Table III.

Number of Ca²⁺-ATPase molecules required (per μm square membrane surface) to account for the observed rates of Ca²⁺ transport into the 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	340	1,000–34,000
	Endodermis plus pericycle	110	330–1,100
Old	Endodermis	480	1,400–48,000
	Endodermis plus pericycle	46	140–4,600
	Exodermis	580	1,700–58,000
	Exodermis plus central cortical layer	14	420–1,400

Pump activity at V_max is 50 Ca²⁺ ions pump⁻¹ s⁻¹. Data also shown for 33% and 1% V_max, and for adding the membrane surface areas of cell layers adjacent to the endodermis and exodermis in young and old root zones.