Increased light triggers accumulation of manganese and zinc in leaf tips and outer tipburn in lettuce

Abstract Background Tipburn (TB) is a complex physiological disorder occurring on edges and tips of young unexpanded leaves (inner TB) and on outer mature leaves (outer TB/outer marginal necrosis) of lettuce and leafy vegetables. Tipburn causes severe food losses and postharvest problems. The aim of this study was to investigate changes in TB and distribution of metals in different leaf types and segments of lettuce ( Lactuca sativa L ‘Frillice’) in response to increased irradiance (150→300 µmol m −2 s − 1 ), with the main focus on outer TB. Results Based on ICP-AES analysis of separately sampled leaf tissues, high irradiance (HI; 300 µmol m⁻²s⁻¹) was associated with elevated calcium (Ca) and manganese (Mn) concentrations in the margins of mature leaves compared with inner leaf tissues. In contrast, young unexpanded leaves exhibited substantially lower Ca and Mn concentrations than mature leaf margins. Laboratory-based micro-X-ray fluorescence (µ-XRF) elemental mapping was used to image the composition and spatial distribution of metals in healthy and necrotic tips of outer leaves. High accumulation of Mn, zink (Zn), and Ca along with depletion of potassium (K) and chlorine (Cl) was observed in injured parts of the tips. Marked differences in the accumulation and distribution of Mn and Zn between healthy and injured parts in increased light indicate a close association between local metal imbalances and development of outer TB. Accumulation of reactive oxygen species was detected in outer tips in response to HI, and callose deposition was identified in injured cells. Conclusions This study highlights the utility of combining spatially resolved XRF with traditional plant physiological analyses to improve our understanding of the mechanisms underlying TB and the potential association with Mn accumulation.