Evidence of mechanical tissue strain in the development of hollow stem in broccoli

The underlying cause of hollow stem in commercially grown broccoli and cauliflower crops has been debated as being either a result of boron deficiency or high plant growth rates. This study provides a detailed account of the timing, site of initiation and pattern of development of hollow stem in broccoli plants grown in the field at standard commercial density (CD; 33,000 plants.m^-2) and high density (HD; 150,000 plants.m^-2). Hollow stem did not develop in plants grown at HD which had relatively tall, slender stems in comparison with plants grown at CD which had relatively short, wide stems. Hollow stem developed in all plants grown at CD just after inflorescence initiation, a time when stems began to rapidly enlarge and starch content in the pith decreased. Cavities first appeared in the upper sections of the stem as a fine fracture either at the edge of or traversing the pith cylinder. Many symptoms associated with a boron deficiency in broccoli and cauliflower were not observed and environmental scanning electron microscopy micrographs of the newly formed cavity revealed cell fracture consistent with mechanical strain. Furthermore, the elliptical shape of stems with hollow stem was consistent with the release of mechanical strain and this may provide a non-destructive method for determining hollow stem occurrence and severity.