Journal of Applied Botany and Food Quality 89, 315 - 321 (2016), DOI:10.5073/JABFQ.2016.089.043 1Department of Food Technology, Pir Mehr Ali Shah, Arid Agriculture University Rawalpindi, Pakistan 2Department of Agricultural Sciences, University of Haripur, Pakistan 3Department of Botany, Pir Mehr Ali Shah, Arid Agriculture University Rawalpindi, Pakistan 4College of Agriculture, Bahauddin Zakariya University, Bahadur Campus Layyah, Pakistan Photo-induced changes in quality attributes of potato tubers during storage Kashif Sarfraz Abbasi1, Tariq Masud1, Abdul Qayyum2*, Asif Ahmad1, Ayaz Mehmood2, Yamin Bibi3, Ahmad Sher4 (Received August 7, 2016) * Corresponding author Summary The retail display of potato tubers is carried out in supermarkets under additional light sources to impart aesthetic value and consu- mer’s attention, however, is associated with potato greening and as- sociated disorders. The objective of this study was to identify a most appropriate light source for potato variety ’Lady Rosetta‘ along with photo-induced changes in different quality parameters. Potato tubers were placed for 27 days at ambient storage (25 ± 2 oC) under different light sources i.e. blue, fluorescent, green, mercury and red along with dark storage, which also served as normal control. In general, quality parameters, such as sugars, chlorophyll, total glycoalkaloids, increase while attributes, such as starch and ascorbic acid decrease during the storage period. The initial increase followed by final decline has been observed in parameters, such as total phenolic contents and radical scavenging activity. The results showed maximum retention of dif- ferent quality attributes in dark potato storage. Amongst different light sources mercury and green light retained appreciable retention of different quality parameters with non-significant difference esti- mated between them in most of the studied parameters. Storage of potato under fluorescent, red and blue light proved to be precarious due to skin discoloration. Overall results revealed tuber sensitivity to different colored light along with their potential storage stability in the retail markets. Keywords: potato; light; quality attributes; storage stability Introduction The worldwide consumption of potatoes is ascribed to its luscious- ness and high nutritive value (Rytel et al., 2005). It produces more dry matter per hectare than the major cereal crops, such as wheat, rice, etc. (GRavoueille, 1999) thus considered as an important staple crop in most parts of the world. Potatoes are a rich source of carbohydrate with starch being the key ingredient or the main form, which serves as an inexpensive source of energy (lachman et al., 2001). In addition, it is also considered as good source of high- quality protein such as lysine (FRiedman, 2004). Four of the six main vitamins required in our daily diet are present in potatoes, which highly indicates the nutritious importance of the crop. The predominant vitamins found in potatoes are thiamin, riboflavin, nia- cin and an antioxidant ascorbic acid. Ascorbic acid is vulnerable to heat and light, thus its loss during storage is considered as a major index of quality deterioration (BuRlinGame et al., 2009). In addition, potatoes are the second most imperative source of vitamin B6 for the elderly who are particularly at risk of chronic diseases. Vitamin B6 influences hormonal synthesis, erythrocyte production, immune mo- dulation and central nervous system functions. In addition, it is also important for treatment of various chronic diseases, such as sickle cell anemia, asthma, and cancer (Kolasa, 1993). Potato is low cho- lesterol, high potassium food with significant antioxidants potential and thus capable of protecting human beings against cardiovascular diseases and cancer (lachman et al., 2000). Polyphenols, in addi- tion, are the most common dietary antioxidants (lachmann et al., 2008) being efficient as reducing agents, metal chelators and reactive oxygen species quenchers in a biological system. It is impossible to prevent potato tubers from light exposure during the marketing chain and commercial processing. The response, how- ever, presents remarkable changes in the physiology of the potato tuber and deteriorates its quality by cold sweetening, sprouting or tuber greening due to excessive chlorophyll accumulation and for- mation of toxic steroidal glycoalkaloids (senGul et al., 2004). Glycoalkaloids are present as alpha-solanine and alpha-chaconine in marginal tuber layers. However, with