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LETTUCE QUALITY MANAGEMENT SYSTEMS
Recommendations for Maintaining Postharvest Quality
Types of Lettuce:
Iceberg Lettuce
Romaine Lettuce
Boston Lettuce
Bibb Lettuce
Looseleaf Lettuce
Batavia Lettuce
Little Gem Lettuce
Oak Leaf Lettuce
Rec Coral
Arugula
Butterhead lettuce
Coral lettuce
Cress
Endive
Escarole
Frisée
Since the early 2000s, plant factory (or vertical farm) technology has been introduced for growing vegetables and soft fruits. With a well-controlled environment, new health benefits, food safety, optimized nutrients and increased shelf-life can be offered to consumers. With the progress of light emitting diode (LED) lighting efficiency and the knowledge of light-plant interaction, a better quality control can now be achieved together with improved energy efficiency. Growth strategies combining crop quality attributes (e.g., color, nutrients, shelf life) with efficient growth are key for economic viability of plant factories. Most research so far has been addressing quality and growth efficiency separately. Several strategies exist from literature to improve quality attributes, but so far not in terms of optimization of the total growth efficiency including space and energy use. We are aiming to achieve a high growth efficiency (in g mol-1) and at the same time fulfill the requirements on crop quality; for example: high yield, good color (high anthocyanin index or chlorophyll index) and texture (firmness), high flavonoids content or controlled nitrate levels. An optimization routine has been used with high technical engineering and plant physiology approach in a state of the art plant factory research center at Philips Research Laboratories. LED lighting with a large variety of spectral composition (from UV to far-red) and dynamic control has been used with a total radiation level dimmable per color up to 600 µmol m-2 s-1. In this presentation we will illustrate our optimization approach (growth recipe) with specific experimental results on three different red lettuce cultivars with results showing the evolution of the anthocyanin accumulation, spacing optimization and yield during growth.
Maturity & Quality
Romaine or cos lettuce is an elongated heading lettuce type. Maturity is based on the number of leaves and head development. A very loose or easily compressible head is immature and a very firm or hard head is overmature. Heads that are immature (
Quality Indices
After trimming outer leaves, the leaves should be a bright to dark green color (tinged with red in the red romaine cultivars) with the inner leaves of the head being yellow or light green. The bright to dark green of romaine leaves is indicative of higher vitamin A and vitamin C contents relative to iceberg lettuce types. Leaves should be crisp and turgid, and free from insects, decay or mechanical damage (U.S. Grade No. 1). Different romaine varieties may vary in sweetness and bitterness.
Temperature & Controlled Atmosphere
Optimum Temperature
0°C (32°F) is required to optimize the postharvest life of romaine lettuce. A shelf-life of around 21 days is expected at this temperature. At 5°C (41°F) a shelf-life of about 14 days can be expected as long as no ethylene is in the environment. Water spray-vacuum cooling or hydrocooling are often used for romaine lettuce, but forced-air cooling may also be used.
Freezing Injury. Freeze damage can occur in the field and cause separation of the epidermis from the leaf. This weakens the leaf and leads to bacterial decay during storage. Freeze damage can occur during storage if the lettuce is held at <-0.2°C (31.7°F). This appears as darkened translucent or water-soaked areas that will turn slimy and deteriorate rapidly after thawing.
Relative Humidity
>95%
Rates of Respiration
Romaine lettuce heads have moderate respiration rates, but they are generally higher than rates for iceberg lettuce:
Temperature 5°C (41°F) 10°C (50°F) 15°C (59°F) 20°C (68°F)
ml CO2/kg·hr 9-12 15-20 19-25 30-38
To calculate heat production multiply mL CO2/kg·hr by 440 to get Btu/ton/day or by 122 to get kcal/metric ton/day.
