Abstract: With soilless culture in greenhouse, this paper studied the effects of three potassium (K) levels (120, 240, and 360 mg·L^-1) on the photosynthetic characteristics and chloroplast submicroscopic structure of muskmelon cv. ‘Tiantian No. 1’ leaves. The results showed that at low (120 mg·L^-1) and high (360 mg·L^-1) K levels, the net photosynthetic rate (P_n) decreased, chloroplast lamellar structure disordered, and lamella number decreased. No significant effects were observed on CO₂ compensation point (70 μl·L^-1), saturation CO₂ for photosynthesis (600 μl·L^-1) and light compensation point (50 μmol·m^-2·s^-1) at the three K levels. The saturation light for photosynthesis, carboxylation efficiency and apparent quantum yield at 240 mg·L^-1 K level were significantly higher, being 1 200 μmol·m^-2·s^-1, 0.1364 and 0.0237, respectively. Under the conditions of this study, the optimal K level in the nutrient solution for muskmelon soilless culture was 240 mg·L^-1.

Key words: Plant, Species diversity, Gap, Fir forest, Tibet