Shaded-Side Supplemental Lighting During Winter Enhances the Overall Productivity of Yellow Pitaya (Hylocereus megalanthus)

Yellow pitaya (Hylocereus megalanthus) is highly photosensitive and therefore strongly influenced by light availability. In winter, insufficient sunlight often induces a “yin–yang effect”, characterized by physiological disparities between the sunlit and shaded sides of east–west-oriented orchards. To elucidate the effects of supplemental lighting parameters on flowering, yield, and fruit quality of shade-grown yellow pitaya, we systematically examined four factors: lighting angle (45°, 60°, 90°), power (12, 15, and 18 W), duration (3, 4, and 5 h per day), and lighting period (10, 20, and 30 days). Compared with no supplemental lighting (net photosynthetic rate = 10.60/11.73 μmol m−2 s−1, yield = 903/3536.5 kg ha−1, net profit = 6435/72,675 CNY ha−1 in two seasons), a 90° angle in the first season and a 60° angle in the second season increased the net photosynthetic rate by 45.87% and 60%, yield by 165.98% and 145.16%, and net profit by 373.82% and 159.42%, respectively. 18 W lighting power raised average yield and net profit by 176.37% and 278.7%, while 5 h lighting duration enhanced them by 161.29% and 267.66%. Meanwhile, a 20-day lighting period increased yield and profit by 128.91% and 240.6% on average. The recommended parameter set of a 60°/90° angle, 18 W power, 5 h duration, and 20-day lighting period markedly improved photosynthetic performance, yield, and net economic returns of shaded-side yellow pitaya. These improvements were attributed to enhanced carbon assimilation and reallocation from source to sink tissues, which contributed to higher fruit yield and quality and effectively mitigated winter shading stress.