Unveiling Foliar-Applied Calcium Translocation in Tomato: Evidence from Leaf-to-Fruit Movement Using Strontium Tracer

Calcium (Ca) deficiency can impair fruit development even under optimal soil Ca levels due to its transpiration-dependent transport. Because fruits exhibit significantly lower transpiration rates than leaves, any further reduction in fruit transpiration during development can limit Ca delivery, leading to lower fruit Ca content and diminished quality. Foliar application of Ca offers a potential strategy to mitigate these effects; however, its low mobility in the phloem often limits treatment efficacy. To better understand this phenomenon, we employed X-ray fluorescence spectroscopy (XRF) to investigate the penetration and transport of foliar-applied Ca, using strontium (Sr) as a physiological tracer. Based on the prevailing paradigm that Ca is largely immobile in the phloem, we evaluated the influence of osmotic regulators, sucrose, mannitol, glycerol, and potassium, on Ca transport. Results showed that Sr was effectively translocated to distal tissues. While potassium and mannitol had no significant impact on transport kinetics, sucrose and glycerol showed a trend toward enhancing Sr movement. XRF imaging of leaf tissue revealed that Sr was primarily transported through the apoplast toward the leaf margin. Moreover, foliar application of Sr combined with sucrose significantly increased Sr accumulation in seeds and in the apical portion of tomato fruits. These findings suggest that sucrose has the potential to actively enhance foliar-applied Ca mobility by acting as an osmotic regulator, accelerating short-range apoplastic movement within leaf tissues and long-distance systemic translocation to the fruit.