Effects And Mechanisms Of Exogenous Nitric Oxide On The Growth Of Mung Bean Seedlings Under Cd～(2+) Stress

Nitric oxide (NO) is a ubiquitous messenger and effective moleculein biology; it is described as a multifunctional second messenger in animals since itplays various functions including the activation of enzyme and transcription factor,gene expression, relaxation of blood vessels, neurotransmission and defense duringimmune response etc. Nitric oxide is also an important signal molecule in plants, and itparticipates in a series of key physiological processes such as seed germination, leafand root development, leaf and fruit senescence, programmed cell death, responses tobiotic and abiotic stresses such as pest and disease challenge, drought, salinity and lowtemperature. However, information is still lacking regarding the effects of nitric oxideon the inhibition of the growth induced by heavy metal stress. In the present study,mung bean seedlings were used as plant material and NO donor sodium nitroprusside(SNP) was used as a supplier of exogenous NO, we investigated the effects ofexogenous NO on the growth of leaf and root of mung bean seedlings under 5μmol/LCd²⁺ stress (seedlings stressed by Cd²⁺ for three days and eight days). To understandthe mechanisms of exogenous NO-promoted seedling growth under Cd²⁺ stress, theeffects of exogenous NO on the membrane lipid peroxidation, cross-linking betweenthe two strands of DNA, DNA content and photosynthesis of mung bean seedlingsunder Cd²⁺ stress were also been investigated. The main results were presented asfollows:1. When the mung bean seedlings were stressed by 5μmol/L Cd²⁺ for three daysand eight days respectively, the fresh and dry weight of the over-ground part and rootswere all decreased, and the net increment of seedling height, main root length, thelongest lateral root length and the net increased number of the lateral root were alsorestrained. However, the above inhibited effects of Cd²⁺ stress could be clearlyalleviated by 50μmol/L SNP (NO donor) treatment. The results indicated thatexogenous NO can alleviate the inhibited effect of Cd²⁺ stress on growth of mung beanseedlings.2. Compared to control treatment, Cd²⁺ stress for three days and eight days respectively caused the raises of MDA content and H₂O₂ level, and the increases inSOD, CAT and POD activities in leaf and root of mung bean seedlings. However,compared to Cd²⁺ stress alone, treatment of SNP+Cd²⁺ caused more increases in SOD,CAT and POD activities, a decrease of H₂O₂ level and MDA content in leaf and root ofmung bean seedlings. These results indicated that exogenous NO induced increases inSOD, CAT and POD activities, which scavenged H₂O₂ and resultantly inhibitedmembrane lipid peroxidation in leaf and root of mung bean seedlings under Cd²⁺ stress.This may be one of the mechanisms that NO alleviate the inhibited effect of Cd²⁺ stresson leaf and root growth of mung bean seedlings.3. Cd²⁺ stress induced the decrease of DNA content and the aggravation ofcross-linking between the two strands of DNA in leaf and root of mung bean seedlings.NO donor SNP could alleviate the effects of Cd²⁺ stress on DNA.4. Compared to control treatment, Cd²⁺ stress caused the reduction of netphotosynthetic rate, stomatal conductance, intercellular CO₂ concentration, apparentquantum yield, carboxylation efficiencies and the enhancement of stomatal limitationvalues in leaf of mung bean seedlings. These results showed that the inhibitive effect ofCd²⁺ stress on photosynthesis of mung bean seedlings was caused by stomatal factorand non stomatal factor together. However, NO donor SNP enhanced netphotosynthetic rate, stomatal conductance, apparent quantum yield, carboxylationefficiencies and photosynthetic capacity, and reduced the intercellular CO₂concentration and stomatal limitation values in mung bean seedling leaves under Cd²⁺stress, indicating that exogenous NO treatment improved ability of light reaction andthe efficiency of carbon assimilation of mung bean seedlings under Cd²⁺ stress, whichresulted in the alleviation of Cd²⁺ stress-induced inhibitive effect on photosynthesis inmung bean seedlings.