Blue LED Induces Cold Tolerance in Tomato Fruit: Comparison
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LED lighting is increasingly applied to increase yield and quality of greenhouse produced crops, especially tomatoes. Tomatoes cannot be stored at cold temperatures due to chilling injury that manifests as quick quality deterioration during shelf life. 

  • tomato fruit
  • blue light
  • chilling injury
  • postharvest

1. Introduction

Tomato (Solanum lycopersicum L.) is one of the most popular consumer fruits often stored at low temperature to extend shelf life [1]. Unknown to many consumers and producers, tomato is a cold-sensitive fruit that suffers from chilling injury (CI) [2]. Exposure to temperatures below 12 °C followed by storage at higher temperatures will result in reduced keeping quality, reduced flavor life and negative consumer appreciation [3][4]. CI is caused by a sequence of events, starting with an increase in cell membrane micro viscosity, followed by increases in reactive oxygen species (ROS) causing further membrane malfunctioning (lipid peroxidation), protein oxidation, enzymatic activity inhibition and, finally, damage occurring to DNA and RNA [5][6][7]. CI symptoms in tomato fruit include the inability to ripen (lack of lycopene synthesis and production of unfavorable volatiles) and accelerated decay (firmness loss, susceptibility to pathogens and water soaking), which reduces consumer acceptability [8]. The effect of chilling on the activities of cell wall degradation enzymes is not very clear [9]. Chilling reduced the activity of PG, β-galactosidase and pectate lyase (PL), but not PME [10]. Enhanced softening during the shelf life of tomatoes was not correlated with PG activity; instead, it was associated with PME activity [11]. However, Rugkong et al. [12] did not find that cold storage retarded PME activity in tomato. Cold tolerance in tomato is mainly determined by the antioxidant capacity [13][14][15]. The extent of oxidative stress can be indicated by the level of malondialdehyde (MDA), which is a product of membrane lipid peroxidation [16]. Ascorbic acid (AsA) and catalase (CAT) are known to scavenge H2O2, a major ROS with a long half-life [17][18]. One of tomato CI symptoms is lycopene degradation in red ripe tomato, which does not only reduce tomato nutritional value, but also decreases its visual quality [19][20]. Furthermore, lycopene is considered the most efficient quencher of ROS among carotenoids [21][22].
Moderate stress, such as temperature, water status or light during cultivation, trigger plants to react by initiating immediate protection against the stressor. As a result to the initial stressor, the plant will develop a certain defence mechanism to confer protection against other stresses simultaneously or subsequently [23][24][25]. For example, water deficit induces production of abscisic acid (ABA) and increases antioxidant levels for superoxide dismutase, CAT, ascorbate peroxidase, and glutathione reductase [26][27][28]. Water deficit also induces dehydrin production, proteins that are suggested to have protective effects against water and temperature stress [29]. Dehydrin genes are expressed under the regulation of ABA-dependent and ABA-independent signaling pathways [30][31]. Cold tolerance can also be facilitated by the accumulation of cryo-protectants, such as soluble sugars, sugar alcohols and amino-acid-derived compounds, including proline and glycine betaine, and activated antioxidant defence system [32].
Recently, the role of far-red LED lighting during cultivation to induce chilling tolerance was examined. In prior to being long cold stored MG-tomatoes cultivated with additional far-red LED light, reduced weight loss, less pitting and faster red color development during shelf life was observed. Red harvested tomatoes, cultivated with additional far-red light were firmer at harvest, showed reduced weight loss and less decay during shelf life after prior cold storage [33]. Far-red LED light (non-photosynthetically active radiation) during cultivation therefore induces postharvest cold tolerance in tomato. Far- red might affect the expression of the C-repeat-binding factors (CBF) genes that regulate the expression of cold responsive (COR) genes leading to membranes stabilization during cold stress [34][35]. Heat tolerance is provided by production of heat shock protein that is known to also provide protection against cold stress [13][36].

2. Effect of Light Treatments on CI in R and MG Fruit

Increased chilling duration resulted in a higher decay index for R tomatoes at the start of shelf life (p < 0.001) and overall higher decay index values during shelf life (Figure 1). Severe decay occurred in R tomatoes after a shelf life of 20 d when prior being cold stored for 20 d. The decay index during shelf life for cold-stored R tomatoes was consistently lower for 12B cultivated tomatoes compared to the other light treatments (Figure 1B–D). For example, for tomatoes chilled for 5 d, the decay index of 12B was more than 74 and 140% lower than that of 24B and 0B after 10 d in shelf life, respectively. This indicates that chilling tolerance was induced for 12B cultivated R tomatoes. In MG tomatoes, chilling symptoms (pitting and uneven coloration) were observed only after 20 d of cold storage with no effects of light treatments (data not shown).
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Figure 1. Average decay index with indicated standard error during shelf life (20 °C) for five red (R) tomatoes per cold storage duration. Red, green and blue symbols indicate cultivation at 0, 12 and 24% additional blue light, respectively. Tomatoes were either non-stored (A) or cold stored at 4 °C for 5 d (B), 10 d (C) or 20 d (D). LSD values, when present, are indicated per panel.

