Water uptake under water stress at panicle initiation stage in upland rice as affected by previous soil water regimes

Abstract

Under water stress conditions, water uptake from the deep soil layers is a key trait for drought avoidance in upland rice (Oryza sativa L.). In this study, upland rice subjected to different soil water regimes during the vegetative growth (low: − 0.20 to − 0.03 MPa and high: − 0.09 to − 0.03 MPa soil matric potential (\\g4s) was investigated to evaluate the daily plant water uptake of the root systems under short-term soil water stress at the panicle initiation stage, by characterizing the distribution of the root length and changes in the water extraction per unit root length q (cm2 d−1) in different soil layers in a column experiment. Irrigation was applied to set the \\g4s at − 0.04 to − 0.03 MPa at a 0 −20 cm depth and − 0.03 to − 0.02 MPa in the layers below a depth of 20 cm in both soil water regimes just before the imposition of the water stress treatment. In both soil water regimes, daily plant water uptake decreased continuously during the stress period, while the leaf water potential decreased substantially. During the 0–2 d period after the last irrigation (DALI), the water uptake rate in the 0–20 cm layer was the largest among the soil layers and coincided with the largest root length in this layer. Subsequently, the water uptake rate in the layers below a depth of 20 cm increased temporarily and then decreased rapidly as \\g4s decreased, while the water uptake rate at a 0–20 cm depth decreased monotonically. The q value responded to a decrease in \\g4s differently depending on the layers in both soil water regimes. In the 0–20 cm layer, the q value decreased continuously with the decrease in \\g4s. On the other hand, the q value in the layers below a depth of 20 cm increased with the decrease in \\g4s from the range of − 0.03 to − 0.02 to the range of − 0.05 to − 0.04 MPa. However, as the \\g4s decreased to below − 0.05 MPa, the q value in the layers below a depth of 20 cm decreased to a larger extent than in the 0–20 cm layer, reflecting the limited root water extraction ability in those layers as water stress developed, irrespective of the soil water regimes. The root length in the 20–40 cm layer increased in the low soil water regime due to the stimulation of the branching of fine and medium roots (42–220 μm). Consequently, the water uptake rate in the 20–40 cm layer was larger in the low soil water regime, which contributed to a larger daily plant water uptake at the beginning of the stress period. During the latter stress period, daily plant water uptake decreased rapidly in both soil water regimes