Effects of zinc deficiency on growth, photosynthetic characteristics, and endogenous hormone content of apple sapling ‘Tianhong 2/Jizhen 2’

Objectives The study investigates the growth response, photosynthetic characteristics, and changes in endogenous hormones in young ‘Tianhong 2 / Jizhen 2’ trees due to zinc deficiency.

Methods One-year-old ‘Tianhong 2 / Jizhen 2’ rootstock-spike combined apple saplings were used as experimental materials. Three zinc concentrations were set up in nutrient solution: 0 µmol/L (Zn0), 2 µmol/L (Zn2), and 4 µmol/L (Zn4, control). We measured the plant height and leaf area of young trees and determined zinc content, photosynthetic characteristics, chlorophyll content, chlorophyll fluorescence parameters and endogenous hormone in leaves at 40 days after treatment. We studied the relative expression levels of the enzyme encoding gene MdHEMA1, chlorophyll degradation key enzyme encoding gene MdPAO, and auxin synthesis genes MdYUCCA4a and MdYUCCA6a.

Results The plant height, leaf area, and leaf zinc content of ‘Tianhong 2/Jizhen 2’ saplings (P<0.05) decreased with decreasing zinc concentration. The lower the zinc concentration, the lower the relative expression of auxin synthesis genes MdYUCCA4a and MdYUCCA6a and IAA and GA₃ levels in plants. Compared with the control, net photosynthetic rate (Pn), transpiration rate(Tr), and stomatal conductance (Gs) (P<0.05) decreased, while intercellular CO₂ concentration (Ci) increased under zinc deficiency. Under zinc stress, chlorophyll a, b, and total chlorophyll content decreased, and the chlorophyll a/b ratio increased. Chlorophyll synthesis enzyme encoding gene MdHEMA1 and chlorophyll degradation enzyme encoding gene MdPAO (P<0.05) increased. The chlorophyll fluorescence parameters Fv/Fm, ΦPSⅡ, qP and ETR of ‘Tianhong 2/Jizhen 2’ seedlings (P<0.05) decreased under zinc deficiency, while Fo and NPQ increased.

Conclusions The expression of auxin synthesis genes MdYUCCA4a and MdYUCCA6a decreased under zinc deficiency, and IAA synthesis was blocked. This decreased IAA and GA₃ levels, inhibiting the young tree growth. The PSⅡ reaction center and PSⅡ oxidation transfer chain were damaged to some extent under zinc deficiency, resulting in decreased PSⅡ activity, chlorophyll degradation, and decreased photosynthesis. Simultaneously, the high expression of the chlorophyll degradation gene MdPAO caused the tree to produce a feedback regulation mechanism to protect itself. It induced the expression of the key enzyme gene MdHEMA1 in the chlorophyll synthesis metabolism pathway. However, the synthesized chlorophyll did not compensate for the loss of chlorophyll degradation caused by zinc deficiency.