Variability of barley radiation-use efficiency

Radiation-use efficiency (e) is a crop-dependent coefficient widely used in crop simulation models and in the physiological interpretation of crop response to the environment and management practices. Our objectives were to: (i) determine the e of spring barley (Hordeum vulgare L.), (ii) analyze the impact of weather variables and fraction of solar irradiance intercepted (f(i)) on e, and (iii) compare reported estimates of e for barley and wheat (Triticum aestivum L.) with our measurements for barley. Field experiments were conducted in 2000 and 2001 at Pullman, WA. Treatments consisted of factorial combinations of two cultivars of spring barley (Baronesse and Steptoe), two seeding densities (250 and 100 plants m(-2)), and two seeding dates (normal and late). Intercepted radiation was measured with tube solarimeters installed below the canopy during the crops life cycle and aboveground biomass obtained from weekly to biweekly samples. The extinction coefficient for solar radiation of both cultivars was 0.43, with no effect of seeding date and density. Cultivar, plant density, and f(i) did not affect e; however, e at the normal seeding date was greater than at the late seeding date in both years (1.15 and 1.19 g MJ(-1), first seeding date, and 0.90 and 0.95 g MJ(-1), second seeding date, Years 2000 and 2001, respectively, P < 0.01). These variations were correlated with daytime vapor pressure deficit (D). The e of barley and wheat reported in the literature and those obtained in this study were linearly related to D (kPa): e = 1.88 - 0.53D (r(2) = 0.70, n = 22). Maximum values of e reported for barley and wheat are near 1.6 g MJ(-1), but our analysis suggests that these high values can only be achieved in low D environments. The effect of the evaporative demand of the atmosphere should be considered in the interpretation of measured e or in the use of e in crop simulation models.