Abstract
“Fuji” apple trees on Malus prunifolia rootstock showed a higher incidence of bitter pit (a physiological disorder of apple fruit) when ammonium salt rather than nitrate was supplied in the nutrient solution. The number of spots induced by bitter pit was intimately associated with a decrease in the calcium content of the fruits.
Proton transport and (K++Mg2+)-ATPase activity were observed in the microsomal fraction of the fruit flesh of apple. Sodium vanadate at 0.1mM decreased (K++Mg2+)-ATPase activity to 8% of the control and inhibited the proton transport. Neither the proton transport nor the (K++Mg2+)-ATPase activity was inhibitedby nitrate at 50mM. On a continuous sucrose density gradient the distribution of the vanadate-sensitive ATPase activity was different from those of NADPH-cytochrome c reductase and cytochrome c oxidase activities. In apples seriously affected with bitter pit, proton transport and the vanadate-sensitive ATPase were reduced respectively to 37% and 55% of the levels in healthy fruit, while fruits slightly affected with bitter pit showed a higher activity of the vanadate-sensitive ATPase and a similar proton transport activity to that in healthy fruit.
The relationship between bitter pit disorder and the changes in the microsomal proton transport and vanadate-sensitive ATPase activities is discussed.
