Inorganic pyrophosphatases in leaves during plant development and senescence

The activities of inorganic pyrophosphatases were determined in developing and senescing leaf tissues as indicators of biosynthetic activity. The level of alkaline inorganic pyrophosphatase varied with leaf age and leaf position on the plant. In younger plants alkaline inorganic pyrophosphatase increased up the plant to a maximum and then decreased with leaf proximity to the apex. In older plants the enzymatic activity decreased in the lower leaves and the maximum level occurred in leaves closer to the apex. Retardation of leaf senescence with benzyladenine maintained the high levels of enzyme activity. In cucumber plants removal of the shoot at any time before cotyledon death rejuvenated the cotyledons leading to excessive growth, regreening, and dramatic increases in alkaline inorganic pyrophosphatase. Alkaline inorganic pyrophosphatase activity correlated highly with 14C-leucine incorporation into protein of bean leaf discs, supporting the view that this enzyme is involved in making biosynthetic pathways irreversibly anabolic. Acid inorganic pyrophosphatase activity was not proportional to the alkaline activity, suggesting that two separate enzymatic proteins are being assayed. It is concluded that the level of alkaline inorganic pyrophosphatase can be used to indicate which plant parts have high biosynthetic capacity. Also, delayed senescence of leaves or cotyledons, with benzyladenine or apex excision, is associated with maintained biosynthesis.