Rates of Ethylene Production
Ethylene production rates are very low:
Responses to Ethylene
Romaine lettuce is sensitive to ethylene. Ethylene damage appears as discolored spots on the midrib. These are generally larger and less defined than those found with ethylene-induced Russet spotting on iceberg lettuce (see physiological disorders). Varieties can vary significantly in their susceptibility to ethylene.
Responses to Controlled Atmospheres (CA)
Some benefit to shelf-life can be obtained with low O2 atmospheres (1-3%) at temperatures of 0-5°C (32-41°F). Low O2 atmospheres will reduce respiration rates and reduce the detrimental effects of ethylene. Intact heads are not generally benefited by atmospheres containing CO2 and injury may occur with >5% CO2 (see physiological disorders, brown stain). Cut Romaine lettuce, however, is commonly packaged in low O2 (<1%) and high CO2 (7-10%) atmospheres because these conditions control browning on the cut surfaces. On salad pieces, cut surface browning occurs more rapidly and more extensively than do symptoms of brown stain caused by CO2. Cut iceberg lettuce tolerates higher CO2 concentrations than cut romaine lettuce.
Disorders
Physiological and Physical Disorders
Several disorders can occur on romaine lettuce. Some very common and important disorders are the following.
Tipburn. A disorder caused in the field and is related to climactic conditions, variety selection and mineral nutrition. Leaves with tipburn are unsightly and the damaged leaf margins are weaker and susceptible to decay.
Ethylene injury. Due to exposure to low concentrations of ethylene gas which stimulates the production of phenolic compounds which in turn leads to brown pigments. Russet spots appear as dark brown spots especially on the midribs. Under severe conditions, russet spots are also found on the green leaf tissue and throughout the head. The disorder is strictly cosmetic but makes the lettuce unmarketable. Ethylene contamination may occur from propane fork lifts, transport in mixed loads, or storage with ethylene-generating fruits such as apples, pears, etc.
Brown Stain. The symptoms of this disorder on romaine lettuce heads are yellowish-reddish-brown large, depressed spots or stains. These are most noticeable on the midribs, and may darken and enlarge with time. Brown stain is caused by exposure to CO2-containing atmospheres, especially at concentrations above 5%. Visual symptoms of brown stain may occur less rapidly on Romaine than on iceberg lettuce.
Pink rib. A disorder associated with heads that are overmature. Higher than recommended storage temperatures can also lead to a increased incidence of pink rib. In this disorder, the midribs take on a generalized pinkish coloration. Ethylene exposure does not appear to affect pink rib and low O2 atmospheres do not control it.
Breakage of the midribs often occurs during field packing, especially on overmature romaine heads, and results in unsightly browning and increased susceptibility to decay. Product harvested early in the morning, when pulp temperatures are lower, is more susceptible to midrib cracking and breakage.
Pathological Disorders
Bacterial soft-rots are caused by numerous bacteria species and result in a slimy breakdown of the infected tissue. Soft-rots may follow fungal infections. Trimming outer leaves, rapid cooling and low temperature storage reduce development of bacterial soft-rots.
Fungal pathogens may also lead to a watery breakdown of lettuce (watery soft-rot caused by Sclerotinia or gray mold rot caused by Botrytis cinerea) but are distinguished from bacterial soft-rots by the development of black and gray spores. Trimming and low temperatures also reduce the severity of these rots.
Special Considerations
Cut or broken midribs of Romaine lettuce may discolor more rapidly than cut pieces of iceberg lettuce. This is probably due to the higher content of phenolic compounds found in romaine leaves compared to iceberg leaves. Romaine varieties can vary greatly in the rate and severity of discoloration of cut pieces.
