Evaluation of Calcium Regulating Role of Calcium Oxalate Crystals in Eddo Corms in Hydroponic Solution Containing Calcium at Different Concentrations

Aims: Involvement of calcium oxalate crystals in the maintenance of calcium homeostasis in the corms of eddo in hydroponic solution containing calcium at different concentrations was investigated.

Study Design: Plants of eddo [Colocasia esculenta (L.) Schott var. antiquorum Hubbard & Rehder] cv. Aichiwase was used in this study. Seed corms were planted in plastic pots filled with vermiculite and the plants were sprouted by watering under natural temperature, light and humidity conditions in the green house at Hirosaki University, Hirosaki, Japan. After 2 months the plants were grown in a growth chamber. Plants with new growing corms were subjected to grown in water culture solutions containing one of the four treatments as 0 mM calcium, 1 mM calcium nitrate (control), 15 mM calcium nitrate and 15 mM calcium chloride.

Results and Discussion: Under scanning electron microscopy and optical microscopy, few number of raphide crystals were observed in the cortex and many druse crystals were observed in the lateral part containing cortex, the connecting part between root and corm, and the peripheral part of stele. Very few number of druse crystals were observed in the center part of stele. The number and size of crystals in the lateral part of corms under 1 mM calcium nitrate treatment were significantly higher than that in 0 mM calcium solution and were significantly lower than that in the 15 mM calcium solutions. Calcium mapping images by energy dispersive X-ray spectrometry showed a positive correlation between the amounts of calcium in the lateral part of corms and calcium concentration in treatment solutions. The weight percentage of calcium relative to the total weight of major constituent elements per crystal idioblast was significantly higher than that per parenchyma cell in the lateral part. However, the weight percentage of calcium per parenchyma cell except idioblasts of the lateral part was stable among the treatments. The results suggest that calcium is accumulated in crystals under calcium-excessive conditions and is released from crystals under calcium-deficient conditions for stabilisation of calcium levels in the tissues other than the idioblasts in the lateral part of eddo corm. In addition, the weight percentage of calcium per storage parenchyma cell of stele was stable among the treatments and was significantly lower than the weight percentage of calcium per parenchyma cell of the lateral part.

Conclusion: The results indicate that calcium delivered from roots is mostly entered at the lateral parts of corms and is accumulated in calcium oxalate crystals in the cortex, the connecting part between root and corm, and the peripheral part of stele. As a result, calcium contents of the storage parenchyma cells of stele may remain stable at a low level under different calcium treatments.