3.2. Light Treatments Affect the Colour and Firmness at Harvest in R Tomatoes

3. Light Treatments Affect the Colour and Firmness at Harvest in R Tomatoes

At harvest, R tomatoes cultivated at 12B showed lower NAI values compared to tomatoes cultivated at 24B (Figure 2A) and higher FI index values compared to tomatoes cultivated at 0B (Figure 2B). Firmness of MG tomatoes was at least 47% higher than that of R tomatoes. The biggest difference in FI between MG and R tomatoes was found in 0B (63%) whereas 12B showed smaller difference (47%). Nevertheless, at harvest, no differences in NAI and FI values were observed for MG tomatoes with regard to the light treatments (Figure 2).
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Figure 2. Average color at harvest (A), expressed as normalized anthocyanin index (NAI) and (B) firmness, expressed as FI index, at harvest of twenty-five MG (solid bars) and R tomatoes (dashed bars) with indicated standard error, respectively. Colors indicate tomatoes cultivated with 0 (red bars), 12 (green bars) and 24% (blue bars) additional blue light. The different letters in each panel indicate significant differences between light treatment.

34. Effect of Light Treatments and Cold Storage on Coloration and Softening of MG Fruit

Non-chilled MG tomatoes cultivated at 12B showed a delayed increase in NAI values (Figure 3A) compared to fruit from the other light treatments, but this effect was not observed during shelf life after cold storage (Figure 3B–D). The softening of MG tomatoes was affected by the cold duration. Longer cold duration resulted in lower firmness at the start of the shelf-life period and a lower apparent softening rate during shelf life. Regardless of the cold storage duration, no effect of BL was observed on the softening of MG tomatoes during storage and during shelf life (Figure 3E–H). In MG fruit, long cold storage (10 and 20 d) resulted in lower weight loss in fruits cultivated at 12B compared to fruits of the other treatments (Figure 3K,L). Weight loss of 12B tomatoes chilled for 10 d was 94 and 43% lower than weight loss of 0B and 24B, respectively, at the start of shelf life. The differences became smaller towards the end of the shelf-life period (35 and 24%). Longer cold storage (20 d) and shelf life resulted in larger weight loss differences between 12B and the other light treatments. At the start of shelf life, the weight loss of 12B was 65 and 13% lower than that of 0B and 24B. Whereas at the end of shelf life, the weight loss of 12B tomatoes was 69 and 28% lower than the weight loss of 0B and 24B tomatoes.
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Figure 3. Average color, firmness and weight loss development of five MG tomatoes during shelf life at 20 °C after cold storage at 4 °C for 0 d (A,E,I) 5 d (B,F,J), 10 d (C,G,K) or 20 d (D,H,L) with indicated standard error, respectively. Colors indicate tomato cultivation with 0 (red symbols), 12 (green symbols) and 24% (blue symbols) additional blue light. LSD values, when present, are indicated per panel.

45. Cold-Stored R Tomatoes Show Colour and Firmness Loss

Non chilled R tomatoes showed constant NAI values, indicating a constant red color during shelf life, irrespective of the light treatment (data not shown). Cold-stored R tomatoes showed lower NAI values and lower FI values the longer the duration of cold storage (Figure 4). The loss of red coloration was higher in R tomatoes cultivated at 12B compared to the other light treatments (Figure 4A). Firmness loss during cold storage was independent of light treatments (p = 0.177, Figure 4B). Longer cold storage duration resulted in lower FI values at the start of the shelf life for R tomatoes. No difference in softening rate during shelf life was observed for R tomatoes when the light treatments were compared for all cold storage durations (Figure S1).
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Figure 4. Average color development, expressed as NAI values (A), and average firmness development, expressed as FI index (B), for five red tomatoes during cold storage (4 °C), with indicated standard error. Red, green and blue symbols indicate cultivation with 0, 12 and 24% additional blue light, respectively. The LSD value in panel A indicates the presence of significant differences between light treatments.

56. AsA, CAT Activity, H2O2 and MDA Content Are Unaffected by BL Treatments

Total AsA, CAT activity, H2O2 and MDA contents at harvest and during cold storage were affected by the maturity at harvest. Significant differences were found between MG and R tomatoes, with lower AsA (Figure 5A) content in MG tomatoes compared R tomatoes at harvest. AsA content in MG tomatoes increased during cold storage. At the end of cold storage, MG tomatoes had a 9% higher AsA content than R tomatoes. Higher CAT activity (Figure 5B), but lower H2O2 (Figure 5C) and MDA (Figure 5D) content, was observed at harvest and during cold storage for MG compared to R tomatoes. The H2O2 production of R tomatoes did not significantly change throughout cold storage, whereas MG tomatoes showed slowly increasing H2O2 levels during cold storage. MDA levels of R and MG tomatoes showed more fluctuation in terms of difference between both maturity throughout cold storage. The differences in H2O2 fluctuated between 52, 44, 18 and 46% at 0, 5, 10 and 20 d, respectively. Despite differences with respect to maturity, levels of these compounds at harvest and during cold storage were not affected by the light treatments. This indicates that the antioxidant status as indicated by these compounds is not affected by the light treatments.
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Figure 5. Changes in total AsA (A), CAT activity (B), H2O2 (C), and MDA (D) content at harvest and cold storage (4 °C) for fifteen MG (green symbols) and fifteen R tomatoes (red symbols). Light treatment effects were not significant and therefore values are shown only per maturity. LSD values are indicated per panel.

67. Summary

The chilling tolerance of red harvested tomato fruit was improved only by moderate blue light addition (12%) on top of a red background during cultivation. This improved cold tolerance for the R fruit was not due to differences in CAT activity, total ascorbic acid, H2O2 and MDA levels, but due to a lower red color at harvest and faster discoloration during cold storage. The red color measurement, measured by remittance spectroscopy, is closely related to the lycopene concentration. It is hypothesized that the lower lycopene content of the R fruit cultivated with moderate blue light levels allows for more lycopene loss during cold storage, thereby creating a higher cold tolerance.

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