**2. Materials and Methods**

### *2.1. Experimental Design and Plant Growth Conditions*

This study was conducted during the summer of 2017 at the Agriculture and Agri-Food Canada's Canada-Saskatchewan Irrigation Diversification Centre in Outlook (51◦29 N, 107◦03 W, 541 m), Saskatchewan. The cultivar "Vigor", a cross between "Agria" and "Wischip" from the Agriculture and

Agri-Food Canada Lethbridge Research and Development Centre, was evaluated for its performance under soil moisture stress during the tuber bulking phase. Prominent characteristics of the cultivar are its yellow-fleshed tubers and pigmented (red-violet) flowers. Plants were grown under optimum soil moisture conditions at 70% field capacity (FC) and restricted soil moisture conditions at 35% FC under two high tunnels using drip irrigation. Treatments were imposed at the start of the tuber bulking phase for gradual exposure to drought stress, thereby mimicking natural field conditions (Figure 1). High tunnels were opened from all sides but covered with plastic film on top to mimic the open field condition while preventing rainfall (Supplementary Figure S1A). Plots were laid out in a randomized complete block design containing four replicates with guard rows on either side. Each plot consisted of 12 hills. The two end hills were considered as guard hills for yield estimation purposes. Seed pieces were spaced 1 m between-rows and 20 cm within-rows and were planted on 30 May 2017. The crop was raised using standard managemen<sup>t</sup> practices (i.e., fertility, irrigation, pest control, etc.). Pre-plant basal fertilizer included urea (46-0-0), mono-ammonium phosphate (11-52-0), and potash (0-0-60). Two applications of ammonium sulphate (21.5-0-0-4) were given at 4 and 7 weeks after planting. Soil moisture was monitored using Watermark sensors (Supplementary Figure S1B). Plots were harvested on 2 October 2017 and graded according to commercial-grade standards.

**Figure 1.** Visualization of potato growth stages and in-season soil moisture trends when soil moisture was maintained at 35% and 70% field capacity (FC).
