Mitochondrial response in the apical and lateral flower buds of the Hanfu apple to cold stress during the dormancy stage

In order to study the different physiological bases of cold tolerance in the apical flower buds (AFB) and the lateral flower buds (LFB) of the Hanfu apple (Malus domestica Borkh), we used 4-year-old grafted Hanfu plants as material and evaluated the physiological characteristics of mitochondria in the flower buds, such as electron transport chains (cytochrome pathway and alternative pathway), H2O2 content, mitochondrial membrane permeability transition (mPT), and MDA content. AFBs and LFBs showed different changes in total respiratory rate (Vt) during low-temperature stress, except that both reached the lowest Vts at −30 °C. The AFB Vt increased to a peak at −25 °C and decreased sharply to its minimal value at −30 °C, and then remained relatively low. In contrast, the LFB Vt decreased to its minimal value at −30 °C and increased sharply to a peak at −35 °C and then decreased again. In both AFBs and LFBs, the cytochrome pathway was still the main electron transport chain throughout the whole process, and the contributions of the cytochrome pathway (ρVcyt/Vt) and of the alternative pathway (ρValt/Vt) showed similar tendencies to those of Vt as temperature changed. Changes in the AFB mPT were different from those of AFB Vt. LFB mPT zigzagged from peaks at −25 °C and 35 °C. The H2O2 content of the LFBs increased from −10 °C to −30 °C, then decreased slightly from −30 °C to −35 °C, and then increased again. H2O2 content in AFBs went up steadily throughout the whole process. During the early stage of low-temperature treatment, before temperatures reached −35 °C, LFB MDA content remained relatively low and later increased. MDA content in AFBs began to increase from the beginning of treatment. It can be concluded that the higher cold tolerance of LFBs relative to AFBs could be closely related to their higher Vt and ρValt/Vt, which may aid adaptations to stress by supplying energy and metabolic substrates under low-temperature stress conditions.