Changes in Spring Wheat Tillering Patterns in Response to Delayed Irrigation

Soil water availability can have a marked effect on the vegetative development of small cereal grains. However, few studies have examined the effects of soil water deficits on the appearance patterns of specific tillers. Field studies were conducted in 1983 and 1985 to evaluate the effects of early-season irrigation timing on tillering patterns of spring wheat (Triticum aestivum L.). The studies were conducted on a Declo fine sandy loam and a Declo silt loam (coarseloamy, mixed, mesic Xerollic Calciorthids) in 1983 and 1985, respectively. In both studies, the first irrigation was applied by sprinkler when either 3, 5, or 7 leaves were present on the main stem. Subsequent irrigations were applied to provide more than 50% of available soil water for the remainder of the growing season. Main stem development was linearly related to cumulative degree-days and was not affected by irrigation timing. Tillers which developed under optimal soil moisture conditions exhibited uniform appearance patterns and reached maximum populations over relatively short degreeday intervals. Soil water deficits decreased the rate of appearance of all main stem tillers and caused appearance to occur over longer intervals. Dry soil conditions also severely reduced development of tillers at the coleoptilar node. When stressed plants were finally irrigated, the appearance rate of affected tillers frequently increased. This increase usually occurred 40 to 70 degree-days (base 0°C) after irrigation. In some cases, the stimulation of tillering was sufficient to compensate for earlier tiller losses. However, periods of stress extending into the latter part of the vegetative period often reduced maximum tiller populations. Grain yields for wheat irrigated at the 3-, 5-, or 7-leaf stage were 7.28, 6.34, and 5.66 Mg ha⁻¹ in 1983 and 4.95,4.46, and 3.92 Mg ha⁻¹ in 1985, respectively. The results of this study illustrate the importance of adequate early-season soil water availability in developing uniform tillering patterns and high yield potential for spring wheat.