the increased amassing of the compounds, significant economic losses coupled with food safety hazards are observed. Presently, high glycoalkaloids (GA) intake is considered as grave food safety concern and with the increasing demand for fresh and processed potatoes the threat has increased manifold (PeRcival, 1999). Excessive glycoalkaloids accumulation leaves a bitter taste, if the concentration exceeds a specific level, with upper safe intake limit for a human is 20 mg 100 g-1 f.w (nema et al., 2008). Excessive glycoalkaloids intake results in diarrhea, vomiting, fever, rapid pulse, colic pain, low blood pressure, gastro- enteritis, and neurological disorders (slanina, 1990). Although Photo-induced chlorophyll formation in potatoes is not a serious threat for human body, but green potatoes appear to be highly un- attractive in comparison to normal red skinned or white skinned potatoes. The unappealing green color of the potatoes reduce their market value and according to research carried out by British potato council, £ 6 million loss has been borne by the grower due to its green pigmentation (anon., 1998). Changes in the quality attributes of potato tubers under different colored light sources have been an important point of interests for the retailers. The present study was designed to identify the best light source along with the transition in the vital attributes of potato variety ‘Lady Rosetta’ at ambient tem- perature storage. Materials and methods Potato variety ’Lady Rosetta‘ was harvested from Potato Research Institute, Sahiwal (Punjab, Pakistan). The experimental material was shifted to the Postharvest Technology Laboratory, Department of Food Technology PMAS-Arid Agriculture University Rawalpindi, Pakistan. Potatoes were washed, sorted and graded into the homo- genous lot before they were subjected to the different analytical tri- als. The selected tubers were cured for one week at a temperature between 15-20 oC. Potato tubers were placed under different light sources in specially designed cabinets. The trial was divided into the following set of treatments: 316 K.S. Abbasi, T. Masud, A. Qayyum, A. Ahmad, A. Mehmood, Y. Bibi, A. Sher T1 = Blue light (BL) T2 = Fluorescent light (FL) T3 = Green light (GL) T4 = Mercury Light (ML) T5 = Red light (RL) T6 = Dark (D) Potatoes were placed under the lamp of 20 W of different light sources maintained at a distance of 1 m during the complete storage duration (27 days) except in T6. All the tubers were barred exposed to different light sources and rotated at 24 hours interval to ensure maximum skin exposure. 1 kg of potato tubers was placed in each replication, and 3 kg tubers were maintained in each treatment for per day analysis. In total, 30 kg of potatoes were left in each treat- ment and were subsequently subjected to different physicochemi- cal analyses at three days interval. All the potatoes were stored at temperature and relative humidity maintained around 25 ± 2 oC and 70 ± 5 % respectively before being subjected to different analytical estimations. Glucose: Glucose estimation was carried out by glucose test strips imported from Snack Food Association, (Arlington, Virginia USA) along with the color chart expressing glucose percentage. The test strip color change was correlated with the color chart, and the val- ues were expressed in percentage. The tests were conducted in trip- licate. Total sugar: Lane and Eynon titration method using Fehling’s solu- tion was employed for the estimation of reducing sugar (RS), non- reducing sugar (NRS) and total sugars (TS) contents as reported in AOAC (1990) method no. 925.35. Ten grams sample was diluted with 100 ml water, stirred thoroughly to dissolve all suspended particles, subsequently filtered in 250 ml volumetric flask. 100 ml of prepared solution was transferred into conical flask along with 10 ml of diluted HCl and boiled for 5 minutes. The resultant solution was cooled and neutralized with 10 % NaOH and made up to volume in 250 ml volu- metric flask. The solution was titrated against Fehling’s solution, and readings were recorded up to brick red end point. The readings were calculated as under: Total sugar (%) = 4.95 (Factor) × 250 (Dilution) × 2.5 / Weight of sample × Titre × 10 × 100 Starch: Starch estimation was carried out by making the tuber sugar free by the repeated extraction with 80 % isopropanol. Tubers were dried at 70 oC and the starch was hydrolyzed by 60 % perchloric acid. The glucose was estimated spectrophotometrically at 620 nm by us- ing anthrone reagent as described by KumaR et al. (2005). Ascorbic Acid: Ascorbic acid (AA) estimation was carried out by the titrimetric method by employing 2, 6, dichlorophenol indophenol dye (redox dye) as explained in AOAC (1990) method no. 967.21. 