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

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.

Effects of Soil Moisture Conditions before Heading on Growth of Wheat Plants under Drought Conditions in the Ripening Stage: Insufficient Soil Moisture Conditions before Heading Render Wheat Plants More Resistant to Drought during Ripening

“…Effi ciency of irrigation water use in grain production would be improved if the plants develop better root systems before heading, even if shoot growth is suppressed to some extent. Much research on irrigation scheduling has been performed, but almost all studies involved fi xed irrigation intervals without consideration of the adaptive responses of plants to drought (Proffitt et al, 1985;Oweis et al, 1998Oweis et al, , 2000. However, it was reported recently that irrigation a few weeks before heading, during which roots grew vigorously, encouraged plants to develop shoots and roots more effectively with increased grain yield under severe soil moisture depletion (Xue et al, 2003).…”

“…It is important for the effi cient use of irrigation water and associated labor to determine how irrigation should be scheduled. Considerable researches have been conducted on the effects of drought stress and irrigation on wheat growth and yield (Proffitt et al, 1985;Kobata et al, 1992;Musick et al, 1994;Zhang et al, 1998;Oweis et al, 1998;2000;Foulkes et al, 2001;Yang et al, 2001;Xue et al, 2003). While soluble carbohydrate reserves before heading might contribute to grain yield to some extent (Yang et al, 2001;Foulkes et al, 2002), grain yield is mainly determined by dry matter production after heading (Schnyder, 1993;Gebbing and Schnyder, 1999).…”

Effects of a Reduction in Soil Moisture from One Month beforeFlowering through Ripening on Dry Matter Production and Ecophysiological Characteristics of Wheat Plants

“…The development of root system can be promoted under moisture-defi cient conditions, even though it depends on the strength of the drought (Eghball and Maranville, 1993) and on both the species and the variety of plants Hida et al, 1995;Huang et al, 1997). A well-developed root system has also been reported in wheat plants grown under lower soil-moisture conditions (Proffi tt et al, 1985;Mian et al, 1993;Nakamura et al, 2003).…”

Effects of Soil Moisture Depletion for One Month before Flowering on Dry Matter Production and Ecophysiological Characteristics of Wheat Plants in Wet Soil during Grain Filling

“…It was reported that, when soil moisture decreased, roots developed well especially in deeper soil, not only in the wheat (Box et al, 1989;Morita and Okuda, 1994;Proffitt et al, 1985) but also in soybean (Hida et al, 1995;Hirasawa et al, 1994Hirasawa et al, , 1998, cowpea, rice (Angus et al, 1983), peanut, millet (Inanaga et al, 1996), and other plants. Wheat roots in the shallower soil in the present experiment were not so long, as in earlier studies by others (Box et al, 1989;Box and Ramseur, 1993;Proffitt et al, 1985), perhaps due to the dry and compacted surface of the soil, light leakage and temperature effects, which resulted from insertion of the minirhizotron tubes (Wiesler and Horst, 1994;Smit et al, 2000). The resistance to water transport in plants in the D-plot, with their better-developed root systems, was smaller than that in the W-plot ( Table 3), indicating that plants in the D-plot had a greater capacity for water absorption (Hirasawa and Ishihara, 1991;Hirasawa et al, 1992a, b).…”