Foliar Application of Mn and Zn Amino Acid Complexes in Coffee Plants: In Vivo XRF Monitoring under Contrasting Relative Humidity

This study evaluated whether amino acid–metal complexes (histidine or glutamate) enhance foliar uptake and translocation of manganese (Mn) and zinc (Zn) in coffee plants (Coffea arabica L.) under contrasting air relative humidity. Coffee seedlings ( cv. Catuaí Amarelo ) were grown under greenhouse conditions and foliar-treated with Mn and Zn as sulfate salts or as histidine- or glutamate-based complexes. Plants were maintained under low (35–45%) or high (70–86%) relative humidity. In vivo uptake and short-distance transport were monitored for 168 h using X-ray fluorescence spectroscopy (XRF), with repeated petiole measurements used to track temporal changes in Mn and Zn signals. Both histidine and glutamate complexes increased Mn and Zn uptake and transport compared with sulfate-based sources. Under high humidity, the histidine complex produced the highest Mn and Zn signals in petioles, whereas the glutamate complex showed superior transport performance, favoring Mn movement toward older leaves and Zn toward younger leaves. Under low humidity, the complexes maintained a positive effect on nutrient uptake, highlighting their potential to mitigate humidity-related limitations to foliar fertilization. Amino acid–metal complexes, particularly histidine and glutamate formulations, improve foliar Mn and Zn efficiency and internal redistribution in coffee, with benefits maintained even under low relative humidity, supporting more productive and sustainable coffee management.