Sensory quality attributes of lettuce obtained using different harvesting performance systems
The objective of this study was to determine the best lettuce cultivar (American 'Graciosa', 'Vanda', 'Marcela' and 'Lavínia') harvesting method. Therefore, quality and shelf-life were evaluated using sensory analyses. Lettuce heads was harvested with the root on by the producer, so that they were cut in different ways in the laboratory. The samples were stored in a cold chamber at 10 °C and 80% ± 2% of relative humidity for nine days, and the sensorial analyses were performed according to Qualitative Descriptive Analysis method on days 1, 3, 6, and 9 of storage by twelve trained testers. The results were evaluated by variance analysis, principal component analysis, and the Tukey test with a reliability of 95%. The results indicate that there was a reduction in the quality of lettuce between the 1st and the 5th day of storage and that after the sixth day of storage the lettuce samples were considered unfit for consumption, except for the 'Lavínia' lettuce cultivar with root and cut treatment 2. On the ninth day of storage all samples were considered inappropriate for consumption.
Lactuca sativa L.; shelf-life; quality
Sensory quality attributes of lettuce obtained using different harvesting performance systems
Denize Cristine Rodrigues de Oliveira*; Paulo Ademar Martins Leal; Sylvio Luís Honório;Eveline Kássia Braga Soares
Laboratório de Tecnologia Pós-colheita, Departamento de Engenharia Agrícola, Faculdade de Engenharia Agrícola, Universidade Estadual de Campinas - UNICAMP, Av. Cândido Rondon, 501, Barão Geraldo, CEP 13083-875, Campinas, SP, Brasil, e-mail: denize.cris@hotmail.com
ABSTRACT
The objective of this study was to determine the best lettuce cultivar (American 'Graciosa', 'Vanda', 'Marcela' and 'Lavínia') harvesting method. Therefore, quality and shelf-life were evaluated using sensory analyses. Lettuce heads was harvested with the root on by the producer, so that they were cut in different ways in the laboratory. The samples were stored in a cold chamber at 10 °C and 80% ± 2% of relative humidity for nine days, and the sensorial analyses were performed according to Qualitative Descriptive Analysis method on days 1, 3, 6, and 9 of storage by twelve trained testers. The results were evaluated by variance analysis, principal component analysis, and the Tukey test with a reliability of 95%. The results indicate that there was a reduction in the quality of lettuce between the 1st and the 5th day of storage and that after the sixth day of storage the lettuce samples were considered unfit for consumption, except for the 'Lavínia' lettuce cultivar with root and cut treatment 2. On the ninth day of storage all samples were considered inappropriate for consumption.
Keywords: Lactuca sativa L.; shelf-life; quality.
1 Introduction
Among the vegetables widely consumed in Brazil is the lettuce cultivar Lactuca sativa L., a leafy vegetable belonging to the Astereceae family (FELTRIM et al., 2005).
In the state of Sao Paulo, the volume of commercialized leafy vegetables increased between 1999 and 2009 from 84 thousand to 136 thousand tons, totaling 63%. The Asteraceae family had the highest growth, from 27 thousand to 52 thousand tons, totaling 93% increase, and the lettuce reached 82% of the volume in 2009. (INSTITUTO..., 2008; SISTEMA..., 2010).
Fruits and vegetables are valued food because they play an important role in human nutrition and health. However, some are highly perishable, especially leafy vegetables, requiring attention and special handling in all stages of the production.
One of the reasons they are extremely perishable, besides their high water content, is that they are still alive even after harvest, thus maintaining their vital processes. Hence, they are forced to use their reserves of substrate (such as sugars and starch) to breathe and to produce necessary energy to stay alive (CHITARRA; CHITARRA, 2005).
When subjected to physical stresses, these products have their metabolism accelerated, leading to an increase in the respiratory rate causing faster deterioration (PORTE; MAIA, 2001).
The lettuce postharvest processing operations in tropical countries such as Brazil presents several challenges to those involved in the production and marketing chain due to the high perishability of this product. Moreover, the value of leafy vegetables is low in most markets, which makes difficult the adoption of more advanced techniques of postharvest (TIVELLI, 2007).
