Effects of Supplemental Lighting on Potassium Transport and Fruit Coloring of Tomatoes Grown in Hydroponics

Abstract

Supplemental blue/red lighting accelerated fruit coloring and promoted lycopene synthesis in tomato fruits. Potassium (K) is the most enriched cation in tomato fruits, and its fertigation improved tomato yield and fruit color. However, the effects of supplemental lighting on K uptake and transport by tomatoes and whether supplemental lighting accelerates fruit coloring through enhancing K uptake and transport are still unclear. We investigated the effects of supplemental light-emitting diode (LED) lighting (SL; 100% red, 100% blue; 75% red combined 25% blue) on K uptake in roots and transport in the fruits as well as the fruit coloring of tomatoes (Micro-Tom) grown in an experimental greenhouse in hydroponics. The use of red SL or red combined blue SL enhanced K uptake and K accumulation as well as carotenoid (phytoene, lycopene, γ-carotene, and β-carotene) content in fruits by increasing photosynthesis, plant growth, and fruit weight. The genes related to ethylene signaling were upregulated by red SL. Quantitative real-time PCR (qRT-PCR) results showed that K transporter genes (SlHAKs) are differentially expressed during fruit development and ripening. The highest-expressed gene was SlHAK10 when fruit reached breaker and ripening. SlHAK3 and SlHAK19 were highly expressed at breaker, and SlHAK18 was highly expressed at ripening. These might be related to the formation of tomato fruit ripening and quality. SlHAK4, SlHAK6, SlHAK8, and SlHAK9 were significantly downregulated with fruit ripening and induced by low K. The expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 were significantly increased by blue SL or red combined blue SL during breaker and ripening. Blue SL or red combined blue SL increased content of phytoene, β-carotene, α-carotene, and γ-carotene and accelerated fruit coloring by enhancing K uptake in roots and transport in fruits during fruit ripening. This was consistent with the expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 during fruit development and ripening. The key genes of photoreceptors, light signaling transcript factors as well as abscisic acid (ABA) transduction induced by blue SL or red combined blue SL were consistent with the upregulated genes of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 under blue SL and red combined blue SL. The K transport in tomato fruits might be mediated by light signaling and ABA signaling transduction. These results provide valuable information for fruit quality control and the light regulating mechanism of K transport and fruit coloring in tomatoes.

Keywords: fruit coloring; potassium; potassium transporters; supplemental lighting.

Figure 1

Plant growth characteristics and K content of tomatoes grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K level. (A) The increase percentage of growth. (B) K content in tomato root, leaf, and fruit at 33, 47, and 61 DAA (days after anthesis). Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). FW: fresh weight, DW: dry weight, SPAD: soil plant analysis development, leaf chlorophyll content, CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 2

Photosynthesis characteristics and photochemistry efficiency of photosystem Ⅱ (PSII) of tomatoes grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K level. (A) Net photosynthetic rates (Pn); (B) Stomatal conductance (Gs); (C) Intercellular CO₂ concentration (Ci); (D) Transpiration rate (Tr); (E) Maximum quantum efficiency of PSII (Fv/Fm); (F) Non-photochemical quenching (NPQ); (G) Whole plant images created using PlantExplorer^TM indicate Fv/Fm. Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 3

K uptake and fruit coloring of tomatoes grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K level. (A) K uptake rate; (B) average K uptake rate during fruit development and ripening stage; (C) hue angle values of tomato fruits at 33, 47, 61 DAA; (D) correlation between K uptake and hue angle; (E) fruit coloring at 47 DAA. Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). The asterisk * and ** indicate a significant difference at p ≤ 0.05, p ≤ 0.01, respectively. CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 4

Carotenoid content analysis of tomatoes grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K level. (A) The carotenoid biosynthetic pathway [26]; (B) carotenoid content in tomato fruit at 47 DAA (days after anthesis) grown under no supplemental lighting or red or/and supplemental lighting. Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). The asterisk * and ** indicate a significant difference at p ≤ 0.05, p ≤ 0.01, respectively.CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 5

Transcriptional analysis of the potassium transporter genes SlHAKs and channel genes at 33, 47, and 61 DAA (days after anthesis) grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K level. (A) SlHAKs expression patterns; (B) K channel expression patterns. The expression was normalized to the 33 DAA CK sample. Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 6

Expression of light interaction transcription factors and light receptors at 33, 42, and 61 DAA (days after anthesis) of tomatoes grown under red or/and blue supplemental lighting or without supplemental lighting at low K or normal K levels. The expression was normalized to the 33 DAA CK sample. Error bars represent standard deviations of the means of three independent replicates. Error bars represent standard deviations of the means of three independent replicates. Different letters indicate significant differences between treatments by Duncan’s multiple range test (p ≤ 0.05). CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.

Figure 7

Transcriptome analysis of differentially expressed genes (DEGs) among supplemental lighting treatments or K levels in tomato fruits at 33 DAA (days after anthesis). (A) Up- and downregulated K channel and transporter related genes in supplemental red or/and blue light and low K treatments compared with CK; (B) DEGs assigned to fruit lycopene metabolism pathways between supplemental red or/and blue light treatment; (C) DEGs assigned to ethylene transduction pathways between supplemental red or/and blue light treatment; (D) DEGs assigned to abscisic acid (ABA) transduction pathways between supplemental red or/and blue light treatment. CK: natural light, R: supplemental 660 nm red light, B: supplemental 430 nm blue light, RB: supplemental red combined with blue light with ratio of 3:1, K: normal potassium supply, –K: low potassium supply.