Effects of Seasonal Variation On Total Phenolics and Quality Attributes of Eight Specialty Leafy Greens in Colorado

Leafy vegetables are particularly good sources of bioactive compounds since in addition to being principal photosynthesis sites, leaves are major accrual areas of metabolites. Plants synthesize various secondary metabolites for functions such as defense mechanisms against insects, pathogens, and photooxidation. Effects of these compounds on human health have been well demonstrated in various retrospective and prospective studies. Among various phytochemicals, dietary phenolics have been of particular interest in recent years due to their contribution to human health and organoleptic attributes of leafy greens. Synthesis of these compounds may vary in response to seasonal variation. This study was designed to investigate the effects of seasonal variation on eight selected specialty leafy greens (arugula, komatsuna, mizuna, pac choi, huazontel, amaranth, tai sai, and red iceberg lettuce).

The eight selected taxa were planted in June, mid-July, and September 2008 in triplicate in clay-based soil under high tunnels at the Horticulture Field Research Center of Colorado State University. Total phenolics content, total soluble solids, dry weight, texture, and color coordinates of the greens were analyzed after planting trials.

Except for one taxon (amaranth), all others were significantly lower in total phenolics in early seasons in relation to mid- and/or late seasons (P<0.05). Comparison of the mid-season and late season crops yielded a similar trend. The majority of mid-season taxa showed significantly lower values of total phenolics compared to late season crops. Among the eight taxa, the difference between early- and late cultivations of red iceberg lettuce were notably high and of practical importance. This taxon showed more than 7 times higher level of phenolics compounds in late planting trials relative to early cultivation. Similar trends were observed for total soluble solids and dry matter of the taxa; six taxa were significantly lower in total soluble solids and dry weight content from the early planting trial relative to mid and/or late season trials. Variation in color and texture analyses was lower among the three planting trials.

In could be concluded that, although visual and textural properties of the planted greens were not meaningfully affected by seasonal variation, total phenolics content, total soluble solids, and dry weight of the selected leafy greens varied considerably in response to seasonal production. It appears than greens grown in the late season trial have higher values of total phenolics, total soluble solids, and dry matter relative to early season trials.