment in Qinyou NO.7, damage of simulated acid rain (AR) of varying pH value and alliviative effects of ExAsA on acute injury were investigated by soil pot cultivation experiments in Suyou NO.1. That will provide theoretical basis for ExAsA applied in the the actual production of rape.1. CdCl₂·2.5 H₂O (25mg·L^-1) solution caused obvious harm to rape seedlings, inhibitted the growth of rape seedlings, induced significant accumulation of glutathione (GSH). Antioxidant enzymes (SOD, POD, CAT) activity and soluble protein content first increased and then decreased significantly. It showed that seedlings can enhance their capacity to resist Cd stress through regulating their own protective mechanisms, but the capacity is finite. With the stress lasted, antioxidant enzymes activity and endogenous non-enzymatic antioxidant-deoxidized ascorbic acid (AsA) content were significantly decreased, the active oxygen species produced over the scavenging capacity of the system, resulting in oxidative damage, for the performance of chlorophyll and carotenoid content decreasing, lipid peroxides MDA content and relative conductivity (RC) rising sharply.2. ExAsA improved the content of antioxidants and activities of antioxidant enzymes, synergetic effect of which could clear away active oxygen and reduce lipid peroxidation and decelerate the chlorophyl under 25mg·L^-1CdCl₂·2.5 H₂O stress. In rape seedlings treated by ExAsA, antioxidant enzymes (SOD, POD, CAT) activities, non-enzymatic antioxidants (AsA, GSH) and soluble protein content were at a higher level while MDA content and RC were maintaining at a lower level compared to those untreated. Lipid peroxidation caused by Cd stress being restrained effectively, which increased the accumulation of dry matter, chlorophyll and carotenoid content in varying degrees. It suppressed obviosly Cd transportantion from root to shoot, Cd content and Cd accumulation per plant in root, Cd accumulation per plant in shoot increased with the decrease of Cd content in shoot. Above all, AsA was an effective scavenger of active oxyen species, ExAsA of low concentration has preferable alleviative effects on Cd stress, and 1 mmol·L^-1ExAsA treated every 12 days was most efficient, while ExAsA of high concentration treated frequently has no obvious effect but did some damage to rape seedlings instead.3. AR of pH≥4.0 stimulated the growth of the rape seedlings, maintained biomass, pigment content in a corresponding or a higher level compared to CK (pH6.0), and there were also no significant difference on antioxidant enzymes activities and non-enzymatic antioxidants content compared with CK. Along with the AR acidity increasing, antioxidant protection system had been damaged, and antioxidant enzymes activities and non-enzymatic antioxidant content had been gradually declining faster and faster. That induced lipid peroxidation, while MDA content and relative conductivity increased significantly (RC), and the growth of seedlings was affected by the performance of biomass, pigment content starting to decrease. When the acidity (pH2.5 and pH2.0) was too strong, there were injury spots on the leaves to be observed.4. pH2.5 is acute acid rain damage threshold in rape seedlings, it restrained the growth of rape seedlings, damaged leaves and induced the appearanc of white needle-point spots, decreased biomass, pigment content, antioxidant enzymes activities, non-enzymatic antioxidants content and soluble protein content significantly, resulted in membrane lipid peroxidation while MDA content and RC increased significantly. Spraying ExAsA of suitable concentration can be an effective way to ease pH2.5 AR damage to rape seedlings. ExAsA stabilized antioxidant enzymes activities and non-enzymatic antioxidants content at a relatively high level, thus inhibiting activity oxygen species attack to cells, MDA content and RC decreased. Above all, with the concentration of ExAsA increasing, the mitigation effects on pH2.5 AR damage first increased then declined, and 4.0 mmol·L^-1 had the best effect.