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.