Spinach (S. oleracea) a crop that can be grown in any season, tends to accumulate nitrate under low light (Okazaki et al., 2008; Proietti et al., 2004). The electrical conductivity (EC) and the ratio of nitrate to other nutrients decrease the concentration of nitrate in lettuce (Lactuca sativa L.) in winter (Gent, 2003). However, it is unclear to what extent the same treatment will reduce nitrate concentrations in spinach.
The rank of nitrate in salad greens including (Eruca sativa, Beta vulgaris, and Brassica oleracea) purchased in the market was arugula > swiss chard > spinach > lettuce > cabbage (Santamaria et al., 1999). For most vegetables, nitrate was related to dry matter. 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). When spinach was grown at low light intensity, the nitrate concentrations were 42 and 13 mmol·kg−1 in leaf blade and 200 and 195 mmol·kg−1 in petiole at dawn and dusk, respectively: there was rapid uptake of nitrate in the first 6 h of night, then it remained constant until dawn (Steingrover et al., 1986a).
Removing nitrate from the solution before harvest is one method to lower nitrate in hydroponic spinach. Transferring spinach to a nitrogen-free solution for 2 or 3 d lowered leaf nitrate and increased ascorbic acid (Mozafar, 1996). A switch to ammonium in a nutrient film production system decreased growth, nitrate, and oxalate, but increased dry matter content (Elia et al., 1998). Supplemental light at night lowered nitrate by 60% and 9% in the leaf blades and petiole, respectively, compared with a control with no supplemental light (Steingrover et al., 1986b). When nitrate was withdrawn for 2 d after reaching market size, it did not affect spinach fresh mass (Fukuda et al., 1999), but sap nitrate was reduced with supplemental light. After 4 d of nitrate withdrawal, leaf blades and petioles had 600 and 4000 mg·kg−1 (10 and 65 mmol·kg−1) nitrate, respectively (Fukuda et al., 1999). When hydroponic spinach in a controlled environment was switched to no nitrate or low light, growth as measured by dry mass ceased at 4 and 5 d with N withdrawal and 4 and 2 d with low light, in the shoot and root, respectively (Buysse et al., 1996).
Spinach and swiss chard have high oxalate concentrations. Oxalate was greater in blades than in petioles and greater in petioles than in roots (Santamaria et al., 1999). Likewise, comparisons of many cultivars of spinach grown in four seasons in the field found differences in nitrate and oxalate concentrations (Kaminishi et al., 2004; Kaminishi and Kita, 2006). Nitrate content differed among seasons. Nitrate was 11% higher in winter than in other seasons and oxalate was 24% higher in winter. This result suggests oxalate will be high when nitrate is high.
There is little effect on yield or quality of hydroponic spinach produced at root-zone temperatures of 20, 24, or 28 °C in summer or 15, 20, or 25 °C in winter. The best yield was at 20 °C for six of nine cultivars in summer and at 25 °C for six of eight cultivars in winter (Ikeda et al., 1995). This variation was caused by increased leaf weight in summer and by leaf width and number in winter.
Differences in the metabolite profiles of hydroponic spinach leaves were measured for different concentrations of nitrate in solution. Soluble amino acids were 5% to 12% of total nitrogen in spinach with free glutamine predominant (Eppendorfer and Bille, 1996). When spinach was grown in a controlled environment and provided 1, 3, or 6 mm nitrate, nitrogen limitation at 1 mm caused a simultaneous rise in foliar levels of phosphate, sucrose, and starch (Robinson, 1997). Under controlled conditions, nitrate increased from dusk to dawn, whereas malate and soluble sugars increased (Steingrover et al., 1986c). There was no change in total osmotic potential in response to changes in sugar concentration in spinach, as there is in lettuce.
We did various experiments to examine the role of irradiance and temperature on the composition of lettuce (Gent, 2014, 2015). Here, we describe the effect of similar studies of composition in spinach grown in hydroponics in a greenhouse. We grew five plantings at different times of year and harvested them in the morning and afternoon to study the effect of irradiance on spinach in hydroponics. We also harvested one planting in summer at 3-h intervals to determine the effects of time of day on metabolites in spinach. We show that spinach metabolism differs from that in lettuce, but it does not affect nitrate concentration.
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