Boron Mobility and Nutrition in Broccoli (Brassica oleracea var. italica

Abstract

Broccoli (Brassica oleracea var. italica cv Premium Crop) plants were germinated in soil, transferred to vermiculite three weeks later and grown in the glasshouse, then either supplied continuously with boron levels ranging from 0.0 (deficient) to 12.5 (toxic) mg l⁻¹ of nutrient solution or transferred from 2.5 to 0.0 mg B l⁻¹ at the initiation of inflorescence development. At commercial maturity the concentrations of various inorganic and organic solutes in phloem exudates and xylem saps, as well as plant characteristics and elemental composition of the various plant parts, were determined. Under deficient B levels leaf midrib and stem corkiness were evident, together with signs of stem pith breakdown, symptoms which resemble the initiation of the hollow stem disorder. The xylem sap B concentration declined by about 50 % when B was not supplied or was removed after a period of adequate supply; the phloem concentration was unaffected. Also, the decreasing B concentration gradients from mature transpiring tissues to young developing sinks disappeared. Therefore, it is concluded that when B is deficient, it is retranslocated from source leaves in the phloem stream supplying the developing leaves and inflorescence. The data also suggested that at toxic levels B undergoes extensive lateral transfer, probably from xylem to xylem, thereby enhancing the B concentration of developing sinks. The B regime influenced dry-matter partitioning, retranslocation of some elements, and the synthesis and distribution of amino acids and sugars, reflecting the general nature of B involvement in plant processes.