Sorghum root growth and water-use as affected by water supply and growth duration

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Early (‘58M’) and late (‘90M’) isogenic lines of sorghum (Sorghum bicolor L. Moench) were grown in a root observation field installation at Temple, Texas in 1976, in order to study the effect of soil moisture and growth duration on root development and plant water-use. The installation consisted of soil-filled steel chambers equipped with a glass panel and installed in retaining structures in the ground. Each chamber was lifted out of the ground for weighing and measuring roots visible on the glass panel. In all chambers, soil was irrigated to field capacity prior to planting. Two soil moisture regimes were then imposed — an irrigated treatment, wherein irrigation was applied whenever soil moisture content was reduced to 50% of available soil water; and a stress treatment, wherein no irrigation was applied and plants grew on stored soil moisture. Transpiration was considerably less for the early than for the late genotype in relation to the respective leaf area of the two genotypes. Transpiration demand was met without apparent plant stress, as long as the soil moisture content of the total soil volume was greater than 20% of available soil water. Below 20% of available soil water, transpiration was controlled mainly by a reduction in leaf area through leaf senescence. Total cumulative root length per plant and root length density were larger and root length density per unit leaf area was smaller for the late than for the early genotype. Root length density per unit of green leaf area increased under conditions of soil moisture stress due mainly to the reduction in green leaf area. Root distribution along the soil profile was highly skewed (higher root concentration at shallow than at deep soil layers) with irrigation and more homogeneous without irrigation. Root growth at shallow soil layers was promoted by irrigation. Root mortality of approximately two-thirds of the total cumulative root length produced by heading was recorded. Root mortality was not only associated with total plant senescence, but with plant age and rooting depth.