The effects of boron and in vitro propagation stress on the morphological and physiological development of broccoli

Boron deficient broccoli plants produce small heads and develop stress symptoms which reduce the marketability of the head. Although the losses in productivity of broccoli due to boron deficiency have been well-documented, the changes in the morphological and physiological development of boron deficient broccoli plants have not been thoroughly examined. The purposes of this project were to examine the influence of boron on the development of broccoli plants and broccoli leaf explants, and to evaluate the potential of using in vitro propagation as a means of regenerating broccoli plants for use in commercial production.

Boron deficient broccoli plants developed deformed young leaves which had reduced chlorophyll levels and low stomatal conductance and transpiration rates. Boron deficiency induced stress symptoms which included scaling of the stalk epidermis, increased phenolic compound and fiber content of the head, and pith deformation. Symptoms of boron deficiency were less severe in plants which were more efficient in boron uptake.

Broccoli plants grown in toxic concentrations of boron had chlorotic leaf margins, reduced chlorophyll content, and low rates of net photosynthesis. However, these plants developed larger heads, which may suggest that the boron demand of developing broccoli heads is not met when plants are grown in non-toxic levels of boron.

Boron deficiency enhanced auxin-stimulated ethylene production, while suppressing root initiation of broccoli leaf explants. Ethylene blocker and promoter studies indicated that the reduction in root initiation was not caused by enhanced ethylene production.

Broccoli plants regenerated from buds were smaller and matured sooner than seed grown plants. Concentration of harvest and uniformity of head size were not improved through clonal propagation. Further, the frequent occurrence of somaclonal variation may limit the commercial use of in vitro propagation.