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AGROCLIMATOLOGY

Transpiration and Yield Relationships of Grain Sorghum Grown in a Field Environment

USDA-ARS Conservation and Production Research Lab., Bushland, TX 79012

^* Corresponding author (Judy.Tolk{at}ars.usda.gov).

The ability of plants to convert transpiration (T) into dry matter has been studied since the early 20th century. Research has compared differences among species using transpiration efficiency (TE), the ratio of biomass yield (Y_b) to T; and m and k, which are the slopes of the linear Y_b/T relationship normalized by atmospheric evaporative demand. The objective of this research was to develop transpiration and biomass relationships (TE, m, and k) and the transpiration and grain relationship (TE_g) of grain sorghum (Sorghum bicolor L. Moench) grown in a field environment. Grain sorghum was grown in 1998 and 1999 in weighable lysimeters using stored soil water only and in a rain shelter facility. This approach limited the irrigation and precipitation evaporation components of E so that T estimated (T_est) using lysimetry in a field environment was emphasized. The TE remained constant through a range of water availability. The TE was similar between the 2 yr, and the combined data produced a slope of the Y_b/T_est relationship of 3.5 g m^–2 mm^–1. This was similar to the 3.3 g m^–2 mm^–1 reported for sorghum in the early 20th century, but smaller than current field studies which ranged from 4.0 g m^–2 mm^–1 to 5.7 g m^–2 mm^–1. The TE_g was 2.6 g m^–2 mm^–1. Normalization of T_est by seasonal averages of reference evapotranspiration and vapor pressure deficit produced significant differences between years in the slopes of the relationships. Transpiration efficiency did not require normalization for variations in climate between years.

Abbreviations: RMSE, root mean square error • T, transpiration • TE, transpiration efficiency • TE_g, transpiration and grain relationship • VPD, vapor pressure deficit • Y_b, biomass yield

¹ The mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

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Received for publication September 3, 2008.