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Keywords:

  • boron remobilization;
  • boron uptake;
  • phloem sap;
  • xylem sap

Abstract

Root active uptake and remobilization of boron (B) have been accepted as mechanisms contributing to nutrient efficiency under low supply of boron. Here, we examined the existence of these mechanisms in eggplant (Solanum melongena L.) supplied either with luxury (100 μM, B+) or low (7.5 μM, B–) B in the growth medium via semihydroponic cultivation. Boron treatment was marginally not limiting growth thus avoiding side-effects and impairment of acclimation mechanisms of plants. The induction of a B-concentrating mechanism was evident in the roots as B concentration in the xylem sap was only decreased by 23% in B– compared to B+ plants, i.e., B– roots concentrated B by a factor of 2.7 relative to the external solution. Leaf B concentration in the B– treatment decreased by 33% and 40% in young fully expanded and mature leaves, respectively. Larger differences were observed in the soluble B fraction that decreased by 65% in mature leaves. However, both total and soluble B concentrations in developing leaves were almost equal for both treatments exhibiting a pattern commonly observed in B-remobilizing plants. On the other hand, amounts of B export in the phloem sap were small compared to other species in which B is highly mobile. The B export rate from source leaves was slightly increased under low B supply while that of sucrose was not affected. We conclude that the root concentrating mechanism contributes to the alleviation of B deficiency in eggplant under low B supply while B remobilization may also contribute to a lower degree.