Abstract: Granny Smith is a high-acid apple introduced abroad and suitable for cultivation in China. This study aimed to explore the light adaptation mechanism of Granny Smith trees in different light conditions. A shading experiment was carried out on Granny Smith trees in Ningxia from July to September 2021. We measured photosynthetic characteristics and fluorescence parameters of Granny Smith trees under different shading treatments, to provide scientific basis for the introduction and cultivation of Granny Smith trees. The results showed that: (1) 15% shading increased but 25% shading obviously decreased the chlorophyll content of leaves. The ratio of chlorophyll a/b firstly decreased and then increased with increasing shading degree. (2) 15% shading weakened the midday depression of photosynthesis. The treatments with ≥25% shading changed the photosynthesis diurnal pattern from “double peaks” to “single peak”, and 35% shading significantly reduced the photosynthesis efficiency. Rational shading would be helpful for the optimized light utilization of Granny Smith trees. (3) Shading increased light compensation point of leaves and decreased light saturation point, dark respiration rate, and quantum efficiency. With increasing shading degree, the maximum fluorescence of leaves increased. Shading improved the absorption capacity of light energy, while decreased the activity of PSII reaction center with the increase of Q_A from PSII and the decrease of Q_B. Shading improved light utilization efficiency by increasing the quantum ratio of electron transfer from PSII but reducing the quantum ratio of heat dissipation.

Key words: light, Granny Smith, photosynthetic pigment, photosynthesis, chlorophyll fluorescence.