Is pH the key reason why some Lupinus species are sensitive to calcareous soil?

Aims: Previous studies have shown that pH, rather than calcium (Ca), is the main reason why some Lupinus species are sensitive to nutrient solutions mimicking calcareous soils; however, a hydroponic system is quite different from soil systems, and plants may respond differently to these two growing conditions. Thus, studies with Lupinus species grown in calcareous soils are needed. Methods: Two calcicole and two calcifuge species were grown in river sand with different Ca forms and amounts, pH levels, and [bicarbonate (HCO₃ ⁻)] (HCO₃ ⁻ concentration, which is produced by calcium carbonate (CaCO₃)). Leaf symptoms, leaf area, gas exchange, biomass, and root morphology were recorded; whole leaf and root nutrient concentrations were analysed. Results: We observed leaf chlorosis of the youngest leaves under high pH (adjusted by KOH) and high pH + high Ca (representing high [HCO₃ ⁻], high pH and high Ca) treatments for all Lupinus species. However, after 2 weeks, leaf chlorosis of all Lupinus species under high pH started to disappear, with calcicole species fully, and calcifuge species only partly recovering. Leaf chlorosis symptoms of calcicole species under high pH + high Ca partly disappeared as well, while those of calcifuge species did not disappear at all. Conclusions: High pH (resulting from either KOH or HCO₃ ⁻) inhibited root growth, and subsequently uptake of some nutrients and shoot growth of Lupinus species. However, the strong buffering capacity of HCO₃ ⁻ is the key factor determining if Lupinus species can survive in calcareous soils. Among all studied Lupinus species, L. pilosus was the most tolerant to high [HCO₃ ⁻] and/or high pH, followed by L. cosentinii and L. angustifolius, while L. hispanicus was the most sensitive.