5-Aminolevulinic acid enhances photosynthetic gas exchange, chlorophyll fluorescence and antioxidant system in oilseed rape under drought stress

This study evaluates the role of exogenous foliar application of 5-aminolevulinic acid (ALA) on water relations, gas exchange, chlorophyll fluorescence, and the activities and gene expression patterns of antioxidant enzymes in leaves of oilseed rape under drought stress and recovery conditions. Seedlings at four-leaf stage were imposed to well-watered condition (80 % of water-holding capacity) or drought stress (40 % of water-holding capacity) and subsequently foliar sprayed with water or ALA (30 mg l⁻¹). Drought suppressed the accumulation of plant biomass and decreased chlorophyll content and leaf water status (relative water content and water potential). The actual quantum yield of photosystem II and electron transport rates were hampered in parallel to net photosynthetic rate. However, drought stress induced the accumulation of malondialdehyde (MDA) and hydrogen peroxide, enhanced the activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and superoxide dismutase and up-regulated the expression of APX and GR. After rehydration for 4 days, the growth of drought-treated seedlings was restored to normal level for most of the physiological parameters. Foliar application of ALA maintained relatively higher leaf water status and enhanced chlorophyll content, net photosynthetic rate, actual quantum yield of photosystem II, photochemical quenching, non-photochemical quenching and electron transport rates in stressed leaves. Exogenous ALA also alleviated the accumulation of MDA and hydrogen peroxide, increased the activities of antioxidant enzymes and enhanced the expression of CAT and POD in drought-treated plants. These results indicate that ALA may effectively protect rapeseed seedlings from damage induced by drought stress.