10 g representative sample was taken in a beaker and made up to volume with 100 ml 3 % phosphoric acid and filtered. 10 ml of filtrate was titrated with the standard dye solution till pink end point. The ascorbic acid contents were quantified as under: AA (mg 100 g-1 f.w) = Dye factor × Titration × Volume made up / Weight of sample × Volume of filtrate × 100 Dye standardization: 5 ml of standard ascorbic acid solution was diluted with 5 ml of metaphosphoric acid (3 %) and titrated with dye solution till pink color endures for 10 seconds. Dye factor was esti- mated (mg AA / ml of dye) as: Dye Factor (D.F) = 0.5 / Titration. Chlorophyll: Chlorophyll extraction from different tuber samples was carried out by spectrophotometric grade 99.5 % acetone (Sigma- Aldrich) and subsequent quantification was done in spectrophoto- meter (CE-2021, 2000 series CECIL Instruments Cambridge, Eng- land) as illustrated by PeRcival (1999). 5 g representative (selected from each side of potato tuber) lyophilized tissues were ground to a fine powder with the help of mortar and pestle. The sample was transferred to test tubes followed by extraction with 10 ml of acetone. The extract was vortexed then stored at 4 oC for 24-72 hours. After storage, chilled extracts were again vortexed and centrifuged for 15 min at around 2500 × g. The supernatant was collected for chlo- rophyll determination. Chlorophyll content was measured in 1 cm cuvettes at A647 and A664.5 using Spectrophotometer (CE-2021, 2000 series CECIL Instruments Cambridge, England). Total chlorophyll concentrations were ex- pressed as mg 100 g-1 d.w from the following equation: Total Chlorophyll = 17.90 (A647) + 8.08 (A664.5) Total glycoalkaloids: The total glycoalkaloids (TGA) determination was carried out by the method described by GRunenFeldeR et al. (2006). Ground lyophilized potato tissue (500 mg) was extracted in 10 ml of 80 % ethanol at 85-90 oC for 25 minutes. The extract was filtered and reduced to 3-5 ml on rotary evaporator at 50 oC. Each extract was rinsed twice with 3 ml of 10 % (v/v) acetic acid and then centrifuged at 10,000 × g for 30 minutes at 10 oC. The pH of the supernatants was adjusted at 9.0 with NH4OH. The extract was re- fluxed at 70 oC for 25 minutes followed by overnight storage at 4 oC temperature. The extract was similarly centrifuged as earlier, after discarding the supernatants the resulting pellets were dissolved in 0.5 ml of 7 % (v/v) phosphoric acid and stored at -20 oC. The total glycoalkaloids were estimated by adding 200 μL of extract in 1 ml of 0.03 % (w/v) in concentrated phosphoric acid. The contents were allowed to settle for 20 minutes, and absorbance was measured at 600 nm. TGA concentrations were quantified based on α-solanine (Sigma-Aldrich) standard curve using a CE-2021, Spectrophoto- meter (CECIL Instruments Cambridge, England) and expressed as mg TGA 100 g-1 d.w. Total phenolic contents: Total phenolic contents (TPC) estimated as Gallic Acid Equivalent (GAE) were carried out by Folin-Ciocal- teu (FC) assay as explained by lachmann et al. (2008) with few modifications. Tubers randomly selected were freeze dried and then extracted with 80 % ethanol. 2 g extract was quantitatively converted into 100 ml volumetric flask and adjusted with 80 % ethanol. In 5 ml of the sample slightly diluted with distilled water, 2.5 ml of FC and 7.5 ml of 20 % solution of sodium carbonate were added. Contents were allowed to settle for 2 hours and absorbance was measured at 765 nm using a CE-2021, Spectrophotometer (CECIL Instruments Cambridge, England). Total phenolic contents were quantified by standard calibration curve derived from the absorbance of known gallic acid concentration (10-100 ppm). Results were articulated as mg GAE 100 g-1 d.w. Radical scavenging activity (RSA): Antioxidant activity was mea- sured as radical scavenging activity (RSA) using the method de- scribed by sinGh and Rajini (2004) that involves electron transfer reaction based assay by employing free radical 2, 2-diphenyl-1- picrylhydrazyl (DPPH). Five mg of freeze-dried potato extract was incubated with 1.5 ml of DPPH solution (0.1 mM in 95 % Ethanol). The reaction mixture was properly shaken and allowed to stand for 20 minutes at ambient temperature. The absorbance of the resultant