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PPFD within the vertical farming system (VFS; closed symbols) and the horizontal hydroponic system (HHS; open symbols) plotted against layers in the growth systems. Values indicated with different letters indicate statistically significant differences, whereas those marked with the same letters show statistically similar values. Error bars represent SE (n = 8). VFS, vertical farming system; HHS, horizontal hydroponic system; PPFD, photosynthetic photon flux density.
