A microscopic investigation of fibre development in Cannabis sativa

Hemp (Cannabis sativa) produces one of the strongest natural fibres and is an important raw material due to its high tensile strength. The physical and chemical properties of hemp fibres are influenced by many factors, including the hemp crop variety, method of cultivation, stage of plant growth, plant age and fibre processing technique. During growth, hemp fibres undergo several developmental stages. The fibre cells initially elongate through intrusive growth, after which point the secondary cell wall starts to form. In other fibre crops, such as flax, the thickening of the secondary cell wall is a slower process than cell elongation and can take up to 60 days. These developmental processes are, however, less well understood in hemp due to the presence of both primary and secondary phloem fibres.

Scanning electron microscopy (SEM) images illustrating the serial procedures of secondary cell wall thickening of the primary phloem fibres of *Cannabis sativa* (A-C). The images in the second and third rows show a higher magnification of the primary phloem fibres shown in the first row. Image credit: Kim *et al.*

In their new study published in AoBP, Kim et al. investigated the thickening process of the galactan-enriched layer in the secondary cell wall during the development of the primary phloem fibre in hemp (Cannabis sativa). They also sought to better understand the process by which this galactan enriched layer is modified into a gelatinous layer, which is the largest portion of the mature secondary cell wall. Stem segments of hemp collected at 17, 29, 52 and 62 days after sowing were comparatively examined using light microscopy, and for the first time, scanning electron microscopy and transmission electron microscopy.

In the study, the authors observed the secondary cell wall of hemp to thicken remarkably between 52 to 62 days after sowing, leading to the formation of three distinctive layers. The gelatinous layer observed in primary phloem fibres at 62 days exhibited a characteristic stratified structure and indicates the recent modification of the galactan enriched layer into this gelatinous layer. Kim et al. highlight that after 62 days; the cytoplasm of primary phloem fibres was concentrated into a smaller space and may be a critical point of programmed cell death in these cells. The authors suggest that future studies may help to elucidate the process of secondary cell wall thickening in hemp through investigation of other cellular organelles involved in cell wall synthesis, including the Golgi apparatus and the multivesicular body.