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Moisture-Sensitive Growth Stages of Dwarf Wheat and Optimal Sequencing of Evapotranspiration Deficits¹

Careful management of a limited water supply would require that unavoidable water deficits coincide with growth stages which influence grain yield the least. This study was conducted to quantify moisture-sensitive growth stages of wheat (Triticum aestivum L.) grown in a coarse loamy Typic Camborthid soil. Treatment variables were water, N and seeding rate. Each variable had five coded levels: –1.682, –1, 0, 1, and 1.682. A central composite rotatable design was used. Parameters included initial soil water, irrigation depth, periodic as well as seasonal evapotranspiration (ET), and grain yield (Y). For ordering the relative sensitivities of growth stages, Y and ET data from those plots were considered in which N and seeding rate were at code O, least limiting to yield. The Y and ET from the plot with 1.682 coded level of water were taken to represent maximum yield (Ym) and maximum ET (ETm,), expressed as (100, 100). All data points below Ym and ETm were considered to represent the lower level of Y and ET. Actual evapotranspiration (ETa); deficit intensity, expressed as % of ETm by which ETm fell short in a time period; and its interrelations to yield responses were sequenced for three selected growth stages: vegetative, booting/heading, and flowering to grain formation. The Y response to evapotranspiration deficit (ETd) timings was expressed as the ratio of percent yield reduction to percent seasonal ETd. The reduction in Y refers to the actual yield (Ya) as percent of Ym.

The ETd sequences produced two ranges of percent Y reduction/percent seasonal ETd ratio. Low ratios of 0.2 to 0.7 denoted the "optimal" timing of ETd, since mean yield reduction was small, 17%. Large ratios of 1.0 to 1.2 represented "suboptimal" timing of ETd, which brought about 51 to 78% Y reduction. Within the range of optimal ETd sequence, the Y reduction versus ETd relation was linear. This relationship yielded a dimensionless slope to the order of 1.29, which being constant can be used for prediction of Ya if Ym is known. Without prior ETd in the vegetative stage, wheat yields were sensitive to water deficit during the critical booting/heading period but were relatively insensitive when the plants were conditioned to some 15% moisture stress in the vegetative stage. This led to the conclusion that in relation to wheat grain yields if water is limited, the deficits should be spread nearly evenly over the previous growth stages and the critical stage.

Key Words: Evapotranspiration deficit intensity • Water deficit tolerance • Yield reduction ratio • Yield-evapotranspiration relation • Triticum aestivum L.

² Senior scientist (agronomy), UTFN/PDY/005 project, c/o UNDP, P.O. Box 1188, Tawahi, Aden, PDR Yemen.

Received for publication November 1, 1978.

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