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Water Use by Wheat and Plant Indicators of Available Soil Water¹

This study was carried out during 1978 on the Highveld plateau of the Republic of South Africa to provide a more fundamental understanding of the water relations of wheat for irrigation management. The aim was to determine the amount of soil water extracted by wheat (Triticum aestivum L.) and to examine the response of leaf growth and predawn leaf water potential to a decline in soil water content. Evapotranspiration was measured with four weighing lysimeters which contained repacked profiles of the two soil types (Rhodic Paleustalf and Typic Ustropept) of the surrounding area. Three watering treatments were applied. One lysimeter was kept well-watered by replenishing the deficit when 50% of the plant available water was depleted. The second treatment maintained the profile in a well-watered state initially; thereafter no further water was applied while the plants dried the soil profile. The third treatment allowed the profile to be dried; a single irrigation was given and again the profile was dried.

Plants with well-developed root systems extracted 80% of the plant available water at or near potential evapotranspiration rates. Plants with less well-developed root systems extracted only 70% before a decline in the rate of water use was apparent.

Measurement of leaf and stem length during the drying cycle indicated that only 52 to 577, of the plant available water was extracted before the plant growth rate in the drying treatment declined below that of plants which were well-watered.

A rapid decrease in predawn or covered plant leaf water potential was evident around the time that the rate of water use began to decline. This indicated that the decline was primarily the result of a large resistance to water movement through the soil rather than a plant induced control.

Key Words: Lysimeters • Potential evapotranspiration • Leaf growth • Leaf water potential • Tritticum aestivum L

² Senior plant physiologist, Water Res. Commission and deputy director, Soil and Irrigation Res. Inst.

Received for publication April 24, 1979.

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