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Effects of Abnormal Light/Dark Cycles and Continuous Lighting on Tomato and Eggplant

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30 June 2023

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04 July 2023

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Abstract
Response of tomato and eggplant to continuous lighting (CL) and abnormal light/dark (L/D) cycles was studied to elucidate the role of the absence of the dark period in the diurnal cycle in the development of leaf injuries under CL. Four light treatments were set: 16/8 h (control), 24/0 h (CL), 6/6 h, and 24/24 h (abnormal L/D cycles). These light treatments provided average daily light integrals of 17.3, 25.9, 13.0 and 13.0 mol m-2 day-1, respectively. Results of this work have shown that in both tomato and eggplant abnormal L/D cycles caused photoinhibition and leaf injuries similar to those in CL-grown plants. Induced defense mechanisms were not strong enough to contend against oxidative stress caused by light treatments that provided plants with even smaller DLI than 16/8 h photoperiod did. Half time during abnormal L/D cycles were dark periods. Despite these facts, abnormal L/D cycles were injurious. It is concluded that photooxidative stress induced by CL can not be attributed to excessive DLI or the continuity of light itself (the absence of dark periods). Therefore, we believe that the hypothesis suggesting circadian asynchrony to be the main triggering factor of CL-induced leaf injuries is the most plausible among many others proposed.
Keywords: 
Subject: 
Biology and Life Sciences  -   Agricultural Science and Agronomy

1. Introduction

As long ago as in the 1930 it was found that tomato plants exposed to continuous lighting (CL, 24 h photoperiod) develop leaf injury symptoms and eventually die [1]. Then in the 1940th it was observed that low temperature may prevent or alleviate CL-induced injuries [2,3]. It was also shown that even low light can cause injury of leaves if tomato plants are illuminated during 24 h per day [3]. In the 1950thHillman [4] reported that symptoms occurred only in leaves exposed to CL from an early stage in development and plants with 4-7 leaves are injured most quickly while older plants are less sensitive to CL. Hillman also demonstrated that sufficiently wide daily temperature changes can prevent or delay the effects of CL, but he suggested that it is the effect of change, rather than any absolute effect of high or low temperature. Later it was reported that there are other plants sensitive to CL [5,6,7], while tomato and eggplant appear to be the most sensitive ones. These sensitive species exhibit multiple responses to CL such as leaf chlorosis and necrosis, decreased photosynthetic efficiency, acceleration of leaf senescence [8,9,10].
This phenomenon attracted attention of the scientists for many reasons. The first one is the remarkable nature of this phenomenon itself. Secondly, as the tomato plant is daylength neutral plant with respect to flowering initiation, it affords an opportunity to study a purely vegetative effect of photoperiod [4]. Thirdly, while some crops exposed to CL displayed photodamages, the others obtained increased yield [8,9]. Thus, CL is considered to be an efficient cultivation strategy, theoretically driving crops to grow better [9,10,11]. With extension to CL savings possibly could be realized by decreasing the number of light fixtures per unit area and prolonging useful lamp life (no on/off deterioration) [12]. Moreover, it is optimal to use low light intensity and long photoperiods to achieve the target daily light integral (DLI)as electricity cost at night is often lower, and heat released by the light fixtures help to meet the heating requirements during the night [13,14]. For plant dry matter production lengthening the photoperiod often is more effective than increasing light intensity [12,15,16,17,18].CL is also used in breeding research as it accelerates plant development and therefore shortens the time for crop selection [8]. Moreover, CL is commonly used in the short term as a pre-harvest strategy for the enhancement of crop nutritional value and yield and decrease nitrate content [19,20,21,22].
Although the phenomenon of CL-induced leaf injury was described more than 90 years ago, its mechanism is not entirely clear yet, although several hypotheses have been proposed. Excessive carbohydrate accumulation, photooxidative pressure, continuous signalling to the photoreceptors and disruption of circadian clock have been suggested [9]. Later it was reported that an unbalanced photosystem I and photosystem II excitation due to downregulation of the type III light harvesting chlorophyll a/b binding protein 13 (CAB-13) gene expression in tomato leaves is responsible for injury development under CL [23]. There were quite many data reported supporting and opposing suggested hypotheses. For instance, there are evidences that hyperaccumulation of carbohydrates cannot be considered as the main and, moreover, the only cause of damage to plant leaves under CL and long photoperiods [24,25,26,27]. On the other hand, the role of this factor in the appearance and development of leaf photodamage cannot be excluded. The CL was suggested to induce photooxidative damage in leaves by the generation of harmful reactive oxygen species (ROS) [9,28], but later it was shown that ROS is not directly involved in CL-induced leaf injury, as both ROS production and scavenging was highest in leaves (without chlorotic symptoms) of plants treated by CL under variable temperature (27/17°C) [25]. While the hypothesis concerning endogenous rhythms suggested in 1954 [29] was attractive for explanation of the phenomenon under consideration and valuable for emphasizing its complex, integrated nature, no physiological and biochemical mechanisms fundamental to it were reported.
In the present study, we studied the response of tomato and eggplants to CL and abnormal light/dark (L/D) cycles to elucidate the role of the absence of the dark period in the diurnal cycle in the development of leaf injuries under CL. The L/D cycle is characterized by two factors that are the length of L/D cycle (period) and the ratio of the illumination time to the dark time (L/D ratio). It is called normal L/D cycle when the L/D cycle period is equal to the diurnal cycle period of 24 h, otherwise it is called abnormal L/D cycle [30].

