Transpiration-use efficiency of barley

Transpiration-use efficiency, the ratio of biomass (Y) produced per unit of water transpired (T) by a crop, depends on crop characteristics and on the environment in which crops develop. Transpiration-use efficiency has been described as Y/T = k_c/D_a, where k_c is a crop dependent constant and D_a is the daytime air vapor pressure deficit. Our objectives were to determine Y/T and k_c of barley grown in Pullman, WA, and to analyze the variation in Y/T and k_c of barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) reported in the literature. Transpiration and biomass accumulation of barley crops were measured in the years 2000 and 2001. The coefficient k_c was estimated as the slope of the regression between cumulative values of biomass and T/D _a. It ranged from 6.6 ± 0.4 to 6.9 ± 0.2 Pa. These figures are greater than 5.8 Pa obtained by applying equations developed by Tanner and Sinclair [Tanner, C.B., Sinclair, T.R., 1983. Efficient water use in crop production: research or re-search. In: Taylor, H.M., et al. (Eds.), Limitations to Efficient Water Use in Crop Production. ASA, Madison, WI, pp. 1-27]. Data on k_c reported in the literature, although scarce, ranged from 3.0 to 5.9 Pa for barley, and from 2.8 to 6.7 Pa for wheat, with the lower values occurring at low D_a (<1 kPa). This variability seems to associate with the response of the internal (leaf) to external (bulk air) CO₂ concentration ratio (c_i/c_a) to changes of the leaf-to-air vapor pressure deficit (D₁), suggesting that k _c rather than a constant could be a function of D₁. The evaluation of more field data on k_c, the field validation of the response of c_i/c_a to D₁, and testing this approach for different species and cultivars is needed to improve the understanding of the Y/T determination at the canopy level.