Use of Pyruvate Kinase Activity of Leaf Extracts for the Quantitative Assessment of Potassium and Magnesium Status of Tomato Plants

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Abstract
The activity of pyruvate kinase (PK) in extracts from tomato leaves has been measured in the absence of added KC1 or MgSO4 and compared with the activity obtained in the presence of optimum concentrations of KC1 and MgSO4. The activity in the absence of added K+ but withadded Mg2+(10μmole MgSO4ml−1), when expressed as a percentage of the activity obtained in the presence of added K+ (50μmole KC1 ml−1) and with added Mg2+ reflected closely the K content of the leaf tissue. However, when assayed under the above conditions the enzyme activity did not change significantly with increases in tissue K levels above 3·5 g K per 100 g d. wt of leaf. By modifying the assay conditions a positive response of enzyme activity to increases in tissue K levels of up to 6 g K per 100 g d. wt of leaf was obtained. Moreover, assayed under these modified conditions, the PK activity of extracts measured in the absence of added Mg2+ but with added K+, when expressed as a percentage of the activity obtained in the presence of added Mg2+ and K+ was closely correlated with the Mg content of the leaf over a wide range of levels (0·05 to 1·5 g Mg per 100 g d. wt of leaf). These ranges of K and Mg levels in leaf tissue exceed those likely to be encountered in tomato crops grown commercially. An indication of sub-optimal K and Mg levels in leaf tissue was obtained before the onset of characteristic deficiency symptoms. The enzyme tests, which can be completed within 2 h, offer an alternative means of assessment of the K and Mg status of tomato plants and are considerably faster than the conventional analytical techniques at present in use.