2. Results

2.1. Photosynthetic pigments

Total chlorophyll (Chl) content was decreased by 24/0, 6/6 and 24/24 h light/dark cycles in tomato and eggplant leaves, while carotenoids (Car) were little affected in both species with a trend to increase under photoperiods differed from 16/8 h (Table 1). All abnormal light/dark cycles increased Chl a/b ratio and decreased Chl/Car ratio compared to photoperiod 16/8 h. Share of Chl in light-harvesting complex II (LHCII) was also decreased by abnormal light/dark cycles.
Continuous lighting (24/0 h) caused typical for tomato and eggplant injuries. Tomato leaves developed interveinal chlorosis and eggplant leaves were chlorotic with necrotic spots (Figure 1). Exposure of plants to other abnormal light/dark cycles also resulted in leaf injuries. They differed by appearance from those induced by continuous lighting, but certainly leaves were not healthy.

2.2. Photosynthetic activity and transpiration

Plants treated by abnormal light/dark cycles had significantly lower values of potential quantum yield of photochemical activity of PSII (Fv/Fm), net photosynthesis rate (An) and transpiration rate (Tr) than untreated plants (Table 2). Stomatal conductance (gs) was decreased only by 6/6 h cycle in tomato. The ratio of intercellular to ambient CO2 concentration (Ci/Ca) was decreased by 6/6 h cycle in tomato and 24/24 h cycle in eggplant. For tomato, leaf mass per area (LMA) values were much higher in 24/0, 6/6, and 24/24 h plants than in their 16/8 h counterparts. For eggplant, LMA was unaffected by abnormal light/dark cycles except for 6/6 h cycle, which significantly decreased LMA.

2.3. Oxidative Stress and Antioxidants

All plants grown under abnormal light/dark cycles (24/0, 6/6, 24/24 h) had higher H2O2 content compared to 16/8 h photoperiod (Table 3). The intensity of lipid peroxidation determined by the malondialdehyde (MDA) content was also higher in these plants. Besides, they had higher proline and anthocyanin contents.
The activities of antioxidant enzymes ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) were increased by all abnormal light/dark treatments in tomato and eggplants. The activity of catalase (CAT) and superoxide dismutase (SOD) was decreased by 24/0 and 24/24 h cycles and increased by 6/6 h treatment.

