The Effect of Harvest Date and Photoperiod On Yield and NO3-N Concentrations in Organic Leafy Greens.

In leafy green crops, plant tissue nitrate (NO₃-N) concentrations are influenced by environmental and management factors. Although evidence is inconclusive, excessive dietary NO₃-N has been implicated in digestive tract cancers and the European Union has set limits on NO₃-N concentrations in leafy greens. A series of field studies evaluated the effect of harvest date,  photoperiod, and plant part on yield and NO₃-N concentrations in organic leafy greens.  Two lettuce (Lactuca sativa) and two spinach (Spinacia oleracea) cultivars were seeded into plots fertilized with dry fish meal (8-3-0) applied at 168 kg N ha^-1.  An unfertilized control was used for comparison.  At harvest plants were separated into three whorls (outer, inner, center), petioles, and leaf blades.  There were significant two-way interactions between fertility and cultivar, cultivar and plant part, and fertility and plant part. Within the fertilized treatment, lettuce had higher fresh weight NO₃-N concentrations than spinach. Petioles had higher fresh weight NO₃-N concentrations than leaf blades. To compare the effect of photoperiod on NO₃-N concentration, a second study was conducted in Pullman, WA, and Fairbanks, AK.  Two lettuce and two spinach cultivars were sampled and analyzed for NO₃-N concentration over three, 24-hour periods.  Light intensity and duration were measured for all sampling periods.  A third study evaluated the effect of planting and  harvest dates on yield and NO₃-N concentration in Asian greens grown in winter.  A commercial mix of five Brassica spp. was direct seeded in a hoophouse on three different dates.  Initial and subsequent regrowth was harvested.  Each species within the mix was evaluated for NO₃-N concentration and yield. Results for this study are pending.