Drought stress often occurs concurrently with heat stress, yet the interacting effect of high vapor pressure deficit (VPD) and soil drying on the physiology of potato plants remains poorly understood. This study aimed to investigate the physiological and growth responses of potatoes to progressive soil drying under varied VPDs. Potato plants were grown either in four separate climate-controlled greenhouse cells with different VPD levels (viz., 0.70, 1.06, 1.40, and 2.12 kPa, respectively) or under a rainout shelter in the field. The VPD of each greenhouse cell was caused by two air temperature levels (23 and 30 °C) combined with two relative humidity levels (50 and 70%), and the VPD of the field was natural conditions. Irrigation treatments were commenced three or four weeks after planting in greenhouse cells or fields, respectively. The results indicated that soil water deficits limited leaf gas exchange and shoot dry matter (DM_shoot) of plants while increasing the concentration of abscisic acid (ABA) in the leaf and xylem, as well as water use efficiency (WUE) across all VPD levels. High VPD decreased stomatal conductance (g_s) but increased transpiration rate (T_r). High VPD increased the threshold of soil water for T_r began to decrease, while the soil water threshold for g_s depended on temperature due to the varied ABA response to temperature. High VPD decreased leaf water potential, leaf area, and DM_shoot, which exacerbated the inhibition of soil drying to plant growth. Across the well-watered plants in both experiments, negative linear relationships of g_s and WUE to VPD and positive linear relations between T_r and VPD were found. The results provide some novel information for developing mechanistic models simulating crop WUE and improving irrigation scheduling in future arid climates.

‘Shine muscat’ grapevines must be cultivated in a protected facility to avoid insects and rain during the rainy season; however, this decreases the light intensity falling on the canopy of grapes. The use of light-emitting diodes (LEDs), with their high efficiency in converting electricity to light, is a useful method to supplement light for plant growth. This study was designed to primarily investigate the effect of the light duration and intensity of supplemental LED lights on grape growth. The photosynthetic and chlorophyll fluorescence measurements of leaves were used to evaluate the performance of photosynthesis. Grape yield and fruit quality were also investigated. Seven different light treatments were utilized to determine the proper light duration and intensity of supplemental LED lights. The results show that the supplemental LED light intensity with the photosynthetic photon flux density (PPFD) of 300 μmol/(m²·s) at 18:00–24:00 showed the highest grape yield, sugar–acid ratio, and economic benefit, with improvement values of 45.1%, 51.4%, and 23.6%, respectively, compared to unsupplemented control vines (CK). The difference between the net photosynthetic rate (P_n), the max net photosynthetic rate (P_max), and the leaf photosynthetic efficiency (α) between the treatments was negligible. Meanwhile, prolonging the light duration at night was more effective in improving the grape yield and fruit quality than increasing the light intensity in the daytime using supplemental LED lights. The results prove that the supplemental LED lights significantly optimized the light environment and improved grape yield and fruit quality.