Effect of genotype, fertilization, and season on free amino acids in leaves of salad greens grown in high tunnels

The free amino acid composition of salad greens is likely to be affected by the wide variation in light and temperature when grown in high tunnels, as well as by method of fertilization. To test this hypothesis, seven species of salad greens were grown in unheated high tunnels in spring and fall. They were fertilized using organic or conventional fertilization; in plots with perlite fertilized with a complete soluble fertilizer; or with leaf compost fertilized with cotton-seed meal. The concentration of free amino acids in leaf samples was analyzed using liquid chromatography and fluorescence to detect phthaldialdehyde derivatives. The sum of free amino acids varied from 40-150 umol g(-1) dry weight due to harvest date, and from 70-160 umol g(-1) due to species. Most of the effects of harvest date were seasonal, and could be predicted by a second-order function of day of the year. Concentrations were high in early spring and late fall, under cool temperatures. Lettuce and mibuna had the fewest amino acids, and minutina had the most. This result was inversely related to their growth rates. Glutamate and/or glutamine were the predominant amino acids in leaves of all species. As a fraction of all free amino acids, the amount of glutamine in minutina, equal to the sum of all other amino acids combined, while only one quarter of this sum was found in mibuna. Species differed in their distribution of many other individual amino acids, but in general the range across species was less than two-fold. Fertilization had no effect on the sum of free amino acids. It had some effects on individual amino acids, but this result depended on harvest date and species.