Apple rootstock seedling M.9-T337 was selected to explore the effect of drought stress. The findings indicated that the relative water content of both the leaf and soil gradually decreased with an increase in drought stress. The water-use efficiency of the leaves increased gradually but decreased sharply after 20 d of drought. Changes in the gas-exchange parameters and chlorophyll fluorescence parameters reflected the gradual decrease in the photosynthetic capacity of the plants with drought stress duration. Infrared thermal imaging showed significant temperature differences between the drought-stressed and control plants after 15 d of drought treatment. When irreversible damage occurred under drought stress, the crop water-stress index and relative water content of the leaf and soil were 0.7, 60.5, and 17.8%, respectively. Based on the results, we formulated a drought stress-grade standard. Further, we established that the best time for irrigation is when drought stress reaches grade 3. Highlights• Infrared thermal imaging was first applied to apple drought detection • A drought stress-grade standard was established in detail • The optimum watering time and critical index of irreversible damage were identified
Red coloration in apples, an important quality trait, is primarily attributed to the accumulation of anthocyanins. Centuries of breeding have produced a wide variety of apples with different levels of anthocyanins in response to genetic and environmental stimuli. The Huashuo apple shows a much darker red color than its sister line, Huarui. Thirteen different anthocyanins were detected in Huashuo and Huarui apples, of which ten were significantly more abundant in Huashuo apples, confirming that the color difference is indeed attributed to high anthocyanins accumulation rather than the types of anthocyanins. In particular, the contents of cyanidin 3-O-galactoside levels were highest among anthocyanins in both cultivars, reaching >5000 μg·g−1 at the last color transition stage in Huashuo apples, while only >3000 μg·g−1 in Huarui apples. Moreover, the expression of most structural genes, especially DFR, CHI, and 4CL associated with anthocyanin synthesis, were higher in Huashuo apples than in Huarui apples. Combined transcriptomics, metabolomics, and qRT-PCR analysis revealed that six transcription factors from the MYB and bZIP transcription factor families likely play key roles in the dark coloring of Huashuo apples. These results provide deeper insights into apple coloring and suggest a series of candidate genes for breeding anthocyanin-rich cultivars.
The red color of apple peel is an important phenotypic and economic trait mainly attributed to anthocyanin accumulation. Apples show a deeper red color at higher altitudes than at lower ones; however, the molecular regulatory network underlying color variation along altitudinal gradients has not been investigated. In this study, the effects of environmental conditions associated with low (124 m) and high (1901 m) altitudes on peel color were assessed through physiological, metabolomic, transcriptomic, and qRT-PCR analyses in Huashuo apple and its sister line, Huarui apple. The content of cyanidin-3-O-galactoside, cyanidin-3-O-arabinoside, and cyanidin-3-O-xyloside was abundant in the high-altitude environment and may contribute to the deeper red color. Transcript levels of structural genes in the anthocyanin synthesis pathway, especially MdCHI, MdCHS, MdANS, and MdDFR, in apple peel were significantly higher at high altitude than at low altitude. Based on the protein interaction prediction and correlation analyses, four transcription factors (MDP0000127691, MDP0000284922, MDP0000758053, and MDP0000074681) could interact with anthocyanin synthesis-related proteins, showing high correlation with anthocyanin accumulation. Therefore, the abovementioned four genes and four transcription factors were predicted to account for the color differences between high and low altitudes. These results provide genetic resources and a theoretical basis for color-oriented fruit improvement.
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