Intercepted radiation and radiation-use efficiency in sunflower crops grown at conventional and wide inter-row spacings: Measurements and modeled estimates of intercepted radiation

The underlying eco-physiological principle behind row spacing choice in many field crops is that early and complete ground cover will maximize yield via greater crop photosynthesis during the critical period for grain number determination in environments with no major water limitations. Recently this principle has been put in doubt: wide row-spacing (1.40 m instead of 0.7 m) did not reduce oil or biomass yield in sunflower at plant densities of between 2 and 7 pl m−2, in spite of incomplete ground cover at the wider row spacing. Here, we report the effect of two row spacings (0.7 and 1.4 m), in sunflower crops of equal plant density, on interception of radiation, radiation use efficiency (RUE), crop biomass and oil yields in three experiments. Two approaches were used to estimate interception of radiation by the crops: i) periodic diurnal measurements during the crop cycle of interception in the inter-row space and on the sides of the hedgerows; and ii) the use of a olive hedgerow model, adapted to sunflower, to estimate radiation intercepted by the two crop arrays. Biomass at anthesis, at physiological maturity, and oil yield in crops grown in the two row-spacings did not differ. The results show that the increased RUE of the 1.4-m spaced crops in pre-anthesis phase was able to compensate for the delay of these crops in achieving close to full interception of irradiance. The daily patterns of irradiance incident on the hedgerow faces, especially the sunlit face, in the pre-anthesis phase and in the early and late hours of the day, were strongly affected by row-spacing. A more favorable irradiance in these periods of the day in the mid- and upper portions of the sunlit face of the 1.4-m spaced hedgerows may have contributed to the greater RUE values of these crops in the pre anthesis phase.The hedgerow model provided descriptions of the seasonal dynamics of intercepted radiation that were fully consistent with measured values for both row spacings.