EFFECT OF 5-AMINOLEVULINIC ACID ON LEAF SENESCENCE AND NITROGEN METABOLISM OF PAKCHOI UNDER DIFFERENT NITRATE LEVELS

Application with 50 mg L-1 5-aminolevulinic acid (5-ALA) to pakchoi (Brassica campestris ssp. chinensis var. communis Tsen et Lee) seedlings showed that when seedlings were cultured in 1/2 Hoagland's solution containing 1/2 of standard (3.75 mmol L-1) or standard nitrate (7.5 mmol L-1), the outer leaves of rosette plants gradually turned yellow, whereas much fewer yellowed leaves were found when 5-ALA was added. Analysis showed that 5-ALA treatment increased the leaf chlorophyll content, root activities, and insomuch plant production. 5-ALA treatment also greatly increased the content of free amino acids and soluble proteins. However, 5-ALA treatment decreased nitrate accumulation in both leaf blades and petioles, which was related with higher activity of nitrate reductase (NR) and higher level of NR gene transcript in pakchoi leaves. These results suggest that exogenous 5-ALA treatment may promote nitrogen absorption and assimilation with less nitrate accumulation in leaf tissues.