Influence of 5-aminolevulinic acid on photosynthetically related parameters and gene expression in Brassica napus L. under drought stress

In the present study, the roles of foliarly applied 5-aminolevulinic acid (ALA) in photosynthetic parameters, ion contents and expression of several photosynthetic genes were investigated in rapeseed (Brassica napus L.) under well watered and drought conditions. Drought stress reduced relative plant growth rate and leaf water content (RWC), and contents of photosynthetic pigments, starch, soluble proteins and ions in roots. This type of stress, however, stimulated the accumulation of soluble sugars and free amino acids, as well as leaf proline contents. The transcript levels of photosynthetic genes were also down-regulated under drought stress. Exogenously applied ALA partially reduced growth inhibition and increased RWC and chlorophyll contents under drought conditions. The expression of photosynthetic genes was induced by ALA under both normal and drought conditions, which may contribute to an increased photosynthetic capacity. The ALA application further improved the accumulation of hexose and sucrose, accompanied by the maintenance of starch contents in the leaves and possibly relatively higher RWC under drought stress. Based on these results, we suggest that ALA promotes plant growth under drought stress via improving photosynthetic assimilation, increasing chlorophyll content and inducing the expression of several photosynthetic genes.