Foliar Calcium Absorption by Tomato Plants: Comparing the Effects of Calcium Sources and Adjuvant Usage

The deficiency of calcium (Ca) reduces the quality and shelf life of fruits. In this scenario, although foliar spraying of Ca²⁺ 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 CaCl₂, Ca-citrate complex, and Ca₃(PO₄)₂ nanoparticles with and without using adjuvant. We also investigated whether Sr²⁺ can be employed as Ca²⁺ 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 (Solanum lycopersicum L., cv. Micro-Tom) plants were used as a model species. After 100 h, the leaves absorbed 90, 18, and 4% of aqueous CaCl₂, Ca-citrate, and Ca₃(PO₄)₂ nanoparticles, respectively. The addition of adjuvant increased the absorption of Ca-citrate to 28%, decreased that of CaCl₂ to 77%, and did not affect Ca₃(PO₄)₂. CaCl₂ displayed an exponential decay absorption profile with half-lives of 15 h and 5 h without and with adjuvant, respectively. Ca-citrate and Ca₃(PO₄)₂ exhibited absorption profiles that were closer to a linear behavior. Sr²⁺ was a suitable Ca²⁺ tracer because of its similar absorption profiles. Furthermore, the use of adjuvant affected the epicuticular crystal structure. Our findings reveal that CaCl₂ was the most efficient Ca²⁺ source. The effects caused by adjuvant suggest that CaCl₂ and Ca-citrate were absorbed mostly through hydrophilic and lipophilic pathways.