The high tolerance of different pomegranate cultivars to the excess of Boron in irrigation water is due to their capacity to limit Boron transport from the root to the leaves

Background: Presently, irrigation waters often have a high concentration of boron (B), and the fruit trees in the Mediterranean areas, in general, are exposed to a high risk of B toxicity. Aims: To test the hypothesis that pomegranate trees are very tolerant to the presence of B in the irrigation water, to elucidate the physiological mechanisms behind this tolerance, and to assess differences between different varieties. Methods: In this study, the physiological and nutritional behavior of three pomegranate cultivars ('Mollar de Elche', 'Valenciana', and 'Wonderful'), treated with five B concentrations (0.25, 1.25, 2.5, 5.0, 10.0 mg L-1), were studied. At day 30 and 60 after the start of the treatments, growth measurements, gas exchange parameters and organic solutes were recorded. Results: Almost no significant differences were observed between the B treatments for any of the cultivars. Increasing the B concentration in the nutrient solution increased B concentrations in the leaves, but they never exceed 40 mg kg(-1) of B. In the roots, however, B concentrations were very high, up to 400 mg kg(-1). Conclusion: Pomegranate trees are very tolerant to B excess due to their ability to accumulate B in great quantities in the root, without causing toxicity to this part of the plant, thus limiting its transport to the shoots.