Resource use efficiency of field-grown sunflower, sorghum, wheat and chickpea

Radiation use efficiency (ɛ) is a crop parameter widely used in crop simulation models, derived here as the slope of the relationship linking crop carbon gain to cumulative intercepted solar radiation. Our objectives were to: (i) determine ɛ of field-grown sunflower, sorghum, wheat and chickpea, at three levels of aggregation-day-time net assimilation (A), daily net carbon gain as difference between A and night-time dark respiration (R) and biomass; (ii) assess the robustness of ɛ parameter in terms of ability to discriminate between C3 and C4 species, pre- and post-anthesis and impact of nitrogen status; (iii) evaluate the opportunity to normalize ɛ for climate. Field experiments were conducted in 1998 and 1999 in southern Italy. All crops were well watered. Sunflower and sorghum had two nitrogen application treatments, wheat only one and chickpea had no added nitrogen. Daily intercepted solar radiation was derived from measurements of daily incident solar radiation and the percentage of mid-day light interception. Closed-system canopy chambers monitored canopy gas-exchange rates. ɛ, expressed in terms of A, A − R or above-ground biomass, was linear over the entire growth cycle of sorghum and wheat, and up to anthesis in sunflower and chickpea, independent of temperature, vapour pressure deficit and radiation regimes. In sunflower, deviation from linearity was observed after anthesis, due to higher carbon cost in yielding oil seeds. No conclusions could be drawn for post-anthesis chickpea due to the interruption of the experiment caused by a thunderstorm. Overall results showed a great variability in ɛ values, independently of classes of species (C4 and C3), crops and nitrogen treatments. This indicates that the robustness of ɛ to predict biomass productivity in crop simulation models is constrained. Attempts to normalize ɛ by vapour pressure deficit (D), for reducing its variability due to climate and overlaps between crops failed.