The effect of day lengths (LD1, LD2, and LD3; natural day length, 1.5 and 2.5 hours light more than the natural day length, respectively) and potassium humate treatment (PH1 as control and PH2: 50% of potassium humate) on photosynthesis traits, chlorophyll fluorescence, carbohydrate and total antioxidant capacity of stevia was studied. Under LD3, the following parameters significantly improved: root/shoot (R/S), leaf water content per unit leaf area (LWCA), and leaf weight rate (LWR), while R/S increased by 16% under PH2. A combined application of LD3 with PH2 increased the morphological parameters including, shoot dry weight (SDW) and root weight rate (RWR). Furthermore, this combination resulted in higher growth compared with the LD1/PH1 combination. Also, an increased day length improved stomatal conductance (gs), transpiration rate (E), WUEi, photosynthesis pigments; chlorophyll fluorescence (Fo, Fm, Fv, Fm/Fv), and antioxidant activity. Besides, regardless of the photoperiod treatment, these parameters improved compared to PH1. The impact of LD3 or PH2 on carbohydrate content tended to be greater than the expected impact of LD2 or PH1. Our results suggest that increasing day length and application of potassium humate, both as individual and combined factors, will change the growth of stevia.

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