mixture was determined at 517 nm against blank. The radical sca- venging activity was determined as a decrease in the absorbance of DPPH using the following equation: Radical scavenging activity (%) = 1 - Asample 517 nm / AControl 517nm × 100 Statistical analysis: Date obtained as the mean of three replications were statistically analyzed by two-factor factorial in Completely Ran- domized Design (CRD), and treatments and storage intervals mean comparisons were carried out by Duncan Multiple Range test using M-Stat-C Statistical software as described by steel et al. (1997). Photo-induced storage of potato tubers 317 Results and discussion The general trend was an increase in glucose contents in all treat- ments except under dark storage. The treatment means exhibited a maximum increase in fluorescent and red lights and were found at par statistically (α-0.05). Minimum glucose retention was demon- strated in dark followed by green and mercury lights. Storage interval means showed minimum glucose contents at the start and maximum at the end of storage period. The interaction between treatments and storage intervals revealed a significant increase in glucose contents during the last week of storage under blue, fluorescent and red lights (Fig. 1). The increase in glucose contents in tubers initiated early under blue, fluorescent and red illuminations and remained two folds than the rest of treatments by the end of 3rd week. Green and mercury lights retained moderate increase in glucose contents during most of the storage period. Glucose contents recorded at the end of storage in the dark were found lower than that recorded in fluorescent and red lights by the end of 2nd week and found lowest throughout the storage period. The glucose contents accumulation in potato tubers is associated with the starch hydrolysis during the storage period (sonnewald, 2001) and accelerated during tuber stress due to exposure to high energy illuminations (PeRcival, 1993). The greater increase in glu- cose contents during storage under fluorescent, red and blue lights as compared to other treatments might be due to the increased relative degradation of starch contents. The results were also in close confir- mation with the findings of chen and setteR (2003) who reported decreased glucose contents in potato tubers under dark storage. Total sugar accumulation in potato tubers under different illumi- nations progressively increased during storage. This increase was found in consistence with the trend observed in glucose contents. The treatment means showed maximum sugar accumulation in fluores- cent followed by storage under blue and red lights (α-0.05). Storage interval means showed a non-significant increase in sugar content during 1st week followed by a prominent increase during the rest of storage period. The interaction between treatments and storage inter- vals showed maximum sugar accumulation under fluorescent light while minimum estimated in dark at the end of the trial (Fig. 2). In general, total sugar increased under all illuminations, however, the increase was highly significant under fluorescent, blue and red lights. In contrast, a steady increase was observed under mercury and green lights. Storage under dark at the end of storage period retained low- est sugar contents than that estimated under all other illuminations. In terms of their sugar accumulation blue and red illumination were found same during most of the storage period. Continuous exposure of potato tubers to different light sources caused sugar accumulation due to starch hydrolysis mediated through tu- ber stress (PeRcival, 1999). The initial increase in sugar contents in response to different illuminations during storage has also been reported by olson (1996). dale et al. (1993) reported elevated sugar contents in potato exposed to continuous illumination as compared to those placed under dark. The moderate sugar contents identified under green and mercury lights might be due to lesser starch hy- drolysis owing to their low energy spectrums, which have also been confirmed by nema et al. (2008). Data related to starch contents in response to different illumination revealed non-significant initial increase followed by a progressive decline with the increase in storage period. The treatment means showed minimum starch contents in blue, fluorescent and red lights and were found similar (α-0.05). Dark storage retained maximum starch contents at the end of storage followed by green and red lights. The storage interval means