Analyses of the photosynthetic characteristics, chloroplast ultrastructure, and transcriptome of apple (Malus domestica) grown under red and blue lights

Abstract

Background: Light quality significantly affects plant growth and development, photosynthesis, and carbon and nitrogen metabolism. Apple (Malus domestica Borkh.) is a widely cultivated and economically important fruit crop worldwide. However, there are still few studies on the effects of different light qualities on the growth and development of apple seedlings.

Results: In this study, we explored the effects of blue and red light treatments on the growth and development, photosynthetic characteristics, leaf chloroplast ultrastructure, and carbon and nitrogen metabolism of apple seedlings. Blue light significantly inhibited apple plant growth and leaf extension, but it promoted the development of leaf tissue structures and chloroplasts and positively affected leaf stomatal conductance, the transpiration rate, and photosynthetic efficiency. The red light treatment promoted apple plant growth and root development, but it resulted in loosely organized leaf palisade tissues and low chlorophyll contents. The blue and red light treatments enhanced the accumulation of ammonium nitrogen in apple seedlings. Moreover, the blue light treatment significantly promoted nitrogen metabolism. Additionally, an RNA-seq analysis revealed that both blue light and red light can significantly up-regulate the expression of genes related to carbon and nitrogen metabolism. Blue light can also promote amino acid synthesis and flavonoid metabolism, whereas red light can induce plant hormone signal transduction. The expression of a gene encoding a bHLH transcription factor (MYC2-like) was significantly up-regulated in response to blue light, implying it may be important for blue light-mediated plant development.

Conclusions: Considered together, blue and red light have important effects on apple growth, carbon and nitrogen metabolism. These findings may be useful for determining the ideal light conditions for apple cultivation to maximize fruit yield and quality.

Keywords: Apple; Carbon and nitrogen metabolism; Growth and development; Red and blue light.

Fig. 1

Treatment of apple seedlings under different light quality conditions. a Plants growing in the growth chamber under different light quality. b Light spectra of different treatments: blue light with a maximum peak emission at 446 nm, red light with a maximum peak emission at 664 nm

Fig. 2

Effects of Blue and Red Light Treatments on the Growth and Leaves Development of Apple seedlings. a Growth status of apple seedlings after 30 days of different light quality treatments. Scale bars = 10 cm b Plant height, c stem thickness and e leaf area after of apple seedlings after 30 days of different light quality treatments. Values are means ± SD of three independent biological replicates. The uppercase letter indicates a very significant difference at the 1 % level (P < 0.01), and the lowercase letter indicates a significant difference at the 5 % level (P < 0.05). d Comparison of apple leaf morphology after 30 days of different light quality treatments. The leaves were taken from the third to fifth functional leaves, as shown by the arrows in Fig. 2a. Scale bars = 2 cm. f Leaf anatomy of apple leaves treated with different light quality. E: epidermis; PT: palisade tissue; VB: vascular bundle; W: white light; B: blue light; R: red light. Scale bars = 100 μm. g Chloroplast ultrastructure of apple leaves under different light conditions. The bars shown are 10 μm, 2.5 μm, 1 μm, and 0.5 μm from left to right respectively. M: Mitochondria; S: Starch grain; O: osmiophilic granules; SL: stroma lamella

Fig. 3

Effects of different light quality on photosynthetic characteristics of apple seedlings. a Chlorophyll a, b chlorophyll b and c carotenoid content in apple leaves after different light quality treatments. Chlorophyll fluorescence parameters of apple seedlings under different light conditions, including: d actual photochemical efficiency (ΦPSII); e the maximum photochemical efficiency of PSII under dark adaptation (Fv/Fm); f net photosynthetic rate; g Intercellular CO₂ concentration; h Transpiration rate and i Stomatal conductance. Values are means ± SD of three independent biological replicates. The uppercase letter indicates a very significant difference at the 1 % level (P < 0.01), and the lowercase letter indicates a significant difference at the 5 % level (P < 0.05)

Fig. 4

Effects of Different Light Quality on Carbon and Nitrogenous metabolism. a and b The content of soluble sugar and starch in apple leaves after different light quality treatments. c and d Sucrose phosphate synthase (SPS) and Sucrose synthase (SS) activities in apple seedlings under different light quality. e and f The content of nitrate nitrogen and ammonium nitrogen in apple leaves after different light quality. g to i Glutamate dehydrogenase (GDH), glutamate synthase (GOGAT), and nitrate reductase (NR) activities in apple seedlings under different light quality. Values are means ± standard deviation (SD) of three independent biological replicates. The uppercase letter indicates a very significant difference at the 1 % level (P < 0.01), and the lowercase letter indicates a significant difference at the 5 % level (P < 0.05)

Fig. 5

RNA-seq Analysis and and screening of differentially expressed genes (DEGs) between Blue-/red-light and white-light treatment. a DEGs between blue light treatment and white light control. b DEGs between red light treatment and white light control. c The Venn diagram of the DEGs between the blue, red, and white light treatments. d KEGG pathway enrichment analyses of up-regulated DEGs between blue light and white light treatment. e KEGG pathway enrichment analyses of up-regulated DEGs between red light and white light treatment

Fig. 6

The screening of up-regulated DEGs and qRT-PCR validation. a Cluster analysis on the DEGs of all comparison groups. Blue box: Clusters of DEGs up-regulated after blue light treatment; Red box: Clusters of DEGs up-regulated after red light treatment. A total of 30 up-regulated genes were screened in blue-light treatment apple seedlings, and 20 up-regulated genes in red-light treatment. b and c Heatmap analysis compared the qRT-PCR verification and RNA-seq results of 30 DEGs selected in blue-light treatment and 20 DEGs selected in red-light treatment. MdActin was used as internal control gene

Fig. 7

Proposed model of how blue light and red light affect the growth and development of apple seedlings