Objectives Copper-based foliar fertilizer has been proved of effectiveness in yield, quality and disease prevention. Amino acids could provide nitrogen nutrition and stress resistance of crops. The effects of adding amino acid into copper-based foliar fertilizers were studied for the production of a novel amino acid-copper-based foliar fertilizer.

Methods A pot experiment was conducted in 2015 and 2016, with celery (Apium graveolens L.) as test crop. There were five treatments in the experiment, including spraying water (CK), spraying 1.0 g/L Cu-based foliar fertilizer (CF1), 2.0 g/L Cu-based foliar fertilizer (CF2), 1.0 g/L amino acid-Cu-based foliar fertilizer (Cu-AA1) and 2.0 g/L amino acid-Cu-based foliar fertilizer (Cu-AA2). At 15, 45, 75 and 105 days after transplanting, spraing 15, 30, 50 and 50 mL, respectibely. At 30, 60, 90 and 120 days after transplanting, the leaf chlorophyll contents and disease index were determined. At harvest, the leaf photosynthetic characteristics were determined, and the yield and nutrition quality were measured.

Results Compared to CK, the yields in other four treatments were increased significantly. The yield in Cu-AA1 treatment was 10.9% higher than that in CF1 treatment, and the Cu-AA2 was 13.8% higher than CF2 in 2015. There was significant difference between CF1 and CF2 treatments ( P < 0.05), while no significant difference between Cu-AA1 and Cu-AA2. The SPAD value and Pn of celery leaves in foliar fertilizer treatments were increased significantly and there was no significant difference in SPAD value among the foliar fertilizer treatments. The Pn in Cu-AA1 treatment was increased by 17.3% than that in CF1, and no significant difference was found between different doses for the same foliar fertilizer. Compared CK , contents of soluble solid, soluble protein and nitrate contents in celery stems were significantly increased in CF1, Cu-AA1 and Cu-AA2 treatments. No significant difference in soluble solid was found among different foliar fertilizer treatments. Cu-AA2 treatments significantly increased soluble protein contents compared with CF2 and there was no significant difference between CF1 and CF2, or between Cu-AA1 and Cu-AA2. The nitrate content in Cu-AA2 treatment was 9.9% lower than that in CF2 and there was significant difference between difference doses for the same fertilizer ( P < 0.05). The disease indexes of celery in foliar fertilizers were significantly decreased than those in CK at 90 and 120 days after transplanting and those in CF1 and Cu-AA1 treatment were lower than those in CF2 and Cu-AA2 treatment. No significant difference was found between different foliar fertilizers with the same dose ( P < 0.05). Compared with CK, the Cu concentrations in celery leaves in all the four treatments were increased significantly. The Cu concentration in CF2 was 60.5% higher than that in CF1, and Cu-AA2 was 26.4% higher than that in Cu-AA1. The Cu concentration in Cu-AA2 was 14.0% lower than that in CF2, and no significant difference was found between Cu-AA1 and CF1 ( P < 0.05).

Conclusions Adding amino acids into copper-based foliar fertilizer could further improve the good effects in celery yield, nutrition quality and disease control. Adding amino acids could also increase the photosynthetic efficiency of celery leaves and decrease the nitrate contents of celery stems. So, amino acid-Cu-based foliar fertilizer is novel in function, and its proper dosage is 1.0 g/L for celery.