3. Discussion

Tomato plant responses to CL are well documented [1,3,4,5,9,18,23,24,25,26,31,32,33,34,35,36,37,38,39,40,41,42]. There are also some reports on the effects of CL on eggplant [7,28,43,44] and some other species belonging to Solanaceae, Cucurbitaceae, Asteraceae, Brassicaceae, Leguminosae, Convolvulaceae, Laminaceae, Rosaceae, Poaceae, Pinaceae and Taxodiaceae families [22,37,45,46,47,48,49,50]. In this study we also observed leaf epinasty and the development of interveinal leaf chlorosis in tomato and necrosis in eggplant leaves of CL-grown plants. These plants had lower chlorophyll content, higher chlorophyll a/b ratio, lower chlorophyll/carotenoid ratio and lower relative chlorophyll content in the light-harvesting complex compared to plants grown under 16/8 h photoperiod. They also had lower values of Fv/Fm, An and Tr. Both H2O2 concentration and the intensity of lipid peroxidation estimated by MDA content were increased by CL. The content of antioxidants such as proline and anthocyanins, as well as the activity of some antioxidant enzymes (APX, GPX) were increased. All these changes are aimed to protect photosynthetic apparatus against light stress due to an excess of absorbed light beyond that utilized in photosynthesis. In our experiment CL-treated plants had greater DLI compared to plants grown under 16/8 h photoperiod (25.9 and 17.3mol m-2 day-1, correspondingly) and therefore the enhancement of photoprotective mechanisms was rather expected. However, plants grown under abnormal light/dark cycles 6/6 h and 24/24 h also exhibited an entire spectrum of responses that can be regarded as protective and adaptive ones to excessive illumination. These included all aforementioned changes in the pigment complex, PSII efficiency and photosynthesis as well as increased content of H2O2 and MDA that evidenced oxidative stress in tomato and eggplants. Increased antioxidative capacity was not enough to cope with the oxidative stress caused by abnormal light/dark cycles, which provided plants with even smaller DLI compared to 16/8 h photoperiod (17.3 and 13.0 mol m-2 day-1, correspondingly). Thus, in our experiments treatments that did not provide excess light energy to plants were injurious for tomato and eggplant causing leaf photodamages similar to those induced by CL.
The causes of leaf injuries and photoinhibition under CL conditions are still the subject of discussion. Some authors state that energy component of light that drives photosynthesis is responsible for the CL-induced injuries [46,51]. They conclude that photooxidative stress induced by CL can be attributed to excessive DLI. The others [24,26,40] suggest that signaling component of light perceived by several photoreceptors is crucial for triggering injury to CL-grown plants. The results reported here were obtained with plants receiving smaller quantities of light in every 48 h period (except for CL-grown plants) compared to non-injurious for tomato and eggplant 16/8 h photoperiod. Plants grown under 6/6 h and 24/24 h light/dark cycles were injured being subjected to the darkness for half the time. Thus, these results clearly show that the cause of leaf injury is not the continuity of light itself and higher DLI is not needed to cause leaf photodamages. This supports previous research [18,24,40] suggested that continuous light signaling, continuous photosynthesis and continuous high oxidative pressure are not prerequisites to induction of leaf photodamages. In fact, all injurious light/dark cycles (24/0 h, 6/6 h and 24/24 h) provided light to the plants during the subjective night, which implies a circadian asynchrony, i.e. the mismatch between the internal (circadian) biorhythms and the external light/dark cycle. All photoperiods longer than 16-18 h would expose plants to light during advanced stages of the scotophile phase (phase of endogenous daily rhythm in which light is inhibitory or has no promoting effect [52]. Therefore, theoretically predicted and observed effects well agreed as there are evidences of developed chlorosis on leaves of cucumber plants grown under 20 h photoperiod [53]. First it was hypothesized [9] that the mechanism by which circadian asynchrony could cause injury involves a mismatch between a fluctuating circadian-controlled protection against photoinhibition and continuous photo-oxidative stress imposed by light. However, later it was shown that light itself upregulates protection to photoinhibition in a circadian-independent way [26].

4. Materials and Methods

4.1. Plant Material and Growth Conditions

The seeds of tomato plants (Solanum lycopersicum L., hybrid Verlioka plus F1) and eggplants (Solanum melongena L., cv. Almaz) were germinated for 2 days in Petri dishes on filter paper moistened with distilled water in the darkness at 28°C. The germinated seeds were planted in 7×7-cm plastic containers with sand and the plants were grown in a Vötsch growth chamber (Germany) at an air temperature of 25°C and air humidity 70%. Plants were supplied with Hoagland complete nutrient solution (pH 6.2–6.4).