showed maximum starch content during 1st week storage and then declined significantly by the end of sto- rage period. The interaction between treatments and storage intervals showed minimum starch contents in blue, fluorescent, and red lights on last day storage (Fig. 3). In general, after an initial increase all the treatments experienced starch depletion in response to different illuminations. However, the decrease was highly prominent in red (17.90 %), blue (17.94 %), and fluorescent (18.10 %) lights as compared to other treatments. The minimum decline in starch contents was reported in the dark (18.59 %) followed by green (18.42 %) and mer- Fig. 1: Effect of different light sources on glucose. Vertical bars show ±SE of means (n=3). The interaction between storage duration and light sources was significantly different at p ≤ 0.05. Fig. 2: Effect of different light sources on total sugar. Vertical bars show ±SE of means (n=3). The interaction between storage duration and light sources was significantly different at p ≤ 0.05. Fig. 3: Effect of different light sources on starch (%). Vertical bars show ±SE of means (n=3). The interaction between storage duration and light sources was significantly different at p ≤ 0.05. 318 K.S. Abbasi, T. Masud, A. Qayyum, A. Ahmad, A. Mehmood, Y. Bibi, A. Sher cury (18.27 %) illuminations at the end of storage. The initial increase in starch contents might be due to the tenden- cy of freshly cure potato to accumulate dry matter primarily in the form of starch (Kaul et al., 2010). The metabolism of carbohydrate contents during post-harvest storage is very important both in table and processing potato varieties (heRRman et al., 1996). The quality of potato tubers keeps on changing due to depletion of starch and formation of corresponding sugars during storage (nouRian et al., 2003). nema et al. (2008) reported that continuous exposure of tuber to high energy wavelengths cause physiological stress characterized by increased rate of respiration followed by starch depletion. Maxi- mum starch depletion in potato under different high energy illumina- tions (fluorescent, blue and red) during the present study confirmed the findings reported above. The general decline in starch content in different treatments might be attributed to the consumption of respi- ratory substrate (starch) during post-harvest storage which has also been reported by aBBasi et al. (2015). The general trend showed reduction in ascorbic acid (AA) contents in response to different illuminations during the storage time. The treatment means showed a significant difference between them with maximum and minimum AA retention in dark and fluorescent light, respectively (α-0.05). The storage interval means showed a signifi- cant difference in AA contents with maximum retention at the start and minimum recorded at the end of the trial. The interaction be- tween treatments and storage intervals showed appreciable retention of AA in dark followed by green during most of the storage period. Significant reduction in AA contents was observed in fluorescent and red lights after one-week illumination (Fig. 4). In the present study, exposure of potato tubers to different illuminations caused a significant reduction in AA. The reduction was highly significant in fluorescent light after mid storage i.e. 21.97 mg 100 g-1 which lasted up to 17.8 mg 100 g-1 by the end of one-month storage. By the end of one-month storage tubers, exposure to blue and red lights retained 19.13 mg 100 g-1 and 19.73 mg 100 g-1 AA contents, respectively. Moderate reduction of AA contents also reported in the case of green (21.70 mg 100 g-1) and mercury (21.40 mg 100 g-1) illumina- tions during the same storage period. Dark storage of potato retained maximum AA contents and the eventual decline observed was only around 22.37 mg 100 g-1. Ascorbic acid is considered as important dietary antioxidant vita- minand known to decline during post-harvest storage period in fruits and vegetables (lee and KadeR, 2000) due to oxidation (PiGa et al., 2002), photodegradation (lesKova et al., 2006) or utilization as a respiratory substrate (KadeR, 2002). Owing to its high degree of sen- sitivity it is considered as an imperative index of quality in fruits and vegetable during storage and food processing (ozKan et al., 2004). In addition, AA depletion has been associated with reduced nutritional quality; therefore their assured stability during storage has been a major concern of the post harvest technologists (aBBasi et al., 