(Comparative Effects of Chemical Nitrogen and Amino Acids on Growth, Yield, and Quality of Cucumber (Cucumis sativus L.) under Greenhouse Conditions)

A two-season greenhouse study (2022/2023 and 2023/2024) was conducted at Sohag University, Egypt, to evaluate foliar-applied amino acids, glycine and glutamine, as partial or full substitutes for mineral nitrogen in cucumber (cv. Sultan). A split-plot design was used, with five nitrogen levels (0%, 25%, 50%, 75%, and 100% of the recommended dose) as main plots and amino acid treatments, glycine and glutamine, individually at 250–500 ppm or combined (250 ppm + 250 ppm), as subplots, applied four times: first at 17 days after transplanting and then at two-week intervals. The results indicated that amino acid treatments significantly enhanced vegetative growth, flowering, photosynthetic pigments, and total yield compared with sole nitrogen fertilization. The combination of amino acids (glycine and glutamine) with moderate nitrogen levels (N50–N75) showed the greatest improvements. The 50% nitrogen + 250 ppm glycine and glutamine treatment produced the highest number of branches (12), the greatest leaf dry matter content (22.7%), and a total yield per m² statistically equivalent to the full nitrogen treatment, while 75% nitrogen + 500 ppm amino acids achieved the highest chlorophyll to carotenoids ratio (4.69 mg/g FW). Correlation analysis revealed that fruit number and total yield were strongly and positively associated with branch number and leaf dry matter, whereas leaf specific area and days to first bloom were negatively associated with yield. Pigment ratios exhibited weak relationships with morphological traits, suggesting that pigment dynamics were primarily regulated by nitrogen availability and amino acid supplementation. Overall, the findings confirm the potential of glycine and glutamine as effective biostimulants capable of improving nitrogen use efficiency, reducing dependence on mineral nitrogen fertilizers, and enhancing sustainable cucumber production under greenhouse conditions.