Monday, November 13, 2006 - 10:15 AM
81-4
Subsoil Water Use by Wheat.
We combined novel field experiments, crop simulation, detailed investigations of root distribution, morphology, and anatomy in the field, and laboratory studies of water relations of plants grown in intact soil cores to investigate subsoil water use by wheat. In field experiments using rainout shelters and irrigation to control water supply under terminal stress, subsoil water (from 1.2 m to 1.8 m) was converted to grain yield with an efficiency of 59 kg/ha.mm, around 3 times that typically expected for total seasonal water use. Simulation studies suggested the value can range from 0 to 83 kg/ha.mm depending on seasonal conditions with a median value of 30 kg/ha.mm. In the field experiment, only 11 of the 39 mm of plant available water in the subsoil occupied by roots in the 1.2 m to 1.8 m zone was used by the crop. Measured subsoil diffusivity (8 to 10 x 10-5cm2/s) was 5 to 10 times larger than commonly reported for dryish soil, and the root length density averaged around 0.1 cm/cm3, enough to extract all the water in the time available if the roots were well dispersed. However, most roots (70-90%) were clumped into cracks and biopores and entangled in ropes of the decaying remnants of previous crop roots, often with poor contact to the biopore wall. In some cases the biopore walls were lined with hydrophobic layers comprising the endodermis of previous crop roots. Subsoil roots were dominated (40%) by small 2nd order branches containing two small (<16 µm diameter) xylem vessels, suggesting high axial resistance to water flow. Thus transport from soil to root and within roots rather than within the soil itself may have limited water uptake. We will review these investigations of subsoil water uptake in wheat in the context of optimising water use by field-grown crops.
Recorded presentation