Transpiration-use efficiency of barley

Abstract

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) CO 2 concentration ratio ( c i/ c a) to changes of the leaf-to-air vapor pressure deficit ( D l), suggesting that k c rather than a constant could be a function of D l. The evaluation of more field data on k c, the field validation of the response of c i/ c a to D l, and testing this approach for different species and cultivars is needed to improve the understanding of the Y/ T determination at the canopy level.