| Oilseed rape (Brassica napus L.) is one of the world's major oilseed crops and the most important source of edible oil. The growth of oilseed rape is always negatively affected by several abiotic stress factors like cold, drought, temperature and salinity. In addition, the increasing world population and urbanization are forcing farmers to utilize marginal lands as well as poor quality water. Salinity is one of the most serious threats to crop growth and yield of oilseed rape. Oilseed rape is also classified as a moderate salinity tolerant crop, thus it is utmost important to exploit its potential against saline soils. One of the promising approaches to increase plant tolerance against salinity is the proper use of plant growth regulators. The present study was carried out to investigate the role of 5-aminolevulinic acid (ALA) regarding plant salt tolerance induction in oilseed rape plants grown under salinity conditions. Plants were grown hydroponically in greenhouse conditions under three levels of salinity (0,100,200 mM NaCl) and foliar application of ALA (30 mg/1), we have investigated the following morphological, physiological, biochemical and ultra-structural changes in oilseed rape plants:(1) Data were recorded on two different leaf positions (1st and 3rd) to have a better understanding of ameliorative role of ALA on NaCl stressed oilseed rape plants. Results have shown that increasing salinity imposed negative impact on relative growth rate (root and shoot) and leaf water relations (osmotic potential and relative water content), whereas enhanced the level of relative conductivity, malondialdehyde (MDA) content, osmolytes (soluble sugar, soluble protein, free amino acid and proline) concentration, reactive oxygen species (ROS), and enzymatic (ascorbate peroxidase, guaicol peroxidase, catalase and superoxide dismutase) and non-enzymatic (reduced glutathione and ascorbate) antioxidants activity in two different leaf position samples. Foliar application of ALA improved relative growth rate (root and shoot) and leaf water relations (osmotic potential and relative water content), and also triggered the further accumulation of osmolytes (soluble sugar, soluble protein, free amino acid and proline) as well as enzymatic (ascorbate peroxidase, guaicol peroxidase, catalase and superoxide dismutase) and non-enzymatic (reduced glutathione and ascorbate) antioxidants activity in both leaf samples, whereas decreased the membrane permeability, MDA content and ROS production. Our results also indicate that osmolytes are preferentially accumulated in younger tissues.(2) In a separate set of experiments, role of ALA regarding photosynthetic gas exchange capacity and ion uptake under salinity stress in oilseed rape (Brassica napus L.) was studied. Exogenous application of ALA increased the leaf chlorophyll concentrations of stressed plants. Foliar application of ALA also maintained leaf water potential of 100 mM salinity stressed plants at the same level as that of the control plants and 200 mM treated plants recovered substantially as well. Net photosynthetic rate and gas exchange parameters were also reduced significantly with increasing salinity, whereas foliar application of ALA partially reversed them. Sodium (Na) accumulation increased with increasing NaCl concentration which induced a complex changeable response regarding the macro and micro nutrients uptake and accumulation in both roots and leaves. Generally, analyses of macro (N, P, K, S, Ca and Mg) and micro (Mn, Zn, Fe and Cu) nutrition showed no accumulation of these ions in the leaf and root under increasing salinity stress except zinc (Zn), and foliar application of ALA enhanced their concentrations except Mn and Cu. These results suggest that under the present short-term salinity-induced stress conditions (ten days), exogenous application of ALA could help the plants to improve growth, photosynthetic gas exchange capacity, water potential, chlorophyll content and mineral nutrition by manipulating the uptake of Na+.(3) In another experiment, role of ALA was also studied with special attention being given to the chloroplast ultra-structures in leaf mesophyll cells under salinity conditions. Two different levels of salinity induced the oxidative stress, which was evident by electron microscopic images, highlighting several changes in cell shape and size, chloroplast swelling, increased number of plastogloubli, reduced starch granules and dilations of the thylakoid membranes. Oxidative stress could be counteracted by reduced chlorophyll content and a transient decline of the photosynthetic efficiency as observed in the NaCl-treated plants. Foliar application of ALA improved the energy supply and investment in mechanisms (higher chlorophyll and carotenoid contents, enhanced photosynthetic efficiency) and reduced the oxidative stress as evident by the regular shaped chloroplasts with more intact thylakoids. On the basis of these results we can suggest that ALA is a promising plant growth regulator which can improve plant survival under salinity.
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