Transpiration rate affects the mobility of foliar-applied boron in Ricinus communis L. cv. Impala

In plant species not containing polyols, boron (B) is regarded as practically phloem immobile. This has been explained by the high membrane permeability of boric acid (BA) resulting in a rapid efflux out of the phloem and re-transport into the leaf in the xylem. The present study investigated how the xylem flow rate affects the phloem mobility of foliar-applied BA in Ricinus communis L. cv. Impala. Xylem flow rates were varied by exposure of the canopy to different levels of relative humidity (RH). In seedlings with severed hypocotyls, i.e. without xylem flow, B was highly mobile. In intact seedlings and plants, the degree of mobility and the within-plant distribution of B were strongly RH-dependent. At RH of 70% or above, up to 16-24% of the B was translocated to other plant parts, whereas at lower RH no significant movement of B was detected. Only at an intermediate RH (70-80%), did leaf-applied B accumulate in roots. At 100% RH, B transport in the xylem was significantly increased, suggesting that the build up of root pressure induced the recycling of phloem delivered B from roots to shoots. These results indicate that in R. communis phloem B mobility is not constant, but strongly affected by transpiration rates.