4.2. Light Treatments

Light was provided by fluorescent lamps (F36W/T8 BRITEGRO, Sylvania, Germany). Light intensity was 300 μmol m-2 s-1 of PPFD. The PPFD value was measured using LI-250A Light Meter (Li-COR Biosciences, USA). All trays were systematically rearranged every day to minimize disproportion in light distribution.
Four light treatments were set: (1) 16-h light/8-h dark (16/8 h), (2) 24-h light/0-h dark (24/0 h), (3) 6-h light/6-h dark (6/6 h), (4) 24-h light/24-h dark (24/24 h). These light treatments provided average DLIs of 17.3, 25.9, 13.0 and 13.0 mol m-2 day-1, respectively.

4.3. Growth Measurements

Fully expanded leaves of six plants from each treatment were sampled on the 30th day after sowing for the measurements.
The values of leaf mass per area (LMA) were calculated as a ratio of a dry mass of the lamina discs to their area. Eight discs were cut from each leaf with an 8-mm in diameter cork borer. The dry weight of the discs was determined after their drying to a constant weight at 105°C.

4.4. Net Photosynthesis, Transpiration and Stomatal Conductance

The net photosynthesis (An) rate, transpiration rate (Tr), stomatal conductance (gs), and ratio of intercellular to ambient CO2 concentration (Ci/Ca) were measured on the youngest mature leaf using a portable HCM-1000 photosynthetic system (Walz, Germany) at a leaf temperature of 23°С, air humidity of 65-70%, CO2 concentration of 400-420 ppm, and PPFD of 300 μmol m-2 s-1. The parameters were measured not earlier than 2 h after the start of a light period.

4.5. Chlorophyll Fluorescence Measurements

Chl fluorescence parameters of the plants were measured using a Pulse Amplitude Modulation Fluorometer (MINI-PAM, Heinz Walz, Germany). The values of potential quantum yield of photochemical activity of PSII (Fv/Fm) were determined after leaves were dark-adapted for 30 min with leaf clips.

4.6. Photosynthetic Pigment Content

Five plants per treatment were randomly selected for the following measurements. Content of Chla and b and carotenoids (Car) was measured in 96% ethanol extracts with a SF2000 spectrophotometer (Spectrum, Russia) and calculated according to the known formulas [54]. The percentage of Chl in light harvesting complex II (LHCII) was calculated by accepting that almost all Chlb is in LHCII, and that the ratio of Chla and b in LHCII is 1.2 [55].
The dynamics of the chlorophyll content was express-monitored with a SPAD 502 Plus chlorophyll meter (Konica Minolta, Japan) in the course of an experiment. We previously demonstrated the applicability of this tool for rapid nondestructive assay of chlorophyll in the leaves with interveinal chlorosis [56].

4.7. Anthocyanins Content

Anthocyanins were extracted from leaves according to Kang et al. [57]. Fresh leaves tissues (0.1 g) were homogenized in 4 mL of 95% ethanol-1.5 N HCl- (85:15, v:v). After overnight extraction at 4 °C in darkness, each sample was centrifuged at 10,000× g for 5 min. The absorbance of the supernatant was measured at 533 nm (peak of absorption of anthocyanin) and 657 nm (peak of absorption of Chl degradation products). The results were plotted as a difference in absorption at 530 and 657 nm relative to tissue fresh weight (∆A∙g−1 FW) and the formula ∆A = A530−1/4A657 was used to deduct the absorbance contributed by chlorophyll and its degradation products in the extract [58].

4.8. Malondialdehyde (MDA) Content

The content of malondialdehyde MDA, the end product of lipid peroxidation, was determined with a standard method based on the reaction of these substances with thiobarbituric acid (TBA) that produces a trimethine complex with an absorption maximum at 532 nm [59]. The value for nonspecific absorption of each sample at 600nm was also recorded and subtracted from the absorbance recorded at 532nm. The concentration of MDA was calculated using an extinction coefficient of 155mM−1cm−1. The lipid peroxidation levels were expressed as micromoles of MDA per gram of FW.

