Evaluation of Calcium Regulating Roles of Guttation and Calcium Oxalate Crystals in Leaf Blades and Petioles of Hydroponically Grown Eddo

We investigated the involvement of guttation and calcium oxalate crystals in the maintenance of calcium homeostasis in leaf blades and petioles of eddo in hydroponic solution containing calcium at different concentrations. Under scanning electron microscopy, two types of crystals were observed in leaf blades and petioles: raphides (bundles of needle-shaped crystals) and druses (multifaceted conglomerate crystals). The number and size of crystals of leaf blades and petioles in 1 mM calcium treatment (control) were smaller than that in 15 mM calcium treatments and larger than that in 0 mM calcium treatment. Calcium contents of leaf blades, petioles and whole plants increased with the increase of calcium concentration in the treatment solution. In addition, calcium-mapping images demonstrated a positive correlation between the amount of calcium in crystal idioblasts and concentration of calcium in treatment solutions. On the other hand, the weight percentage of calcium per mesophyll cell (spongy cell and palisade cell) of leaf blades and per normal parenchyma cell of petioles was stable irrespective of calcium treatment conditions. These results suggest that calcium accumulates in crystals under calcium-excessive conditions and is released from crystals under calcium-deficient conditions to stabilize calcium levels in leaf tissues other than the idioblasts. A positive correlation was observed between the calcium concentration of guttation fluid (µg mL-1), the total amount of calcium in guttation fluid (µg leaf-1 night-1) and the calcium concentration of treatment solutions. These results suggest that guttation eliminates excess calcium and would be involved in maintaining calcium ion homeostasis in eddo.