2016 a). dale et al. (2003) reported that AA contents decrease in horticul- tural products due to light exposure, heat; low relative humidity and prolonged storage time, hence, are critical considerations in optimiz- ing suitable post-harvest storage conditions. Similar results were re- ported in the present study by maximum retention of AA in dark storage as compared to different illuminations. Chlorophyll contents continue to increase significantly during the storage time in response to different light exposures. The treatment means showed maximum chlorophyll accumulation under fluores- cent followed by red and blue lights (α-0.05). Considerable chloro- phyll contents were estimated in red followed by restrained contents identified under green and mercury lights. Chlorophyll contents remained at a minimum level under dark storage. The interaction between treatments and storage intervals demonstrated minimum contents witnessed in most of the treatments by the end of 1st week storage, whereas maximum contents were estimated in blue, fluores- cent and red lights by the end of last week (Fig. 5). Chlorophyll contents increased progressively under fluorescent, blue and red illuminations and showed steady increase under dark during the storage period. The contents started to increase in high energy illuminations within 1st week of storage particularly in fluorescent light, i.e. 1.283 mg 100 g-1. In contrast, lowest chlorophyll contents were estimated in the dark i.e. 0.721 mg 100 g-1 during the same storage period. A similar trend continued with maximum contents that were recorded under fluorescent light i.e. 3.64 mg 100 g-1. This significant linear increase in the chlorophyll contents was more than six folds over the complete storage period under fluorescent light. The response of potato to exposure to red (3.219 mg 100 g-1) and blue (3.231 mg 100 g-1) illuminations accumulated considerable chloro- phyll contents as compared to mercury (1.99 mg 100 g-1) and green (1.66 mg 100 g-1) lights at the end of storage. In general, all different kind of illuminations caused an increase in chlorophyll contents at erratic pace with no indication of termination during the complete trail. Exposure of potato tubers in response to light caused the forma- tion of chlorophyll in cortical parenchyma due to the conversion of amyloplast into chloroplast (Pavlista, 2001).The extent of greening in retail outlets emphasizing the development and realization of the appropriate light source to maintain desirable quality attributes in Fig. 4: Effect of different light source on ascorbic acid. Vertical bars show ±SE of means (n=3). Interaction between storage duration and light sources was significantly different at p ≤ 0.05. Fig. 5: Effect of different light sources on chlorophyll. Vertical bars show ±SE of means (n=3). The interaction between storage duration and light sources was significantly different at p ≤ 0.05. Photo-induced storage of potato tubers 319 potato tubers. PeRcival (1999) compared the response of different tuber varieties to various light sources and reported irrespective of variety maximum chlorophyll accumulation under fluorescent and sodium illuminations as compared to mercury light and dark. GRunenFeldeR et al. (2006) exposed different colored potato varie- ties to fluorescent lights of similar intensity and found discoloration of periderm in all of them due to a significant increase in chlorophyll contents. The present study demonstrated significant chlorophyll accumulation in potato tubers due to various illuminations. Our re- sults concluded that the replacement of fluorescent light with green or mercury illuminations in retail displays, might cause a reduction in potato greening as also reported by researchers above. Dark po- tato storage demonstrated appreciable tuber quality due to minimum chlorophyll accumulation and thus might be recommended for pro- longed tuber storage, which has also been concluded by different researchers like nema et al. (2008) and machado et al. (2007). Data related to total glycoalkaloids (TGA) showed progressive in- crease under different illuminations during storage. The treatment means showed a significant difference between their TGA contents with maximum retention in fluorescent followed by blue and red lights, while minimum TGA contents were identified under dark storage (α-0.05). The storage interval means showed minimum TGA contents during 1st week and then progressively increased until the end of storage. A significant interaction was