Quantitative Aspects of Leaf Acid Phosphatase Activity and the Phosphorus Status of Tomato Plants

The relation between tomato leaf acid phosphatase activity and leaf tissue P content has been examined, and a study made of the effects of leaf development, variation in nitrogen supply, and variation in the growing medium on this relationship. Tomato plants were grown in sand and given various concentrations of phosphate. Plants were also grown for an initial period in peat containing an adequate level of phosphate, then transferred to peat to which was added 0 or 2.3 kg superphosphate m−3 and supplied with either 50 of 300 μg N ml−1. Expressed on a unit tissue f. wt basis, acid phosphatase activity in the control plants in sand (given 41 μg P mlminus;1) was highest in extracts from the expanding leaves and decreased with leaf maturity. However, when given a reduced supply of phosphate, the enzyme activity in the more mature leaves was equal to, or greater than, that in the expanding leaves. The phosphatase activity increased first in the young, fully-expanded leaves and in the mature leaves (with 4.1 μg P ml−1), but did not increase in the expanding leaves until the supply was restricted to 2.1 μg P ml−1. On closer examination, the increase in enzyme activity appeared to be associated with the P level in the leaf tissues, the activity increasing when the level fell below about 0.25 per cent (g P per 100 g dry wt tissue). The same relation was found with the plants grown in peat, and was independent of the concentration of nitrogen supplied to the plants. The fully expanded leaves showed the best enzyme response when the phosphate supply was restricted and the activity reflected closely the local levels of tissue P. The assay of the enzyme in unpurified leaf extracts is simple and rapid, and could be used in a test to detect P-deficiency in tomato plants.