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 kc 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(u)) to changes of the leaf-to-air vapor pressure deficit (D-1), suggesting that k(c) rather than a constant could be a function of D-1. The evaluation of more field data on k(c), the field validation of the response of c(i)/c(a) to D-1, and testing this approach for different species and cultivars is needed to improve the understanding of the Y/T determination at the canopy level. (c) 2005 Elsevier B.V. All rights reserved.