We conducted indoor experiments on a local variety of water spinach in Taiwan to analyze the effect of the irradiation ratio of blue (
B) to red (
R) light on the morphological characteristics of stem tissue. The light spectrum was regulated by using a combination of different fluorescent lamps, and 4 different irradiation treatments were applied for the change of the energy ratio of
B (450 ± 10 nm) /
R (660 ± 10nm) from 2.0 to 23.9. The photosynthetic photon flux density at the top of the shoot was maintained at approximately 200 μmol m
-2 s
-1. Seedlings were grown for 10 days after the expansion of the cotyledon in each plot. Cross-sections of the centre of the first internode were observed microscopically; the thickness of the cortex and pith and the diameter of the stem and cavity were measured. The stem diameter increased significantly at the higher
B/R ratio, while the cortex thickness did not change. The pith thickness increased significantly at the higher
B/R ratio, and was positively correlated to the stem diameter (r = 0.808,
P < 0.001). The increase of the pith thickness and stem diameter was associated with the increase in the cell density and number of cells in the pith with tissue. The cavity diameter was significantly small when the
B/R ratio was lowest. These results indicate that a higher
B/R ratio enhanced the increase of the stem diameter, pith thickness and the density and number of cells in the pith in water spinach.
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