4.9. Hydrogen Peroxide Content

Hydrogen peroxide content was determined according to Velikova et al. [60]. Leaf tissues (0.1 g) were homogenized in ice bath with 2 mL 0.1% (w/v) TCA. The homogenate was centrifuged at 12,000× g for 15 min at 4 °C and 0.5 mL of supernatant was added to 0.5 mL potassium phosphate buffer (pH 7.0) and 1 mL 1M KI. The absorbance of supernatant was measured at 390 nm. The content of H2O2 was calculated by comparison with a standard calibration curve.

4.10. Proline Content

Free proline content in the leaf tissues was estimated according to Bates et al. [61]. Fresh leaf samples (0.3 g) were homogenized in 6 mL of 3% sulfosalicylic acid and the homogenate was centrifuged at 5100× g for 5 min. Then, 2 mL of the supernatant was mixed with 2 mL freshly prepared ninhydrinreagent (1.25 g ninhydrin in 20 mL of 6M phosphoric acid and 30 mL of glacial acetic acid) and 2 mL glacial acetic acid. The colour reaction developed after incubation of the samples for 1 h in a boiling water bath. After warming to 25°C, the absorbance was measured at 520 nm. The concentration of proline was estimated by referring to a standard curve of L-proline and expressed in µmol g−1 FW.

4.11. Antioxidative Enzyme Activity Assays

For protein and antioxidant enzyme assays, leaf tissues (0.1 g) were ground in 4 mL of 50 mM potassium phosphate buffer (pH 7.8) using a chilled pestle and mortar. The extraction buffer contained 0.1 mM EDTA and 1% (w/v) polyvinylpyrrolidone. The homogenate was centrifuged at 14,000× g for 20 min at 4°C, and the supernatants thus collected were used for the assays of catalase (CAT, EC 1.11.1.6), superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), and guaiacol peroxidase (GPX, EC 1.11.1.7), and protein determinations.
CAT activity was determined using spectrophotometer SF-2000 (OKB Spectr, Russia) by measuring the rate of H2O2 disappearance at 240 nm [62]. The reaction mixture contained 50 mM potassium phosphate buffer (pH 7.0) and 10.5 mM H2O2. The reaction was run at 25 °C for 1 min, after adding the enzyme extract and rate of decrease in absorbance at 240 nm (E  =  39.4mM−1cm−1) was used to calculate the enzyme activity. CAT activity was expressed in µmol H2O2 per minute per mg of protein.
SOD activity assay was based on the measurement of inhibition in the photochemical reduction innitroblue tetrazolium (NBT) spectrophotometrically. The reaction mixture contained 50mM K-phosphate buffer (pH 7.8), 13 mM methionine, 75 μM NBT, 0.1 μM EDTA, 4 μM riboflavin, and the required amount of enzyme extract. The reaction was triggered by adding riboflavin and placing the tubes underfluorescent lamps for 30 min. A complete reaction mixture without enzyme served as control. A non-irradiated complete reaction mixture served as a blank. One unit of SOD activity was defined as the amount of enzyme required to cause 50% inhibition of the reduction in NBT as monitored at 560 nm according to [63].
APX was assayed by the method described by Nakano, Asada [64]. The reaction mixture contained 50 mM potassium phosphate buffer (pH 7.0), 0.2 mM EDTA, 0.5 mM ascorbic acid, and 0.25 mM H2O2. The reaction was triggered by the addition of H2O2 after adding the enzyme extract. The decrease in absorbance at 290 nm for 1min was recorded and the amount of APX was calculated from the extinction coefficient 2.8 mM−1cm−1.
The activity of GPX was determined spectrophotometrically by measuring the increase in absorbance at 470 nm [65] (Maehly, Chance, 1954). The reaction mixture contained 80 mmol/L guaiacol and 10 mmol/L H2O2 in 0.066 mol/L phosphate buffer (pH = 7.4). The enzymatic reaction was started by adding 0.05 mL of the extract to 2 mL of reaction mixture. The SOD, APX, and GPX activity values are expressed as unit U per mg of protein.
The concentration of protein was determined according to Bradford [66] using bovine serum albumin (BSA) (Dia-M, Russia) as a standard.

4.12. Data Analysis

The experiment was performed twice overtime for each species and data were pooled across replications. The figures show mean values and standard errors. Significant differences between the means were revealed at p< 0.05 using one-way ANOVA analysis (the least significant difference test).

