Soil water content and soil water potential

After removing the shoot from the soil surface, the soil column was cut into top and bottom halves from the middle (Fig. 1A). After root tissue was removed, each part of the column was weighed (W_original), oven-dried at 80 °C for about a week, and weighed again for dry weight (W_dry). Then the soil water content (%, w/w) was calculated by [(W_orignial−W_dry)/W_dry]×100%.

(A) Soil columns from the well-watered and soil drying treatments on Day 6 after the last watering; (B) soil water content in top and bottom parts of well-watered (WW) and soil drying (SD) treatments (Days 0–6). Pre-germinated maize seeds (Earligold F1) were transplanted into pots filled with sieved soil (John Innes No.2). Seedlings germinated from the soil surface after 1 d. All pots were weighed and watered to the pot capacity every day until the 15th day, except on the seventh day after transplantation when 50 ml of Hoagland’s nutrient solution (pH 5.8–6.0) were supplied to each pot. The third leaf was fully expanded on the 15th day after transplantation, and this day was set as the last watering day (Day 0). Plants at a similar growth stage were selected. The same experiments were conducted twice, and data presented here are the combined result. After Day 0, control plants were watered daily to pot capacity while watering ceased in the soil drying treatment for 6 d. Pots of each treatment were destructively harvested every day during Days 1–6. Each soil column was cut into top and bottom halves from the middle to measure the soil water content in the top and bottom parts. Points and bars are means ± SEs. Data were analysed using one-way ANOVA with Tukey’s post-hoc test, and different letters indicate a significant difference on the same day at P<0.05 (n=13 on Day 0 and n=9 on other days). Values in brackets are estimated soil water potentials (MPa) based on the soil water content values and the soil water characteristic curve (Supplementary Fig. S1).

A soil water characteristic curve can be found in Supplementary Fig. S1 at JXB online. The soil water potential was measured by a thermocouple psychrometer (Wescor Inc., UT, USA) when the soil water content was >25% (water potential higher than –0.37 MPa) and by the WP4-T Dewpoint Potentiometer (Decagon Devices, WA, USA) when the water content was between 5% and 25%. The soil water potential result was estimated from this soil water characteristic curve based on soil water content values.