Chlorine Modulates Photosynthetic Efficiency, Chlorophyll Fluorescence in Tomato Leaves, and Carbohydrate Allocation in Developing Fruits

Abstract

Chlorine (Cl) is an essential nutrient for higher plants, playing a critical role in their growth and development. However, excessive Cl application can be detrimental, leading to its limited use in controlled-environment agriculture. Recently, Cl has been recognized as a beneficial macronutrient, yet studies investigating its impact on plant growth and fruit quality remain scarce. In this study, we determined the optimal Cl concentration in nutrient solutions through a series of cultivation experiments. A comparative analysis of Cl treatments at 1, 2, and 3 mM revealed that 3 mM Cl^- significantly enhanced chlorophyll content, biomass accumulation, and yield. Furthermore, we examined the effects of 3 mM Cl^- (supplied as 1.5 mM CaCl₂ and 3 mM KCl) on leaf photosynthesis, chlorophyll fluorescence, and fruit sugar metabolism. The results demonstrated that Cl^- treatments enhanced the activity of Photosystem I (PS I) and Photosystem II (PS II), leading to a 25.53% and 28.37% increase in the net photosynthetic rate, respectively. Additionally, Cl^- application resulted in a 12.3% to 16.5% increase in soluble sugar content in mature tomato fruits. During fruit development, Cl^- treatments promoted the accumulation of glucose, fructose, and sucrose, thereby enhancing fruit sweetness and overall quality. The observed increase in glucose and fructose levels was attributed to the stimulation of invertase activity. Specifically, acidic invertase (AI) activity increased by 61.6% and 62.6% at the green ripening stage, while neutral invertase (NI) activity was elevated by 56.2% and 32.8% in the CaCl₂ and KCl treatments, respectively, at fruit maturity. Furthermore, sucrose synthase (SS-I) activity was significantly upregulated by 1.5- and 1.4-fold at fruit maturity, while sucrose phosphate synthase (SPS) activity increased by 76.4% to 77.8% during the green ripening stage. These findings provide novel insights into the role of Cl^- in tomato growth and metabolism, offering potential strategies for optimizing fertilization practices in protected horticulture.

Keywords: chlorine; photosynthesis; quality; sucrose metabolism; yield.

Figure 1

Effects of Cl⁻ on plant height and stem diameter of tomato plants. (A) Plant height and (B) stem diameter. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 2

Effects of Cl⁻ on Pn–light response curve of tomato plants. Plants were subjected to three treatments: control (0 mmol·L⁻¹ Cl⁻), CaCl₂ (3 mmol·L⁻¹ Cl⁻), and KCl (3 mmol·L⁻¹ Cl⁻). The values are the means ± SDs (n = 3). Different letters above bars indicate significant differences at p < 0.05.

Figure 3

Effects of Cl⁻ on Fv/Fm and Pm of tomato plants. (A) Maximum photochemical efficiency, Fv/Fm, and (B) PS I maximum oxidation state, Pm. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 4

Effects of Cl⁻ on PS II reaction center of tomato plants. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). (A) Quantum efficiency of PS II photochemistry (Y(II)). (B) Quantum efficiency of non-regulated energy dissipation in PS II (Y(NO)). (C) Quantum efficiency of regulated energy dissipation in PS II (Y(NPQ)). Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 5

Effects of Cl⁻ on PS I reaction center of tomato plants. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). (A) Photochemical quantum yield of PS I in the light Y(I). (B) Quantum yield of non-photochemical energy dissipation at PS I due to donor-side confinement (Y(ND)). (C) Quantum yield of non-photochemical energy dissipation at PS-I due to the acceptor side (Y(NA)). Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 6

Effects of Cl⁻ on photosynthetic rETR-PAR response curves of tomato plants. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). (A) Photosynthetic rETR(I)–PAR response curve and fitting parameters of tomato plants. (B) Photosynthetic rETR(II)–PAR response curve and fitting parameters of tomato plants. Data are expressed as means ± SD. **** above the curve indicates significant differences at p < 0.01. Within each row, means followed by different letters are significantly different (p < 0.05).

Figure 7

Effects of Cl⁻ on sugar content of fruits. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 8

Effects of Cl⁻ on sucrose, glucose, fructose content of fruits in different growth stages. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). (A) Effect on sucrose content of fruits. (B) Effect on glucose content of fruits. (C) Effect on fructose content of fruits. (D) Effect on glucose and fructose content of fruits. Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 9

Effect of Cl⁻ on the activities of key enzymes of sucrose metabolism. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). (A) Effect on sucrose synthetase (SS-I) activity. (B) Effect on acidic invertase (AI) activity. (C) Effect on neutral invertase (NI) activity. Data are expressed as means ± SD. Different letters above bars indicate significant differences at p < 0.05.

Figure 10

Effect of Cl⁻ on sucrose phosphate synthase activity. Plants were subjected to three treatments: control (Hoagland solution), 1.5 mM CaCl₂ (3 mmol·L⁻¹ Cl⁻), and 3 mM KCl (3 mmol·L⁻¹ Cl⁻). Data are expressed as means ± SD. * above the line in the line graph indicate significant differences at p < 0.05.