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 Ca(2+) 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(2), Ca-citrate complex, and Ca(3)(PO(4))(2) nanoparticles with and without using adjuvant. We also investigated whether Sr(2+) can be employed as Ca(2+) 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 CaCl(2), Ca-citrate, and Ca(3)(PO(4))(2) nanoparticles, respectively. The addition of adjuvant increased the absorption of Ca-citrate to 28%, decreased that of CaCl(2) to 77%, and did not affect Ca(3)(PO(4))(2). CaCl(2) displayed an exponential decay absorption profile with half-lives of 15 h and 5 h without and with adjuvant, respectively. Ca-citrate and Ca(3)(PO(4))(2) exhibited absorption profiles that were closer to a linear behavior. Sr(2+) was a suitable Ca(2+) tracer because of its similar absorption profiles. Furthermore, the use of adjuvant affected the epicuticular crystal structure. Our findings reveal that CaCl(2) was the most efficient Ca(2+) source. The effects caused by adjuvant suggest that CaCl(2) and Ca-citrate were absorbed mostly through hydrophilic and lipophilic pathways.