Continuous sensing of water status in the root zone of wheat plants reveals features of circadian-dependent water uptake under mild water deficit

Abstract

Drought studies mostly rely on field capacity (FC) to maintain an imposed soil water deficit. However, they frequently overlook in situ water availability changes, hindering full understanding of soil water uptake by plants. Soil water potential (Ψ_S) may cover the limitation of relying solely on a defined percentage of FC. We conducted a pot experiment in which wheat growth stage- and diurnal-dependent water dynamics was concurrently and continuously monitored using volumetric water content (VWC) and Ψ_S sensors. Low-frequency fertigation in alternate days was applied to maintain mild water deficit (65 % FC) with a mean Ψ_S value of −16.1 kPa. The combined installation of VWC and Ψ_S sensors in the potted root zone disclosed fine features of diurnal oscillations in water uptake rate, endogenously controlled by the plant circadian rhythm, with a maximum rate ratio of five between daytime and nighttime. Flag leaves showed 20 % decrease in both biomass and area, while proline content increased 60 %. Osmotic adjustment favoured keeping daytime stomatal conductance, photosynthesis and WUE. At milk development, day-to-day root water uptake remained unchanged between two consecutive days despite lower water availability every second day. A decline in Ψ_Leaf between consecutive days led to nitrate accumulation. At maturity, grain yield and plant biomass showed parallel reductions in ca. 40 %, reflecting the difference in water use between water treatments. This approach connecting soil water availability with the plant’s circadian-dependent root water uptake using VWC and Ψ_S sensors went beyond their conventional use and is recommended to explore plant adaptation to water deficit.