There are few similar studies related to this proposal since most of the studies on crop systems available in the literature are focused specifically on the product and productivity. There are very few studies on differentiated crop systems of lettuce plants or on the optimization and improvement of crop systems. Therefore, the present study aimed to compare the different processes of lettuce plant harvesting in order to verify the best system to prolong the useful life of lettuce plants.
Sensory analysis, a tool widely used due to its ease and speed of implementation, can be used for quality control. It is carried out in different stages starting from the receipt of raw materials up to finished products. The Qualitative Descriptive Analysis method (QDA) evaluates all the sensory attributes of food products, namely appearance, aroma, flavor, and texture.
In the sensory evaluation of lettuce, appearance has a major influence in choosing the product since brown spots on the leaves and extremities are factors that lead to the refusal to purchase the product (KADER, 2002; HEIMDAL et al., 1995). Thus, physiological parameters are associated to sensory quality, which are essential requirements for the acceptance and success of these products. This study aimed to evaluate the lettuce shelf life harvest process (cut in different ways - on the stem or with the whole root - using the traditional method of cutting in the control sample) under controlled conditions of temperature and relative humidity using sensory analyses.
2 Materials and methods
The experiment was conducted at the Laboratory of Postharvest Technology (LTPC) of the School of Agricultural Engineering (FEAGRI), University of Campinas (UNICAMP), located at the University City "Zeferino Vaz," Barão Geraldo district, Campinas/SP, in September 2011. During that time, the maximum and minimum temperature of 27.8 °C and 15.8 °C, respectively, were detected in Campinas.
Four varieties of lettuce (American ''Graciosa' (Tecnoseed), 'Vanda' (Sakata), 'Marcela' (Hortec), and 'Lavinia' (Sakata)) were used. The plants were obtained from a commercial plantation located on Sítio Noda s/n, Colônia Tozan, Mogi Mirim km 10, Campinas-SP.
All lettuce plants collected were grown under the same conditions the lettuce beds dimensions were 1.00 m/100 m, the number of vegetables per bed was 1.200 units, and the spacing between rows was 12 seedlings per m2. Around 45 days after planting, they were harvested in the morning with the root on by the producer. The cut was performed ± 1 cm above the soil level using a knife. The most damaged outer leaves were discarded. Another cut was made (±3 cm) aiming at the removal of part of the stem that did not had leaves in order to obtain cleaner product.
The plants were transported to the LTPC in plastic boxes (40 × 60 × 23 cm; ten units per box). Next, the plants were rinsed with tap water and stored in cold chamber (Profrio Modular Serie 34512) under temperature and relative humidity of 10 °C ± 1 °C and 80% ± 2%, respectively.
Some samples were harvested with the root on and were cut in different regions at the laboratory: cut 1, root - stem transition, and cut 2, from the beginning of the root zone and the first leave. A caliper was used to standardize the cutting height with the distance of 1mm above the root (cut 1) and the distance of 15 mm (cut 2). The cuts were made by carbon steel blade, like a scalpel, sterilized.
The plants were analyzed on days 1, 3, 6, and 9 after harvest by a team of twelve trained testers.
The following visual attributes: color, freshness, mechanical damage, brightness, and general appearance were analyzed using the Qualitative Descriptive Analysis (QDA) (KADER, 2002; MEILGAARD; CIVILLE; CARR, 1999).
For the choice and definition of the concepts and attributes to be analyzed, the most important product qualities were taken into consideration.
The purpose was to train the testers using the same vocabulary to describe the sensory properties of the lettuce plants (Table 1). During the training, the concept of each attribute was also introduced through photos and lettuce samples that were selected according to the end-points of the scale used.
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Table 1. Profile Attributes used in conventional lettuce, definitions, and references.
Next, an evaluation sheet composed of nine-point hedonic scale was used, in which the end points were lack of quality and excellence in quality. The same sheet model was used to each vegetable.
