EFFECT OF RADIATION ENVIRONMENT ON RADIATION USE EFFICIENCY AND GROWTH OF SUNFLOWER

The level of incident radiation and the proportion of radiation that i s diffuse affects radiation use efficiency (RUE) in crops, However, th e degree of this effect, and its importance to growth and yield of sun flower (Helianthus annuus L.) have not been established. A field exper iment was conducted to investigate the effects of radiation environmen t on RUE, growth, and yield of sunflower. A fully irrigated crop was s own on an alluvial-prairie soil (Fluventic Haplustoll) and was exposed to three distinct radiation environments. In two treatments, the leve l of incident radiation was reduced by 14 and 20% by suspending tao di fferent types of polyethylene plastic films well above the crop. In ad dition to the reductions in incident radiation, the proportion of radi ation that was diffuse was increased by about 14% in these treatments. Lower incident radiation and increased proportion of diffuse radiatio n had no effect on total biomass, phenology, leaf area, and the canopy light extinction coefficient (k = 0.89). However, yield was reduced i n shaded treatments due to smaller grain size and lower harvest index. Although crop RUE measured over the entire crop cycle (1.25 g/MJ) did not differ significantly among treatments, there was a trend where RU E compensated for less intercepted incident radiation. Theoretical der ivations of the response of RUE to different levels of incident radiat ion supported this finding. Shaded sunflower crops have the ability to produce biomass similar to unshaded crops by increasing RUE, but have lower harvest indices.