Iron Supplement-Enhanced Growth and Development of Hydrangea macrophylla In Vitro under Normal and High pH

Hydrangea macrophylla is a popular perennial ornamental shrub commercially grown as potted plants, landscape plants, and cut flowers. In the process of reproduction and production of ornamental plants, the absorption of nutrients directly determines the value of the ornamental plants. Hydrangea macrophylla is very sensitive to the content and absorption of the micronutrient iron (Fe) that affects growth of its shoots. However, the physiological activity of Fe as affected by deficiency or supplementation is unknown. This work aimed at preliminary exploring the relationship between Fe and photosynthesis, and also to find the most favorable iron source and level of pH for the growth of H. macrophylla. Two Fe sources, non-chelated iron sulfate (FeSO₄) and iron ethylenediaminetetraacetic acid (Fe-EDTA), were supplemented to the multipurpose medium with a final Fe concentration of 2.78 mg·L^-1. The medium without any Fe supplementation was used as the control. The pH of the agar-solidified medium was adjusted to either 4.70, 5.70, or 6.70, before autoclaving. The experiment was conducted in a culture room for 60 days with 25/18 °C day and night temperatures, and a 16-hour photoperiod provided at a light intensity of 50 mmol·m^-2·s^-1 photosynthetic photon flux density (PPFD) from white light-emitting diodes. Supplementary Fe increased the tissue Fe content, and leaves were greener with the medium pH of 4.70, regardless of the Fe source. Compared to the control, the number of leaves for plantlets treated with FeSO₄ and Fe-EDTA were 2.0 and 1.5 times greater, respectively. The chlorophyll, macronutrient, and micronutrient contents were the greatest with Fe-EDTA at pH 4.70. Furthermore, the Fe in the leaf affected the photosynthesis by regulating stomata development, pigment content, and antioxidant system, and also by adjusting the expression of genes related to Fe absorption, transport, and redistribution. Supplementation of Fe in a form chelated with EDTA along with a medium pH of 4.70 was found to be the best for the growth and development of H. macrophylla plantlets cultured in vitro.