Biochemical and physiological responses of Brassica napus plants to humic acid under water stress

This study examines the effects of humic acid (HA, control, 3 and 6 mg/L) on some biochemical and physiological parameters of rapeseed (Brassica napus L.) plants under different water supply conditions (60, 100, and 140 mm evaporation from class A pan). Water stress decreased chlorophyll a (Chl a) and total chlorophyll (ChlT) content in plants but proline content partly increased with increasing water stress severity. Plants treated by HA had more Chl a and ChlT content under both well and limited water conditions. Application of HA improved the PSII and peroxidase activity of rapeseed plants under all irrigation treatments. Ascorbate peroxidase activity under severe water stress condition increased by 70 and 95%, compared with that under moderate and well watering conditions, respectively. Catalase activity was 51 and 69% less under well watering than that of moderate and severe water stress conditions, respectively. The highest activity of ascorbate peroxidase was recorded in plants treated by 6 mg/L HA. HA-treated plants had 42, 8.5, and 15% more soluble protein content under well watering, moderate and severe water stress conditions, respectively, compared with control plants. Malondialdehyde increased with increasing the severity of water stress, in contrast, application of HA significantly reduced the amount of this trait under water stress conditions. It was shown that application of HA increased the activity of antioxidant enzymes, improved PSII activity and consequently decreased lipid peroxidation in rapeseed plants.