5. Conclusions

Results of this work with sensitive to CL tomato and eggplants have shown that abnormal light/dark cycles (6/6 h and 24/24 h) caused photoinhibition and leaf injuries similar to those in CL-grown plants. Induced defense mechanisms were not strong enough to contend against oxidative stress caused by light treatments that provided plants with even smaller DLI than 16/8 h photoperiod did. Half time during abnormal light/dark cycles were dark periods. Despite these facts, these light/dark cycles were injurious. It is concluded that photooxidative stress induced by CL can not be attributed to excessive DLI or the continuity of light itself (the absence of dark periods). Therefore, we believe that the hypothesis [26] suggesting circadian asynchrony to be the main triggering factor of CL-induced leaf injuries is the most plausible among many others proposed. However, the underlying reason of why a light interruption during the scotophile phase is inhibitory remains to be uncovered.

Author Contributions

Conceptualization, T.G.S.; methodology, T.G.S. and E.G.S.; formal analysis, investigation, T.G.S., E.G.S., E.N.I. and A.V.M.; data curation, T.G.S., E.G.S., E.N.I. and A.V.M.; writing—original draft preparation, T.G.S.; writing—review and editing, T.G.S., A.V.M., E.G.S., E.N.I., and A.F.T.; funding acquisition, A.F.T. and T.G.S. All authors have read and agreed to the published version of the manuscript.

Funding

Support for this study was provided by RNF 23 23-16-00160.

Acknowledgments

This research was carried out using the equipment of the Core Facility of the Karelian Research Center of the Russian Academy of Science.

Conflicts of Interest

The authors declare no conflict of interest.

