Measurement of plant water use under controlled soil moisture conditions by the negative pressure water circulation technique

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Abstract
A negative pressure water circulation system was developed so as to supply water quantitatively to a water-consuming soil column while maintaining the matric potential of soil water constant. The system consisted of a vacuum chamber with an air pressure gauge, a vacuum pump with a magnetic valve, a water reservoir with a scale, a motor for the circulation of water, a porous ceramic tube, and Tygon tubes to connect the components. Usefulness of the technique for the measurement of the water uptake by intact roots was tested for maize growing on a clayey soil. The soil packed in polyvinyl chloride cylinders (7.5 cm i.d. and 5 cm deep) was treated as follows; non-compacted (bulk density 1.0 g·cm-3), compacted (BD 1.3 g·cm-3), compacted plus 20 holes 1.2 mm in diameter, and compacted plus 20 holes 2.0 mm in diameter. The ceramic tube which supplied water circulating under the pressure of -9.7 (experiment I) or -24.3 kPa (experiment II) was embedded in the center of the treated soil. Maize seedlings were grown for two weeks at the day time temperature of 23 (experiment I) or 27°C (experiment II). The technique allowed the continuous measurement of the water uptake from the soil without any accumulation of air trapped in the system. Water under negative pressure was satisfactorily supplied to the soil layers; in experiment I in which the maximum rates of water uptake from the soil layers amounting to 1.1 to 3.5 mm/cay were observed, the quantity of water extracted from the soil during the whole plant growing period ranged from 1.2 to 1.5 mm, while the total water uptake from the soil ranged from 7.1 to 22.1 mm. In experiment II in which the maximum water uptake rates of 3.5 to 6.5 mm/day were observed, the amount of soil water depletion ranged from 1.5 to 2.4 mm for a total water uptake of 25.0 to 39.2 mm. It was shown that water use by the plant and water use per gram of the roots were significantly greater for all the compaction treatments than for the control. Enhancement of the water uptake by compaction seemed to be due to the increase of the unsaturated hydraulic conductivity of the soil. The negative pressure water circulation technique can be used for precise studies of the effects of various environmental factors on soil water consumption.