The experimental design consisted of a completely randomized in a 4 × 4 factorial arrangement (four varieties of lettuce - 'Graciosa', 'Vanda', 'Marcela' and 'Lavínia') and four different cuttings - with root on (WR); producer's cut (PC); cut on the stem -root transition (C1); and between the beginning of the root zone and the first leave (C2).
A total of 320 lettuce plants were analyzed, and they were acquired according to the number of replications and treatments (4 × 4 × 5 = 80); four varieties of lettuce, four harvest systems, and 5 repetitions, totaling 80 units per cultivar.
The results were subjected to descriptive analysis, and when it was confirmed that the data followed a normal distribution, the analysis of variance was performed and, if significant, the mean comparison between the treatments by the Tukey test with confidence level of 95% using the statistical ASSISTAT® version 7.6 beta (SILVA, 2011).
A multivariate analysis method, called principal component analysis (PCA), was also used; the data obtained in this study were analyzed using the R® software version 2.14.1 (R DEVELOPMENT..., 2011).
3 Results and Discussion
According to Almeida (1995) and Gonzales, Burin and Buena (1999), the instrumental color can be used as a parameter to establish the quality standard of an in natura or processed product. It can also be used as a factor to determine the shelf life of a product, when variation during storage is studied.
The parameters freshness and brightness are also essential for product acceptance by the consumer, and they are directly associated with the water present in vegetables. Moisture stress begins with the loss of moisture to the environment, which is characterized by the loss of turgidity, and therefore it is necessary to control the temperature and relative humidity in an attempt to maintain this parameter (THOMPSON, 2004).
Another important factor is the preservation of vegetables with regards to mechanical damage, which can be caused by many factors and at any time during the life cycle of raw materials. Any damage to the vegetable tissue induces physiological and biochemical activities resulting in deterioration of the product (PORTE; MAIA, 2001). Thus, good handling practices are essential throughout the production chain to assure food quality.
Considering all of the attributes mentioned above: color, brightness, freshness, and damage; it can be concluded that the general appearance is given by the grouping of these parameters perceived by the consumer. All of these factors and the customer's quality expectations are taken into account before purchasing.
The sensory profiles of the lettuce samples obtained by quantitative descriptive analysis are shown in the radar chart (Figures from 1 to 4). The center of the figure represents the point 1 of a range of attributes, while the intensity increases from the center to the periphery.
Lettuce Grades and Standards
U.S. Fancy consists of heads of lettuce which meet the following requirements:
a. Basic requirements:
1. Similar varietal characteristics;
2. Fresh;
3. Green;
4. Not soft;
5. Not burst;
b. Free from:
1. Decay;
2. Russet spotting;
3. Doubles;
c. Free from injury by:
1. Tipburn;
2. Downy mildew;
3. Field freezing;
4. Discoloration;
d. Not damaged by any other cause.
e. Each head shall be fairly well trimmed unless specified as closely trimmed.
f. For tolerances see §51.2513.
U.S. No. 1 consists of heads of lettuce which meet the following requirements:
a. Basic requirements:
1. Similar varietal characteristics;
2. Fresh;
3. Green;
4. Not soft;
5. Not burst;
b. Free from:
1. Decay;
2. Doubles.
c. Not damaged by any other cause.
d. Each head shall be fairly well trimmed unless specified as closely trimmed.
e. For tolerances see §51.2513..
U.S. No. 2 consists of heads of lettuce which meet the following requirements:
a. Basic requirements:
1. Similar varietal characteristics;
2. Not burst;
b. Free from decay;
c. Not seriously damaged by any other cause;
d. Unless otherwise specified each head shall be reasonably trimmed.
e. For tolerances see §51.2513.