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Preprints 78199 g001
Figure 1. Second true leaf of (a) Solanum lycopersicum plants; (b) Solanum melongena plants grown under different light treatments: 16-h light/8-h dark (16/8 h), 24-h light/0-h dark (24/0 h), 6-h light/6-h dark (6/6 h), 24-h light/24-h dark (24/24 h).
Figure 1. Second true leaf of (a) Solanum lycopersicum plants; (b) Solanum melongena plants grown under different light treatments: 16-h light/8-h dark (16/8 h), 24-h light/0-h dark (24/0 h), 6-h light/6-h dark (6/6 h), 24-h light/24-h dark (24/24 h).
Preprints 78199 g001
Table 1. Photosynthetic pigment contents and ratios in tomato and eggplant grown under different light/dark cycles.
Table 1. Photosynthetic pigment contents and ratios in tomato and eggplant grown under different light/dark cycles.
Parameter Tomato Eggplant
16/8 h 24/0 h 6/6 h 24/24 h 16/8 h 24/0 h 6/6 h 24/24 h
Chla+b,
mg/g DW		 15±1.2
A		 11.5±0.6
B		 9.4±1.4
C		 6.2+0.4
D		 10.6±0.6
a		 9.4±0.2
b		 6.7±0.9
c		 7.1±1.3
c
Сhl a/b 1.5±0.1
C		 1.8±0.2
C		 2.4±0.2
B		 4.5±0.2
A		 2.1±0.1
c		 2.6±0.02
b		 2.4±0.2
b		 3.6±0.1
a
LHCII, % 87±4
A		 80±3
B		 65±4
C		 41±1
D		 71±3
a		 62±1
b		 66±3
b		 48±1
c
Carotenoids,
mg/g DW		 0.9±0.1
B		 0.9±0.2
B		 1.1±0.1
AB		 1.3±0.1
A		 1.0±0.1
a		 1.2±0.1
a		 1.0±0.1
a		 1.2±0.2
a
Chl/Car 17.3±1.3
A		 14.4±1.5
B		 8.6±0.9
C		 4.9±0.2
D		 10.3±0.7
a		 7.7±0.2
b		 6.6±0.5
c		 5.8±0.3
d
Different letters for each plant species indicate significant differences between the mean values at p< 0.05.
Table 2. Photosynthetic parameters of tomato and eggplant grown under different light/dark cycles.
Table 2. Photosynthetic parameters of tomato and eggplant grown under different light/dark cycles.
Parameter Tomato Eggplant
16/8 h 24/0 h 6/6 h 24/24 h 16/8 h 24/0 h 6/6 h 24/24 h
Fv/Fm 0.824±0.005
A		 0.648±0.038
C		 0.768±0.030
B		 0.678±0.031
C		 0.814±0.010
a		 0.760±0.012
b		 0.709±0.016
c		 0.677±0.007
d
An, µmol m-2 s-1 10.5±0.5
A		 9.9±1.6
A		 6.9±0.8
B		 8.9±0.6
A		 11.8±0.4
a		 3.4±0.2
c		 1.9±0.5
d		 7.4±0.5
b
gs, mmol m-2 s-1 112.1±7.5
A		 117.3±21.9
A		 43.6±0.78
B		 109.9±9.7
A		 211.2±20.5
a		 143.2±20.2
b		 83.6±6.3
d		 107.0±6.9
c
Tr, mmol m-2 s-1 1.55±0.06
A		 1.35±0.07
B		 0.80±0.09
C		 1.30±0.07
B		 10.3±0.7
a		 7.7±0.2
b		 6.6±0.5
c		 5.8±0.3
d
Ci/Ca 0.62±0.02
A		 0.64±0.03
A		 0.22±0.05
B		 0.58±0.04
A		 0.76±0.03
b		 0.88±0.01
a		 0.87±0.03
a		 0.58±0.03
c
LMA, mg cm-2 2.2±0.2
C		 3.6±0.2
A		 3.2±0.3
AB		 2.9±0.2
B		 2.3±0.3
a		 2.5±0.1
a		 1.3±0.1
b		 2.1±0.1
a
Different letters for each plant species indicate significant differences between the mean values at p< 0.05.
Table 3. The malondialdehyde (MDA), H2O2, anthocyanin, proline contents and catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) enzyme activities in tomato and eggplant grown under different light/dark cycles.
Table 3. The malondialdehyde (MDA), H2O2, anthocyanin, proline contents and catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) enzyme activities in tomato and eggplant grown under different light/dark cycles.
Parameter Tomato Eggplant
16/8 h 24/0 h 6/6 h 24/24 h 16/8 h 24/0 h 6/6 h 24/24 h
MDA,
µmol g-1 DW		 154±14
C		 230±24
B		 293±28
A		 265±25
AB		 126±7
d		 238±38
a		 189±15
b		 143±17
c
H2O2,
µmol g-1 FW		 0.51±0.08
C		 0.98±0.10
B		 1.19±0.12
B		 2.15+0.31
A		 0.79±0.16
c		 1.39±0.07
a		 0.97±0.28
b		 1.06+0.22
b
Anthocyanins,
∆A∙g−1 FW		 1.3±0.1
C		 2.5±0.3
A		 2.7±0.2
A		 2.2±0.5
B		 0.33±0.03
c		 0.46±0.08
b		 0.44±0.02
b		 0.69±0.05
a
Proline, µmol g-1 FW 5.3±0.6
C		 10.3±2.1
B		 24.3±2.8
A		 29.1+1.7
A		 2.5±0.2
d		 3.4±0.2
c		 4.7±0.7
b		 11.1+0.6
a
CAT, µmol mg-1 protein min-1 51.6±5.2
A		 18.4±3.6
B		 61.8±13.1
A		 21.4±2.2
B		 118.7±5.6
b		 37.8±5.3
c		 243.3±25.7
a		 26.0±2.9
d
SOD, U mg-1
protein		 21.5±3.8
B		 6.8±0.7
C		 34.7±3.6
A		 2.6±0.4
D		 13.7±2.2
b		 6.6±1.1
c		 73.6±12.0
a		 4.2±0.5
d
APХ, µmol mg-1 protein min-1 101±17
D		 392±15
A		 215±27
B		 144±16
C		 139±13
d		 360±3
a		 216±21
c		 315±10
b
GPХ, µmol mg-1 protein min-1 52±5
D		 258±15
A		 95±11
C		 130±8
B		 45±9
c		 278±10
a		 58±14
c		 71±13
b
Different letters for each plant species indicate significant differences between the mean values at p< 0.05.
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