A Comparative Study of Root Distribution and Water Extraction Efficiency by Wheat Grown Under High‐ and Low‐Frequency Irrigation1

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Abstract
A number of field experiments have separately evaluated plant responses to water stress, root distribution and soil water extraction patterns, consumptive water use, grain yield, and water use efficiency (WUE); few studies have investigated water movement through the soil‐plant‐atmosphere continuum as a whole. A field study was therefore conducted on spring wheat (Triticum aestivumL. cv. SST33), sown on a Rhodic Paleustalf soil in the Republic of South Africa, to relate the response of the plant and its root system to different water application frequencies. Special emphasis was on a soil dryingout period. Treatments consisted of high‐ and low‐frequency irrigation, with one treatment from each being maintained as a wellwatered control (HFc, and LFc, respectively), and one treatment from each being subjected to a period of water stress after anthesis (HFsand LFsrespectively). Soil water content, root length and distribution, and leaf water potential (ΨL) were monitored before and after irrigations and during the soil drying‐out period. Plants in the HFsand LFstreatments developed shallower rooting systems than plants in both the LFcand LFstreatments. Soil water extraction patterns were correlated to rooting distributions. Deeper roots became increasingly efficient at extracting water as the soil became progressively drier from the surface downwards, but the total water uptake was insufficient to enable the plants to transpire at their potential rate. The departure of the evapotranspiration/potential evapotranspiration (ET/PET) ratio from its well‐wateredvalue occurred at 14% (HFs) and 17% (LFs) depletions from field capacity, and this, coupled with the concomitant decrease in ΨL, suggests that plants can maintain a constant ET/PET ratio only when water is freely available in the upper soil layers. Frequent light applications of water resulted in reduced fluctuations in ΨLthus, higher yields and an improved WUE were obtained.