Low Red to Far‐Red Ratios Reaching the Stem Reduce Grain Yield in Sunflower

The red light (R) to far‐red light (FR) ratio reaching the stem of crop plants is reduced by increasing canopy densities. We sought to investigate whether this light signal that increases resource allocation to the stem affects yield in sunflower ( Helianthus annuus L.) plants. The stem of sunflower plants grown outdoors in pots at very low densities was exposed to low R/FR ratios by placing selective plastic filters around this organ. Control plants had a clear filter around the stem and received the R/FR ratio of unfiltered sunlight. In experiments with the open pollinated cultivar Guayacán II, the hybrid ‘Dekalb G100’, and the line MA 566, low R/FR ratios promoted stem extension growth and dry matter accumulation without affecting the ratio between stem dry matter and length. In comparison with the sunlight R/FR ratio controls, low R/FR ratios reduced grain yield and grain number per plant, without affecting grain dry weight. No significant interaction of the R/FR ratio with cultivar or sowing date was observed. The negative correlation between stem growth and yield did not extend to the genetic variability observed for plant stature and grain yield. We propose that the promotion of stem growth by the low R/FR ratios typical of dense stands could reduce the resources available for grain yield in sunflower crops.