5-Aminolevulinic acid alleviates the salinity-induced changes in Brassica napus as revealed by the ultrastructural study of chloroplast

5-Aminolevulinic acid (ALA) is an important plant growth regulator which is derived from 5-carbon aliphatic amino acid. The present study investigates the interaction of increasing NaCl-salinity and ALA on plant growth, leaf pigment composition, leaf and root Na+/K+ ratio and chloroplast ultrastructure in mesophyll cells of oilseed rape (Brassica napus) leaves. The plants were treated hydroponically with three different salinity levels (0, 100, 200 mM) and foliar application of ALA (30 mg l−1) simultaneously. Ten days after treatment, higher NaCl-salinity significantly reduced the plant biomass and height. However, ALA application restored the plant biomass and plant height under saline conditions. A concentration-dependent increase in Na+ uptake was observed in the aerial parts of B. napus plants. On the other hand, ALA reduced Na+ uptake, leading to a significant decrease in Na+/K+ ratio. Accumulation of Na+ augmented 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 thylakoids. Foliar application of ALA improved the energy supply and investment in mechanisms (higher chlorophyll and carotenoid contents, enhanced photosynthetic efficiency), 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.