The effect of plant age on asparaginase activity and amino acid levels in developing leaves of Pisum sativum

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Abstract
Asparaginase activity in young leaves of Pisum sativum was determined at various stages of plant development. Diurnal variation of asparaginase activity (with an increase in the light period) in emerging and half-expanded leaves was detected in young plants, but a much greater diurnal variation took place in young leaves of older plants. Levels of total .alpha.-amino nitrogen in both emerging and half-expanded leaves varied during the photoperiod (decreasing in the light) and there was a progressive decrease in overall levels in successive leaves at the same developmental stage; the most pronounced changes were noted in the levels of asparagine. The concentrations of asparagine and homoserine varied in xylem sap exudates, decreasing during the light period and also decreasing overall as plants aged. Since the activity of asparaginase in both emerging and half-expanded leaves is highest in mature plants, when levels of .alpha.-amino nitrogen are the lowest, the amount of asparaginase in developing pea leaves may be subject to a form of nitrogen catabolite repression by amino supplied to, and present within, the leaf.
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