Relationship between growth rate and the development of hollow stem in broccoli

Boron deficiency is widely accepted as the dominant cause of hollow stem in broccoli (Brassica oleracea L. var. italica Plenck), although high growth rate has also been mentioned in the literature as a possible cause of the disorder. In this study, we investigated the role of growth rate and boron in the development of hollow stem. Two experiments were conducted with broccoli cv. Marathon, using planting density to manipulate growth rate. In the first experiment, plants were grown at 3 planting densities (19 512, 32 520, and 69 686 plants/ha) and either supplemented or not supplemented with foliar boron applications (0.35 kg B/ha) on two occasions. In the second experiment, broccoli plants at 2 planting densities (32 520 and 100 000 plants/ha) were treated in factorial combination from inflorescence initiation onwards with complete trace element fertiliser (1.5% B), 57% light interception, and paclobutrazol (480 g a.i./ha). The incidence and severity of hollow stem were assessed using digital image analysis and were found to increase with plant spacing but were not affected by boron application. The probability of hollow stem occurring increased with absolute growth rate in the first trial, but not the second. Absolute growth rate, measured as dry weight accumulation, was suppressed independently of planting density by the shading treatment, but did not influence the incidence or severity of hollow stem. The application of paclobutrazol did not influence absolute growth rate but reduced the severity of hollow stem at the lowest planting density. Our data support the hypothesis that the rate and pattern of growth are involved in the development of hollow stem, and that a boron deficiency is not the major causal factor.