observed between treat- ments and storage intervals with highest TGA contents estimated in fluorescent during last week (Fig. 6). The TGA contents increased throughout the storage period irrespective of treatments, however, the rate of increase in TGA contents was higher in fluorescent, blue and red light illuminations. The increase in TGA contents under fluo- rescent light was maximum (70.40 mg 100 g-1 d.w) and estimated around 8-folds as compared to the 2.5-folds increase in the dark by the end of storage period. The increase in TGA contents under fluo- rescent illumination at the end of storage surpassed the safe limits described for human intake i.e. 20 mg 100 g-1 f.w (mensinGa et al., 2005). In general, percentage increase in TGA within different sto- rage interval was found highest in last week. Minimum TGA accu- mulation was identified in the dark (18.90 mg 100 g-1 d.w) followed by mercury (28.03 mg 100 g-1 d.w) and green (30.93 mg 100 g-1 d.w) illuminations. TGA contents estimated in the present study regarding solanine equivalent are affected by different illuminations and are also as- sociated with the elevated chlorophyll contents. GRunenFeldeR et al. (2006) studied the increase in TGA and chlorophyll contents under fluorescent light and concluded parallel but independent de- velopment of both compounds. machado et al. (2007) compared TGA accumulation in potato tubers under different illuminations during postharvest storage. She found maximum and minimum TGA contents under fluorescent light and dark respectively as also observed in the present study. Another investigation carried out by PeRcival et al. (1999) observed light-induced TGA accumulation in potato tubers under four different illuminations. He declared maxi- mum contents under fluorescent and sodium lights and minimum under dark. Our results in the present study also confirmed the pre- vious finding as steady TGA contents in the dark as compared to significant increase under fluorescent light. The trial also exhibited a parallel association between chlorophyll and glycoalkaloids accumu- lations in potato tubers. The process is however found independent due to increased TGA contents in blue to red and green to mercury lights having less corresponding chlorophyll contents during the same storage period. Total phenolic contents (TPC) increased steadily during the storage period in response to different illuminations. However, the increase is followed by a considerable decline in blue, fluorescent and red lights. In general non-significant difference in treatment means was observed except in mercury and dark storage (α-0.05). The storage interval means, however, showed significant difference with maxi- mum and minimum TPC identified at the end and the start of the experiment. Highly significant interaction was observed between treatments and storage intervals after the mid storage period (Fig. 7). Total phenolic contents increased during the start of sto- rage in all the treatments, and the initial rise was found indepen- dent of different illuminations. In general non-significant difference was observed in most of the treatments till 9th day storage. TPC con- tents significantly increased in all the treatments by the end of 2nd week storage, which was followed by progressive decline in fluores- cent, blue and red illuminations during the last week of storage. TPC increased continuously under green, mercury and dark storage with no sign of cessation during the storage period. However, the rate of TPC increase during the 1st half of storage was found slower as com- pared to high energy illuminations (blue, fluorescent and red lights). Total phenolic contents showed varied retention under different il- luminations during the storage period. Light is considered as an important factor causing the biosynthesis of phenolic compounds facilitated by the activity of phenylalanine ammonia-lyase enzymes (lewis et al., 1998). In addition, it is also known to initiate the an- thocyanin and chlorogenic biosynthesis pathways contributing to the total phenolic contents in potato tubers (GRiFFiths, 1995). The swift initial increase in TPC under different light sources and steady TPC Fig. 6: Effect of different light sources on total glycoalkaloids. Vertical bars show ±SE of means (n=3). The interaction between storage du- ration and light sources was significantly different at p ≤ 0.05. Fig. 7: Effect of different light sources on total phenolic content. Vertical bars show ±SE of means (n=3). Interaction between storage duration and light sources was significantly different at p ≤ 0.05. 