Light spectra modify nitrogen assimilation and nitrogen content in Quercus variabilis Blume seedling components: A bioassay with 15N pulses

The light spectra that reach plants change across different shading conditions, may alter the pattern of nitrogen (N) uptake and assimilation by understory regenerations that are also exposed to N deposition. We conducted a bioassay on Chinese cork oak (Quercus variabilis Blume) seedlings subjected to five-month N pulsing with 15NH4Cl (10.39 atom %) at 120 mg 15N plant-1 under the blue (48.5% blue, 33.7% green, and 17.8% red), red (14.6% blue, 71.7% red, 13.7% green), and green (17.4% blue, 26.2% red, 56.4% green) spectra provided by light-emitting diodes (LEDs). Half of the seedlings were fed twice a week using a 250 ppm N solution with added phosphorus, potassium, and micro-nutrients, while the other half received only distilled water. Neither treatment affected growth of height, diameter, or leaf area. Compared to the red light spectrum, the blue light treatment increased chlorophyll and soluble protein contents and glutamine synthetase (GS) activity, root N concentration, and N derived from the pulses. The green light spectrum induced more biomass to allocate to the roots and a higher percentage of N derived from internal reserves compared to the other two spectra. The 15N pulses demonstrated no interaction with spectra but weakened the reliance on N remobilization from acorns, strengthened biomass allocation to shoots, and induced higher chlorophyll content, GS activity, and N concentration. In conclusion, the red light spectrum should be avoided for Q. variabilis regenerations whose biomass allocation to underground organs are weakened under this condition.

. Typical performance of Chinese cork oak (Quercus variabilis Blume) seedlings exposed to blue (48.5% blue, 33.7% green, and 17.8% red), red (14.6% blue, 71.7% red, and 13.7% green), and green (17.4% blue, 26.2% red, 56.4% green) colors of light-emitting diode (LED) spectra. Tanks for potted seedlings were 40 × 60 cm. Green tanks contained seedlings subjected to 15 N pulses and black tanks contained seedlings subjected to water addition of the same volume. This study was conducted as a split-block design with the main block as three LED spectra, each of which 158 harbored two 15 N pulse treatments. Three iron shelves with LED panels emitting blue, red, and green lights were 159 assigned to one block, and three blocks of shelves were assigned as three replicates that were randomly placed.
160 Four seedlings in one tank (either 15 N pulse or not) per shelf level were assigned as the basic unit of sampling 161 and measurement. Three levels of tanked pots of seedlings were grouped to average the observations for 162 combined spectrum and N treatment.
163 Sampling and Measurement

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All four seedlings per tank were sampled and measured for height and root collar diameter (RCD). Four 165 seedlings were assigned to two groups, with two randomly chosen seedlings per group. One group of seedlings 166 were separated into shoots (leaves and woody stems), roots, and acorn. Roots were washed three times, by tap 167 water once and distilled water twice, to carefully remove substrates without causing damage to the fine roots.
168 All three parts of the seedlings for each group were dried in oven at 65 °C to constant mass then weighed, 169 ground, and measured for total N concentration and 15 N enrichment using a stable isotope ratio mass

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Either lighting spectra or 15 N pulse had a significant effect on chlorophyll-a and chlorophyll-b contents 234 (Table 2). Both chlorophyll-a and chlorophyll-b contents were higher in the blue light spectrum than in the red 235 light spectrum (Figure 2A,B). The 15 N pulse increased the contents of both chlorophyll-a and chlorophyll-b 236 relative to the control ( Figure 2D,E). No effect was found on carotenoid content, which ranged between 0.15 237 and 0.25 mg g -1 (Figure 2C,F).

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The LED spectra had a significant effect on leaf protein content (Table 2), which was higher in the blue 239 light spectrum than spectra from red and green lights ( Figure 3A). The 15 N pulse did not have any impact on leaf 240 protein content ( Figure 3C). Both LED spectra and 15 N pulses had a significant effect on GS activity (Table 2).
241 Again, the blue light spectrum resulted in higher GS activity than red and green spectra ( Figure 3B). The 15 N 242 pulse also produced a significant increase in GS activity compared to the control ( Figure 3D). 243 Biomass Accumulation and Allocation

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Both LED spectra and 15 N pulse had a significant effect on biomass in shoot and root parts ( Table 3). The 245 green light spectrum led to lower shoot biomass than the blue and red spectra ( Figure 4A). In contrast, root 246 biomass in the green light spectrum was higher than in the other two spectra ( Figure 4B)

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Both LED spectra and 15 N pulse treatments had a significant effect on N concentration in most seedling 261 tissue parts except for acorn (Table 3). Shoot N concentration was higher in the green light spectrum than in the 262 red light spectrum ( Figure 5A). However, root N concentration was higher in the blue light spectrum than in the 263 red light spectrum ( Figure 5B). The 15 N pulse resulted in higher N concentration in both shoots ( Figure 5E) and 264 roots ( Figure 5F) compared to the control. In contrast, the 15 N pulse lowered acorn N concentration relative to 265 the control ( Figure 5F).