Lettuce Quality Control Techniques and Related Factors for Hydroponic Leafy Vegetables
Hydroponics has been an increasingly important field of vegetable production. However, a big issue with hydroponics is that certain crops can quickly accumulate high levels of nitrate-N (NO3 ± -N) from the hydroponic system. The objective of this research was to decrease NO3 accumulation and increase the nutritional value and yield of vegetable crops using lettuce and oilseed rape as a model under hydroponic production. In this study, two technologies were applied to leafy vegetable production: 1) using supplementary lighting (blue-violet diode) by manipulating illumination and 2) removing fertilization before harvest for a short term (3 or 5 days), thus providing a practical experiment for improving yield and edible qualities of hydroponic leaf vegetable production. Illumination was applied 4 hours a day (0500–0700 hr and 1700–1900 hr) during good weather, or 12 hours a day during bad weather with insufficient natural light (<2000 lux) during the autumn and winter seasons. Results showed that the lettuce cultivar Ou-Luo and the oilseed rape cultivar Ao-Guan Pakchoi had increased yield (50.0% and 88.3%, respectively), decreased NO3 content (26.3% and 30.8%, respectively), and increased total soluble solids (24.1% and 30.6%, respectively). The 5-day fertilizer-free treatment before harvest resulted in 19.2%, 6.4%, and 16.5% yield increases; and 26.0%, 24.3%, and 47.8% NO3 decreases in oilseed rape cultivar Ao-Guan Pakchoi and lettuce cultivars Da-Su-Sheng and Ou-Luo, respectively.
Keywords: lettuce; oilseed rape; supplementary light; fertilization; nutritional value
Hydroponics is an increasingly important field for counterseason vegetable production because of its efficiency in fertilization, water, and space use. Furthermore, it can overcome the disadvantages of soil culture, such as continuous cropping obstacles, diseases, and pests (Sardare and Admane, 2013). According to the market research report by Transparency Market Research (2018), the global hydroponics market is anticipated to reach a value of US$12.1 billion by the end of 2015 from US$6.9 billion in 2016 (Mordor Intelligence, 2018). The market is likely to register a promising 6.50% compound annual growth rate between 2017 and 2025. Green-leaf vegetables are considered to be a good source of ascorbic acid (vitamin C), beta carotene, iron (Fe), calcium, folate, and fiber; they are also low in calories and sodium; and all varieties are free of fat and cholesterol (Jones, 1982). Compared to hydroponic fruit, green-leaf vegetables are easy to plant and their production is low in cost because of the relatively short cultivation period (35 d) and simple cultivation facilities (Jones, 2016). Therefore, this segment is projected to lead the global market in coming years.
However, fast accumulation of high level of nitrate (NO3-)N in plants from mineral fertilizer is a big issue with hydroponic vegetable production (Colla et al., 2010). Human uptake of NO3 is mainly derived from the consumption of raw vegetables (80%) and may be detrimental to one’s health (Rathod et al., 2016). NO3 itself is relatively harmless, because the fatal adult dose is considered to be ≈100-fold greater than the acceptable daily intake of NO3– set by the European Union. Contrary to the relatively nondeleterious effect of the nitrate ion, when nitrite accumulates in the human body to a certain extent, it can form a strong carcinogen—nitrosamine—which may lead to carcinogenesis of the digestive system (Mensinga et al., 2003). The direct contribution of vegetables, fruits, and herbs to nitrite intake is relatively low (Riens and Heldt, 1992); however, the reduction of NO3 to nitrite is ubiquitous in the organism when it was mediated by the endogenous: about 5% of the NO3 is converted to nitrite after being ingested (Santamaria, 2006). Therefore, the accumulation of NO3 was considered to be a crucial factor in reducing the edible qualities of some vegetables. The NO3 content in leafy vegetables is related mainly to species and varieties, followed by environmental factors (e.g., light, soil, and moisture) and management (e.g., water, fertilization, and harvest) (Colla et al., 2018; Santamaria, 2006).