320 K.S. Abbasi, T. Masud, A. Qayyum, A. Ahmad, A. Mehmood, Y. Bibi, A. Sher under dark partially confirmed the findings reported by different researchers above. These results, however, negated the investigation reported by Reyes and zevallos (2003) who reported no consider- able effect of light on the TPC accumulation in purple-fleshed potato tubers. The possible reason might be due to the varietal difference and lack of their comparative study under different illuminations. Tubers placed under green and mercury lights retained appreciable TPC at the end of storage probably due to lack of tuber stress and moderate respiration rate. The decline in TPC contents under high energy illuminations like fluorescent and blue at the end of storage might be attributed to the tuber stress due to high metabolic rate, increased rate of respiration causing an eventual decline in TPC con- tents. Radical scavenging activity (RSA) estimated as % inhibition of DPPH showed slight initial increase followed by gradual decrease under all illuminations except in the dark. Treatment means showed that dark storage maintained maximum activity followed by green and mercury lights, while minimum RSA activity was shown un- der fluorescent followed by blue lights (α-0.05). Storage interval means showed maximum activity till 2nd week and then steadily de- clined till the end. The interaction between treatments and storage intervals was initially found less significant and then became highly significant by the end of storage period. The maximum activity was expressed in green and dark during 3rd and 2nd week storages, re- spectively (Fig. 8). In general RSA activity exhibited initial increase till mid storage period and finally declined to form a sort of para- bolic curve. However, the rate of increase and decrease in activity was considerably affected by different illuminations. Radical sca- venging activity increased during the first week and attained maxi- mum by the second week in blue (45.43 %), fluorescent (45.80 %) and red (47.57 %) lights followed by progressive decline afterward. The trend remained same at variable pace in green (48.73 %), mercury (47.33 %) and red (49.47 %) where they showed maximum activity by the end of the third week followed by a steady decline onwards. Overall results revealed maximum retention of RSA in dark storage (45.57 %) followed by green (43.60 %) and mercury (42.77 %) lights at the end of storage period. It is eventually observed that, except in dark storage, potato tubers placed under different illuminations lost considerable RSA activity after the completion of storage period. RSA activity corresponds to the antioxidant potential retained by the potato tubers and is asso- ciated with the presence of different functional components, such as phenolic compounds, carotenoids, ascorbic acid and tocopherols (aBBasi et al., 2016 b). In the present study, storage under different illuminations conferred diverse effects on these antioxidant com- ponents as attributed to the parallel accumulation of phenolics and depletion of ascorbic acids. The prolonged exposure of potato tubers to high energy illuminations might confer tuber stress consequently resulted in the loss of phenolic substrate due to the increased poly- phenol oxidase activity (BRyant, 2004). The significant retention of RSA activity at the end of storage under dark as compared to dif- ferent illuminations might be due to the appreciable retention of dietary antioxidants, such as ascorbic acid, total phenolic contents etc. hejtmanKova et al. (2009) and lachmann et al. (2008) have also expressed this significant correlation between RSA activity and these functional components. Conclusion It is concluded that during thirty day storage (25 ± 2 oC) under diffe- rent illuminations (blue, fluorescent, green, mercury, red, dark), po- tato tubers presented variable response regarding their investigated quality parameters. Best results were identified in potato tubers kept under dark with minimum loss of quality parameters. Storage under different illuminations showed maximum retention of quality para- meters in green and mercury lights with better storage life. Tuber stability was found highly susceptible to fluorescent light with least retentions of quality parameters. Storage under red and blue light also resulted in decreased storage life with the loss of quality at- tributes studied. 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