Calcium oxalate formation in Lemna minor: physiological and ultrastructural aspects of high capacity calcium sequestration.

•  The function of calcium oxalate (CaOx) raphide crystal formation, and structural features related to regulation of crystal formation, were studied in Lemna minor fronds using physiological and microscopy techniques. •  Specialized crystal-forming cells (crystal idioblasts) increased in number and size; CaOx, but not soluble oxalate, increased in response to increasing calcium in the growth medium. Size and number of idioblasts had a distinct upper limit. •  The CaOx crystals are formed in membranous 'chambers' and connected in rows by parallel membrane sheets, both forming de novo in the vacuole. The chambers, but not parallel membranes, had calcium associated with them. A calcium-binding matrix protein was associated with idioblast vacuoles and crystal formation. •  Lemna crystal idioblasts function as calcium-inducible, specialized high-capacity but saturable sinks for bulk regulation of calcium, and crystal deposition is a highly controlled process requiring intravacuolar membrane systems and calcium-binding organic matrix materials.