EFFECTS OF NITROGEN FERTILIZER ON NUTRITIONAL QUALITY AND ROOT SECRETION ACCUMULATION OF HYDROPONIC LETTUCE

A glasshouse experiment was carried out to investigate the effects of nitrogen levels (5 and 10 mmol/L) and forms (nitrate nitrogen and nitrate plus ammonia nitro-gen) on growth, photosynthetic pigment content, nutritional quality and root secretion accumulation of hydroponically cultivated lettuce. The results showed that nitrogen level of the same nitrate form did not or slightly affect shoot, root and total biomass of lettuce, while nitrate plus ammonia nitrogen (1:1, mol/mol) treatments decreased shoot, root and total biomass of lettuce compared with entire nitrate nitrogen treat¬ments. Two nitrogen levels of nitrate nitrogen or nitrate plus ammonia nitrogen did not make difference in leafy chlorophyll a, b and carotenoid contents of lettuce, but nitrate nitrogen plus ammonia nitrogen significantly increased chlorophyll a, b and carotenoid contents in leaves compared with entire nitrate nitrogen treatments. Both nitrogen level and form affected vitamin C content in leaves. Vitamin C contents in leaves were improved with the increase of nitrate nitrogen supply level, while ammonia nitrogen plus nitrate nitrogen treatments had higher vitamin C contents in leaves than those of entire nitrate nitrogen treatment at the same nitrogen level. Nitrate nitrogen supply levels did not altered nitrate content in leaves of lettuce, but ammonia nitrogen plus nitrate nitrogen treatments decreased nitrate content com¬pared with the treatments with the same level of entire nitrate nitrogen supply. Entire nitrate nitrogen supply gave similar total organic carbon (TOC) concentration and content regardless of nitrogen level, but they were significantly lower than those of nitrate plus ammonia nitrogen treatments. Additionally, lower nitrogen level treat¬ment with ammonia nitrogen plus nitrate nitrogen presented greatest TOC concentra¬tion and content.