Changes in cucumber root adenine nucleotide contents during calcium starvation

Ca²⁺ starvation causes a remarkable increase in the level of alkaline phosphatase-isozyme which effectively hydrolyzes nucleoside di- and triphosphates but not nucleoside 5'-monophosphates, in cucumber roots (12). A comparison of adenine nucleotide contents between roots grown with or without Ca²⁺ was therefore attempted to see whether increased alkaline phosphatase plays a role in the regulation of nuc1eotide contents. ATP content of the Ca²⁺-starved roots did not significantly change during the first 3 days fo11owing the removal of Ca²⁺, and then rapidly decreased. On the other hand, the ATP content of the control roots increased, up to towfold, during the 3-day period, and thereafter decreased to the original level. The levels of both ADP and AMP decreased rapidly after Ca²⁺ starvation, while those in the control roots hardly changed throughout growth. With Ca²⁺ deficiency, the phosphohydrolyzing activities toward ATP and ADP greatly increased, but not toward AMP. Re-addition of Ca²⁺ to the deficient medium, which reverses the increase in the activity, had no effect on the ATP content. Thus, Ca²⁺ starvation caused a great reduction in a11 adenine nucleotide contents of the cucumber roots. It seems, however, that the changes in the nucleotides may not be directly regulated by the alkali ne phosphatase induced by Ca²⁺ starvation.