In hydroponic crops, relative research on manipulating light and fertilization has increased significantly, and systematical studies have provided integrated and elaborate information to enlighten and guide hydroponic production (Colla et al., 2018). However, most studies were conducted in a laboratory or at a small scale, lacking reports of researching experiments were carried out on real and large-scale production conditions, because of the limit in labor, time, and facilities (Craker and Seibert, 1983; Kitaya et al., 1998; Li and Kubota, 2009; Rajapakse and Shahak, 2008).
Therefore, in our study, we implemented operability improvement on the management technologies of lighting and preharvest fertilization in the actual production process of commercial greenhouses to obtain practical and feasible measures for controlling the accumulation of NO3 and total soluble solids in hydroponic vegetables. We used lettuce (Lactuca sativa L.) and oilseed rape (Brassica napus L.) as model species in the research, which are the most commonly grown hydroponic leafy vegetables in North American and East Asia, respectively (Fitt et al., 2006; Resh, 2016). The objective of our research was to decrease NO3 accumulation and increase vitamin C and soluble sugar content, without losing yield in vegetable crops, by manipulating lighting and using two short-term (3 d or 5 d) fertilization breaks before harvest. About 18,480 lettuce and 5280 oilseed rape plants (including controls) were involved in our study, thus providing a theoretical basis for improving the qualities of hydroponic leaf vegetables in a practical case.
Materials and Methods
One oilseed rape cultivar, Ao-Guan Pakchoi; and two lettuce cultivars, American Da-Su-Sheng and Ou-Luo were used as examples of leaf vegetable crops in our study (Fig. 1A). The nutrient film technique (NFT) was applied to the experiments as hydroponic technology. The seedlings of the three cultivars were planted in the cultivation tank using the intensive plug-seeding method (Han, 2016), and the nutrient solution was circulated and flowed on the bottom of the container so the root system could absorb nutrients and water continuously, with a sufficient oxygen supply.
Fig. 1.View Full Size
Fig. 1.
(A) The varieties of leafy greens used in the experiment (from top to bottom): lettuce of Da-Su-Sheng, oilseed rape of Ao-Guan Pakchoi, and lettuce of Ou-Luo. (B) The plants and light-emitting diode (LED) arrangements. SL3, single light 75 cm from the vertical light (VL) hole; SL2, single light 50 cm from the VL hole; SL1, single light 25 cm from the VL hole; OL1, overlapping light at 25 cm from the VL hole; OL2, overlapping light at 50 cm from the VL hole; OL3, overlapping light at 75 cm from the VL hole. (C) Light intensities in different positions. SW, southwest; NW, northwest; SE, southeast; SW, southwest.
The lettuce bluish violet (370–480 nm) LEDs (LX1330B; Sampo, Shanghai, China) were arranged evenly above the hydroponic shelves, with 18 hydroponic cells per LED on average. When daytime natural lettuce lighting was more than 2000 lux, the LEDs were turned on from 0500 to 0700 hr each morning and from 1700 to 1900 hr in the evening (4 h/d). When daytime natural lighting was less than 2000 lux, the LEDs were turned on all day (0500–1900 hr). Light intensities were detected every hour at different positions under four LEDs: southwest, northwest, southeast, and northeast, as shown in Fig. 1B. The position of the LEDs and hydroponic cells have three lighting modes: single lighting (SL), vertical lighting (VL), and overlapping lighting (OL). Details of the illumination model from left to right in Fig. 1B are as follows: SL3, single light 75 cm from the vertical light (VL) hole; SL2, single light 50 cm from the VL hole; SL1, single light 25 cm from the VL hole; VL hole 40 cm below the LED perpendicularly; OL1, overlapping light at 25 cm from the VL hole; OL2, overlapping light at 50 cm from the VL hole; OL3, overlapping light at 75 cm from the VL hole.
After 30 d of growing, the growth traits and edible qualities of hydroponic lettuce were recorded to compare with the control groups, which were grown without illumination. Two durations (3 d and 5 d) of fertilization break were applied before vegetable harvest. The nutrient solution was replaced with water to cut all nutrient supplies. After harvest, growth traits and edible qualities were estimated for each treatment.
