Boron distribution and retranslocation in field-grown broccoli (Brassica oleracea var. italica)

Abstract

A 2-yr study was conducted to investigate the distribution and retranslocation of boron (B) in field-grown broccoli (Brassica oleracea var. italica). Two broccoli cultivars were grown under various B treatments (no supplemental B, broadcast B, and broadcast B together with one to several liquid applications to the surface of the soil immediately surrounding the stem base) at three Ontario locations where the soils differ markedly in both texture and hot-water-soluble B concentration. Yield responses to B application and external symptoms of B deficiency were not observed. In the absence of supplemental B, the B concentrations of the different plant parts were not correlated with either extractable soil B or plant-organ age (which would have reflected primary translocation via the transpiration stream). Supplemental B at rates of 3.0–4.5 kg ha⁻¹ enhanced the B concentration in xylem sap and eliminated the increasing B gradient up the shoot or induced the formation of a decreasing gradient up the shoot. Floret B concentration and content were rarely affected by the B treatments. The ratios of B concentrations in the sink regions (florets or young leaves) to those in the source leaves suggest that B was effectively retranslocated in plants with a relatively low boron status. Higher concentrations of B in phloem sap than in xylem sap are consistent with B being a phloem-mobile element. Key words: Boron nutrition, Brassica, broccoli, nutrient distribution, retranslocation