Effect of the Calcium on Polygalacturonase (PG) Activity and Synthesis at Different Ripening Stages of Tomato Fruits The Role of Polygalacturonase (PG) in Tomato Fruit Ripening and Effects of Divalent Metal Ions and Ethylene on PG Activity Effects of bagging on light use efficiency of tomato fruit photosystem Ⅱ. Effects of water and fertilizers on nitrate content in tomato fruits under alternate partial root-zone irrigation