Table IV.

Ca²⁺-ATPases (percent of total membrane protein particles) required for Ca²⁺ movement into onion root stele

Root Zone	Cell Layer(s) Involved	Activity of Ca2+-ATPase Is Vmax	Activity of Ca2+-ATPase Ranges from 33% Vmax to 1% Vmax
Young	Endodermis	11	34–1,100
	Endodermis plus pericycle	5.7	17–570
Old	Endodermis	16	48–1,600
	Endodermis plus pericycle	2.9	8.8–290
	Exodermis	19	59–1,900
	Exodermis plus central cortical layer	0.94	2.9–94

Activity of Ca²⁺-ATPase pump at V_max is 50 Ca²⁺ ions pump⁻¹ s⁻¹. Effects of using 33% V_max and 1% V_max, and of adding the membrane surface areas of cell layers adjacent to the endodermis and exodermis are also considered.