Protein and Nonprotein Amino Acids in Citrus Leaves as Affected by Sample Preparation and Species Differences1

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Abstract
Samples were taken from grapefruit Citrus paradise Macf. cv. Marsh, Valencia orange Citrus sinenesis L. Osbeck, and lemon Citrus limetta, ‘Risso’ trees growing in the field in the Citrus Research Center, Riverside, California. Quantitation of 17 protein and 18 nonprotein amino acids of citrus leaves were evaluated for comparative effects of sample drying-methods on the 3 citrus species. Freeze-dried leaves contained significantly higher amounts of nonprotein, and lower amounts of protein amino acids, than the analogous leaves that were oven-dried. This indicates that citrus leaves must be enzymatically deactivated immediately after sampling and kept in a frozen condition in preparation for analysis. Otherwise, accurate assessment of amino acids in the leaves at sampling would not be obtained. Grapefruit and ‘Valencia’ orange leaves contained higher concentrations of glutamic acid, aspartic acid, leucine, lysine, and arginine than of other protein amino acids. Lemon leaves contained more substantial amounts of glutamic acid, arginine, aspartic acid, leucine, and glycine. The nonprotein amino acids proline, serine, alanine, and aspartic acid were found in more substantive amounts in grapefruit and ‘Valencia’ orange leaves, 77 and 69 percent, respectively, of the sum of nonprotein amino acids determined. In lemon leaves, proline, arginine, lysine, serine, and alanine were found to be more concentrated. These 5 nonprotein amino acids constituted 79% of the total of those determined. Proportionally, lemon leaves contained a larger fraction of nonprotein amino acids than grapefruit or ‘Valencia’ orange leaves.

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