The role of Ca2+ in the systemic resistance of cucumber young leaf induced by phosphate nutrition

The second leaves of cucumbers were collected every day after spraying the surface of leaves with 75mmol/L K2HPO4. Four kinds of solutions, i.e., water, 1mol/L NaCl, 2% CH3COOH and 5% HCl were used in the sequence to extract different forms of calcium compounds in these leaves. The content of water soluble calcium and calcium pectate decreased, but calcium phosphate increased. Meantime, the activity of resistance related enzymes increased. The changes of Ca2+ localization in cells were investigated with calcium antimonate precipitate electromicroscopic cytochemical methods. When the second leaves were sprayed with K2HPO4, deposits of calcium antimonate became more in the vacuole and chloroplast stroma. Calcium antimonate became much denser in the middle lamella and the X ray microanalysis showed that the Ca/P ratio in the cell wall decreased obviously. It is, therefore, concluded that increased Ca2+ in vacuole and chloroplast and the deposits of calcium phosphate in leaves broke the calcium binding of middle lamella. This process might induce the increase of resistance related enzymes and made cell walls decomposed into oligagalacturonide, which, in turn, induced systemic resistance of cucumber plants to disease.