| Oilseed rape (Brassica napus L. cv. Zheshuang758) seedlings was used to investigate the effects of ALA on reactive oxygen species (ROS) scavenging system, photosynthesis, water relations, ion uptake, and chlorophyll synthesis under drought stress through physiological and molecular biological experiments. The results are as follows:1. The interaction of5-aminolevulinic acid (ALA:0,0.1,1,10and100mg L-1) and water-deficit stress induced by polyethylene glycol (PEG6000:0,-0.3MPa) on growth and ROS scavenging system in oilseed rape (B. napus L.) seedlings was investigated. Water-deficit stress reduced chlorophyll content, and non-enzymatic antioxidants levels, while it enhanced the malondialdehyde (MDA) content, ROS production, enzymatic antioxidants activities, oxidized glutathione ratio and ascorbate ratio. Application of ALA with lower dosages (0.1-1mg L-1) by root soaking significantly increased shoot fresh weight and chlorophyll content in seedlings, whereas moderately higher dosage of ALA (10mg L-1) hampered the growth. The study also indicated that0.1and1mg L-1ALA improved chlorophyll content and reduced/oxidized glutathione ratio and ascorbate ratio as compared to the seedlings under water stress. Compared to those of PEG treatment alone,1mg L-1ALA decreased the MDA and ROS content, further enhanced the activities of antioxidant enzymes, and up-regulated the expression of POD, CAT and GR under water stress conditions. These results indicated that0.1to1mg L-1ALA could alleviate the negative effects of water-deficit stress through maintaining a relative high ratio of GSH/GSSG and ASA/DHA, enhancing the activities of the specific antioxidant enzymes and inducing the expression of the specific antioxidant enzyme genes.2. The effects of ALA on the accumulation of reactive oxygen species (ROS), and activities and gene expression patterns of antioxidant enzymes in four-leaf stage seedlings under dehydration and rehydration conditions were studied. Seedlings were imposed to drought stress (40%of water holding capacity) and subsequently were sprayed with ALA (30mg/L), and then were rehydrated for four days. Dehydration increased MDA and H2O2contents, enhanced activities of CAT, POD, APX and GR, and up-regulated the expression of APX and GR. After rehydration, MDA and H2O2contents, and the expression of APX and GR were restored to normal level. Exogenous ALA application alleviated the accumulation of MDA and H2O2, increased the activities of CAT, POD, APX and GR, and enhanced the transcript level of CAT and POD in drought treated plants, compared to those of plants sprayed with water. Our results demonstrated that ALA could effectively activate antioxidant enzyme system through improving the activities of CAT, POD, APX and GR, and the expression of CAT and POD under drought stress.3. The effects of exogenous foliar application of ALA on plant photosynthesis were investigated through analyzing changes in gas exchange parameters, chlorophyll fluorescence parameters, and expression levels of some Calvin cycle related genes of oilseed rape under drought stress and recovery conditions. Drought suppressed the growth of shoots and roots, decreased chlorophyll, starch and soluble protein contents, declined net photosynthetic rate (Pn) as well as maximum and actual quantum yield of photosystem II (PSII), and down-regulated the transcript levels of Calvin cycle related genes, while it stimulated the accumulation the hexose and sucrose. After rehydration for4d, the growth of drought-treated seedlings was restored to normal level for the most physiological parameters. ALA partially alleviated the growth inhibition of shoots and roots. Foliar application of ALA enhanced Pn and gs, increased the photochemical quenching (qP) and non-photochemical quenching (NPQ), induced the expression levels of Calvin cycle related genes, further improved the accumulation of hexose and sucrose accompanied with a maintenance of starch and soluble protein contents in leaves, compared to those of sprayed with water, under drought stress. These results indicate ALA may enhance the photosynthesis through increasing gs and chlorophyll content, improving photoprotection capacity and up-regulating the expression of Calvin cycle related genes.4. The effects of ALA on water relations, osmolytes accumulation, and ion uptake under drought stress were investigated in oilseed rape seedlings. Drought stress decreased leaf relative water content (RWC) and water potential (WP), accumulating soluble sugars, free amino acid and proline in leaf and root, and inhibited the ion uptake by root. After rehydration for4d, the proline and soluble sugars contents were restored to normal level, while the ion uptake was not recovered except for Mg and Fe. Foliar application of ALA may help leaf maintain relatively higher leaf RWC and WP, decrease proline content and improve K+/Na+ratio by reducing Na+content in leaves to alleviate the osmotic stress induced by drought. However, compared to those of sprayed with water, ALA application has no significant effect on root ion uptake under drought stress.5. The effects of exogenous ALA on chlorophyll and endogenous ALA biosynthesis in six-day-old etiolated oilseed rape cotyledons during the de-etiolated stage were investigated. ALA treatment with low dosage enhanced the accumulation of endogenous ALA and chlorophyll, while the expression level of HEMA was not affected. The decrease of chlorophyll biosynthesis induced by water-defict stress was assumed to be related to the down-regulation of HEMA and ChlH and up-regulation of FC. However, exogenous ALA has no effects on the expression of chlorophyll synthesis related genes under PEG treatment. These results suggested that exogenous ALA might not be involved in the regulation of transcriptional level in chlorophyll biosynthesis. |
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