Iron Source and Medium pH Affect Nutrient Uptake and Pigment Content in Petunia hybrida ‘Madness Red’ Cultured In Vitro

Abstract

Deficiency or excess of iron (Fe) and improper medium pH will inhibit the growth and development of plants, reduce the transfer and utilization of energy from the root to the leaf, and affect the utilization efficiency of inorganic nutrients. The most common symptom of Fe deficiency in plants is chlorosis of the young leaves. In this study, the effects of the iron source, in combination with the medium pH, on plant growth and development, plant pigment synthesis, and nutrient uptake in a model plant Petunia hybrida cultured in vitro were investigated. Iron sulfate (FeSO4·7H2O) or iron chelated with ethylenediaminetetraacetic acid (Fe-EDTA) were supplemented to the MNS (a multipurpose nutrient solution) medium at a concentration of 2.78 mg·L−1 Fe, and the treatment without any Fe 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 carried out in an environmentally controlled culture room with a temperature of 24 °C with 100 µmol·m−2·s−1 photosynthetic photon flux density (PPFD) supplied by white light emitting diodes (LEDs) during a photoperiod of 16 h a day, 18 °C for 8 h a day in the dark, and 70% relative humidity. Regardless of the Fe source including the control, the greatest number of leaves was observed at pH 4.70. However, the greatest lengths of the leaf and root were observed in the treatment with Fe-EDTA combined with pH 5.70. The contents of the chlorophyll, carotenoid, and anthocyanin decreased with increasing medium pH, and contents of these plant pigments were positively correlated with the leaf color. The highest soluble protein content and activities of APX and CAT were observed in the Fe-EDTA under pH 5.70. However, the GPX activity was the highest in the control under pH 4.70. In addition, the highest contents of ammonium (NH4+) and nitrate (NO3−) were measured in the FeSO4-4.7 and EDTA-5.7, respectively. More than that, the treatment of Fe-EDTA combined with pH 5.70 (EDTA-5.7) enhanced nutrient absorption, as proven by the highest tissue contents of P, K, Ca, Mg, Fe, and Mn. The genes’ ferric reduction oxidase 1 and 8 (PhFRO1 and PhFRO8), iron-regulated transporter 1 (PhIRT1), nitrate transporter 2.5 (PhNRT2.5), and deoxyhypusine synthase (PhDHS) were expressed at the highest levels in this treatment as well. In the treatment of EDTA-5.7, the reduction and transport of chelated iron in P. hybrida leaves were enhanced, which also affected the transport of nitrate and catalyzed chlorophyll level in leaves. In conclusion, when the medium pH was adjusted to 5.70, supplementation of chelated Fe-EDTA was more conducive to promoting the growth and development of, and absorption of mineral nutrients by, the plant and the expression of related genes in the leaves.