| Summary: | The deficiency of calcium (Ca) reduces the quality and shelf life of fruits. In this scenario, although foliar spraying of Ca<sup>2+</sup> has been used, altogether with soil fertilization, as an alternative to prevent deficiencies, little is known regarding its absorption dynamics by plant leaves. Herein, in vivo microprobe X-ray fluorescence was employed aiming to monitor the foliar absorption of CaCl<sub>2</sub>, Ca-citrate complex, and Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> nanoparticles with and without using adjuvant. We also investigated whether Sr<sup>2+</sup> can be employed as Ca<sup>2+</sup> proxy in foliar absorption studies. Moreover, the impact of treatments on the cuticle structure was evaluated by scanning electron microscopy. For this study, 45-day-old tomato (<i>Solanum lycopersicum</i> L., cv. Micro-Tom) plants were used as a model species. After 100 h, the leaves absorbed 90, 18, and 4% of aqueous CaCl<sub>2</sub>, Ca-citrate, and Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> nanoparticles, respectively. The addition of adjuvant increased the absorption of Ca-citrate to 28%, decreased that of CaCl<sub>2</sub> to 77%, and did not affect Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>. CaCl<sub>2</sub> displayed an exponential decay absorption profile with half-lives of 15 h and 5 h without and with adjuvant, respectively. Ca-citrate and Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> exhibited absorption profiles that were closer to a linear behavior. Sr<sup>2+</sup> was a suitable Ca<sup>2+</sup> tracer because of its similar absorption profiles. Furthermore, the use of adjuvant affected the epicuticular crystal structure. Our findings reveal that CaCl<sub>2</sub> was the most efficient Ca<sup>2+</sup> source. The effects caused by adjuvant suggest that CaCl<sub>2</sub> and Ca-citrate were absorbed mostly through hydrophilic and lipophilic pathways.
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