Diurnal Fluctuation in Tissue Nitrate Concentration of Field-grown Leafy Greens at Two Latitudes

Abstract

<jats:p>Tissue nitrate (NO<jats:sub>3</jats:sub>) concentration (TNC) in leafy greens generally decreases with increasing light intensity and photoperiod in controlled environment studies. Harvesting late in the day has been recommended as a way to produce leafy greens with lower TNC, although data from field research do not support this recommendation. This study investigated the effect of time of day of harvest on TNC in lettuce (<jats:italic>Lactuca sativa</jats:italic> L.) and spinach (<jats:italic>Spinacia oleracea</jats:italic> L.) grown in the field during the summer at Pullman, WA (lat. 46° N) and Fairbanks, AK (lat. 64° N). Whole plants were sampled every 2 h on three separate, 24-h harvest dates at each latitude. Plants were dried, ground, and analyzed for NO<jats:sub>3</jats:sub>-N. At the high-latitude location, TNC decreased linearly during the day (1000 to 2300 <jats:sc>hr</jats:sc>) on all three dates for spinach and one for lettuce. At the low-latitude location, TNC decreased linearly during the day (1000 to 1900 <jats:sc>hr</jats:sc>) on one date and increased linearly during the night (2000 to 0400 <jats:sc>hr</jats:sc>) on two dates for lettuce. The TNC (average 287 to 607 mg NO<jats:sub>3</jats:sub>-N/kg fresh weight for lettuce and 141 to 189 mg NO<jats:sub>3</jats:sub>-N/kg fresh weight for spinach) and magnitude of diurnal fluctuation (generally less than 25%) should not pose a human health risk regardless of when plants are harvested.</jats:p>

Composition of hydroponic lettuce: effect of time of day, plant size, and season

“…This increased by 5, 15, and 5 mmol kg −1 fw on three nights, and decreased through the day 24. For field lettuce of 65 or 73 g fw grown at 46° and 64°N latitude under 8 or 5 h of night, average nitrate concentration was 20 and 43 mmol kg −1 fw, respectively 15. Under an 8 h night, nitrate increased linearly during two of three nights and decreased during one day, whereas under a 5 h night there was a linear decrease only during one day.…”

“…The magnitude of diurnal variation in nitrate and sugars is also likely to depend on light intensity and temperature. A study comparing field‐grown lettuce and spinach grown at similar temperatures but different day lengths showed that nitrate varied more when grown under a longer day length 15…”

A Review of Environment Effects on Nitrate Accumulation in Leafy Vegetables Grown in Controlled Environments

“…During the daytime or under light conditions, nitrates stored in the vacuole are exported to the cytosol via members of the nitrate transporter 1 (NRT1)/peptide transporter (PTR) family (NPF), including NPF5.12 and NPF1.2, and are reduced by NR [ 50 , 51 ]. Thus, harvesting at the end of the light period has been recommended as a way of producing vegetables with low nitrate concentration [ 52 ]. When lettuce was grown under red (638 nm) light via LEDs under a day/night temperature of 21/17 °C and light intensity of 150–160 μmol m −2 s −1 , continuous light (CL) for 22 days was beneficial for decreasing the nitrate concentration in ‘Solvano’ baby leaf lettuce, whereas it led to excessive nitrate accumulation in ‘Multired 2’ and ‘Multired 4’ red-leaf lettuce [ 52 ].…”

“…Thus, harvesting at the end of the light period has been recommended as a way of producing vegetables with low nitrate concentration [ 52 ]. When lettuce was grown under red (638 nm) light via LEDs under a day/night temperature of 21/17 °C and light intensity of 150–160 μmol m −2 s −1 , continuous light (CL) for 22 days was beneficial for decreasing the nitrate concentration in ‘Solvano’ baby leaf lettuce, whereas it led to excessive nitrate accumulation in ‘Multired 2’ and ‘Multired 4’ red-leaf lettuce [ 52 ]. Zhou et al [ 53 ] reported that CL for 48 h using a mixture of red and blue LEDs (red:blue = 4:1) with a photosynthetic photon flux density (PPFD) of 200 μmol m −2 s −1 significantly reduced the nitrate concentration in hydroponic lettuce.…”

Effect of Irradiance and Temperature on Composition of Spinach

“…A diurnal pattern occurred for nitrate uptake and reduction in spinach. Diurnal variation of nitrate in spinach was more common than for lettuce in studies of the two crops grown at 46°and 64°N latitudes (Neely et al, 2010). Nitrate in the shoot increased over a 14-h dark period and decreased in the light in a controlled environment (Scaife and Schloemer, 1994).…”