Effect of Plant Density on Broccoli (<i>Brassica oleracea</i> L. var. <i>italica</i>) Stem Elongation Using Shade Avoidance Response in Autumn–Winter Cropping

The demand for broccoli (Brassica oleracea L. var. italica) in Japan is increasing, driven by both the fresh market and processing industries. In this study, we attempted to clarify the influence of plant density on broccoli stem elongation in autumn–winter cropping because longer stem lengths and higher head positions have the potential to reduce the labor costs associated with harvesting and thereby increase broccoli production. We set up plant density treatments with approximately 2000, 3000, 4000, 5000, and 6000 plants·10 a−1 (expressed as 2000D, 3000D, etc., hereafter) and measured the stem lengths of two cultivars, ‘Pixel’ and ‘Grandome’ (from the ground surface to the apical bud) over time. Significant differences were observed among the treatments after approximately 800°C of accumulated temperature (AT), which coincided with the onset of leaf overlap among adjacent plants in 6000D. At approximately 1250°C of AT, the stem lengths of ‘Pixel’ under 4000–6000D showed increases compared to those under 2000D and 3000D in response to the accumulation of solar radiation (ASR). Additionally, the vegetation coverages in 2000D and 3000D were 62% and 85%, respectively, indicating room for increase, whereas those in 4000–6000D almost reached their saturation level of over 94%. Further, stem length under 6000D was 11–15% higher than that under 2000D, suggesting that the stem elongation observed in dense planting was due to a shade avoidance response caused by mutual shading. As the plant density decreased, the proportion of branches and leaves increased in ‘Pixel’ and ‘Grandome’, respectively, indicating that the parts that developed under favorable light conditions differed among cultivars. The number of harvested heads decreased as plant density decreased, but there were no significant differences in yield among plant densities in ‘Grandome’. However, the yield of ‘Pixel’ was significantly lower in the lowest plant density (2000D) than in moderate conditions (4000D). In conclusion, there was no restriction on plant density from the perspective of yield, especially for the cultivar ‘Grandome’, which produces a large head, and the density could be adjusted within a range of 2000–6000D to achieve the desired stem length and head size.