IF YOU CAN UNDERSTAND THE FOLLOWING, I NEED YOUR HELP.
Calcium oxalate is not a nutrient (nor a beneficial source of calcium), and it can be toxic in large doses. Duckweeds can contain up to 2 — 4 percent oxalic acid equivalents by weight. However, oxalate also is found in a great many leafy and very nutritious vegetables, including spinach, swiss chard and others. In these edible vegetables, calcium oxalate is found in at levels up to 0.5 — 1 percent. So, minimizing oxalate has the potential to make duckweeds more nutritious and digestible.
However, published reports of calcium oxalate levels in duckweeds are likely to be misleading. The late Vincent Franceschi (Washington State University, see reference below) demonstrated that the calcium oxalate content of Lemna minor depends greatly on the calcium content of the water on which they are growing. Elevated calcium in the water favors formation of calcium oxalate crystals, and their content can be lowered by growth on low-calcium medium. It seems likely that placing duckweed on soft water for a reasonably short period could lower oxalate content significantly in a practical setting.
The chemical and physical form of oxalate differs in the various duckweeds. Lemna and Spirodela accumulate calcium oxalate crystals. These crystals (raphides and druses) are easy to see with the microscope, especially under polarized light, and are found in certain specialized cells called idioblasts. Wolffia and Wolffiella form oxalic acid, but do not exhibit calcium oxalate crystals. Raphide crystals occur as bunches of long-thin crystals, while druses are shaped like a morning star.
Above: Section of a frond of Spirodela (Landoltia) punctata from Landolt (1986).
Dr = druses, Pi = pigment cells, Ra = raphides
Oxalate in plants is made in the idioblast cells
The metabolic precursor of oxalate is L-ascorbic acid (Vitamin C). Recent research in the duckweed relative Pistia stratiotes indicates that L-ascorbate and oxalate are synthesized within the crystal idioblast cells (Kostman, 2001). To do obtain these results, developing crystal idioblasts were isolated and then incubated with various 14C-labeled compounds and then examined by micro-autoradiography for incorporation of 14C into calcium oxalate crystals. [14C]oxalate gave heavy labeling of crystals, indicating the isolated idioblasts are functional in crystal formation. Incubation with [1-14C]ascorbate also gave heavy labeling of crystals, whereas [6-14C]ascorbate gave no labeling. Labeled precursors of ascorbate (L-[1-14C]galactose; D-[1-14C]mannose) also resulted in crystal labeling, as did the ascorbic acid analog, D-[1-14C]erythorbic acid.