Boron accumulation and tolerance in sweet basil (Ocimum basilicum L.) with green or purple leaves

There is a wide variability in plant tolerance to boron (B) toxicity, which is often associated with the ability to limit B accumulation. This study was conducted on two cultivars of sweet basil (Ocimum basilicum L.) with different B tolerance: 'Tigullio', less tolerant and with green leaves; 'Red Rubin', more tolerant and with purple leaves. The main goal was to verify whether the greater B tolerance of 'Red Rubin' is attributable to an exclusion mechanism. In three greenhouse experiments, plants were grown hydroponically with solution B concentration ranging from 0.25 (control) to 25 mg L-1. Tissue B concentration increased with increasing B supply. Boron concentrations in root and leaf tissues were comparable in 'Tigullio' and 'Red Rubin' or even higher in the purple cultivar. Boron supply did not affect the leaf concentration of total phenolic compounds and other nutrients. Leaf concentrations of total phenols and rosmarinic acid were remarkably higher in 'Red Rubin' than in 'Tigullio'. The greater B tolerance of 'Red Rubin' was associated with the ability to withstand higher concentrations of this element in plant tissues rather than to reduced B accumulation in the shoot. The high phenolic content was thought to contribute to the B tolerance of 'Red Rubin'.