Allelopathic effects of 24-Epibrassinolide on nitrogen assimilation in cucumber seedlings under acid stress and aluminum toxicity

Soil acidification due to excess use of nitrogen fertilisers in Chinese greenhouses is threatening the vegetable production through aluminum (Al) toxicity and low pH stress. We examined the changes in N assimilation in cucumber seedlings (Cucumis sativus L. cv. LuMing No. 1) under Al toxicity and H+ stress in presence/absence of 24-epibrassinolide (EBL), a potent allelochemical. Al toxicity suppressed the activities of nitrate reductase (NR), glutamine synthetase (GS) and glutamate synthase (GOGAT) but not glutamate dehydrogenase (GDH), causing the accumulation of nitrate (NO3-) and ammonium (NH4+) in plants and reducing the shoot dry weight, root dry weight and plants height. H+ stress did not affect the plant growth but slightly altered the N assimilation pathway with significant increase in NR and GS/GOGAT activity and drastic decline in GDH activity. The spray of EBL on plant shoots, overcame the growth inhibition of Al toxicity through elevating the antioxidant capacity and enhancing the N assimilation enzymes, but had no effect on N assimilation in plants subjected to H+ stress. In acidic soils, Al3+ stress instead of H+ stress significantly suppressed the nitrogen assimilation by vegetable crops and exogenous EBL alleviated the adverse effects of Al toxicity on nitrogen assimilation.