“Voluntary U.S. lettuce grade standards are issued under the authority of the Agricultural Marketing Act of 1946, which provides for the development of official U.S. grades to designate different levels of quality. These grade standards are available for use by producers, suppliers, buyers, and consumers. As in the case of other standards for grades of fresh and processed fruits, vegetables, and specialty crops these standards are designed to facilitate orderly marketing by providing a convenient basis for buying and selling, for establishing quality control programs, and for determining loan values.” (From the United States Standards for Grades of Greenhouse Leaf Lettuce)
Analysis of lettuce variance was performed using the general linear model of JMP Genomics 7. Student’s t test at α = 0.05 was used for multiple comparisons of the least square mean among the genotypes. The correlation coefficient for the traits was calculated using JMP Genomics 7 software.
Result
Supplementary lighting treatment and light intensities of different cells.
As shown in Fig. 1B, under the four LEDs, the cell (hole) in the vertical light got the highest intensity, reached 1990.32 lux without natural light. Those cells farther away from the lights experienced gradually decreasing light intensity, and the value for each point is shown in Fig. 1C and Table 1. All light intensities for each cell in the illumination group were greater than the control.
Table 1.
Light intensities and increases at different positions under four lamps.
Table 1.View Table
Supplementary lighting impact on growth traits.
To evaluate the lighting impact on growth and development of hydroponic vegetables, we evaluated the following growth traits: plant height, chlorophyll amount, leaf length, leaf width, fresh weight, and yield (Table 2). Plant height, chlorophyll amount, leaf width, and fresh weight of illuminated ‘Ou-Luo’ were greater than the controls, especially for the plants in VL cells, which had a 50.0% increase in yield. These values declined with a decrease in light intensity. Similar impacts were also found in ‘Ao-Guan Pakchoi’. Plant height with supplemental lighting was significantly greater than the control. The best yield was 7751.7 kg/acre in VL cells, which was an 88.31% increase over the control. The gradient changes in growth traits in both lettuce and oilseed rape revealed that proper illumination could promote the growth and development of leafy vegetables.
Table 2.
Growth traits under different lighting treatments.
Table 2.View Table
Supplementary lighting impact on edible qualities.
Under lighting conditions, ‘Ou-Luo’ displayed considerable variation in all nutrition traits compared to the control (Table 3). NO3 content deceased by 26.30%; total soluble solids and sugar contents increased as much as 24.05% and 33.5%, respectively; and vitamin C showed a gradual decline with light intensity increase. The NO3 content in ‘Ao-Guan Pakchoi’ with supplemental lighting was reduced as much as 30.76% than plants with insufficient lighting. Total soluble solids and sugar contents increased by 30.6% and 30.5%, respectively; but vitamin C content decreased under supplemental lighting.
Table 3.
Edible qualities in different lights.
Table 3.View Table
After being subjected to lighting treatments, the accumulation of NO3 in lettuce correlated negatively with soluble sugar content and total soluble solids (r = –0.699 and r = –0.787, respectively); a positive correlation between NO3 content and vitamin C content was seen (r = 0.788) (Table 4). The same phenomena were also seen in oilseed rape: NO3 content correlated negatively with total soluble solids and soluble sugar content (r = –0.956 and r = –0.813, respectively); and a positive correlation between NO3 and vitamin C content was seen (r = 0.741) (Table 4). These results indicate that lighting treatments can decrease NO3 content and simultaneously increase total soluble solids and soluble sugar contents, but not vitamin C content.
Table 4.
Correlations of nitrate, vitamin C, soluble sugar, and total soluble solid contents under light supplementation.
Table 4.View Table
Fertilization-break impact on growth traits.
In our study, “fertilization break” was defined as cutting off all nutrient supply in the short term. The nutrient solution in the hydroponic system was replaced by pure water 3 d or 5 d before harvest. As shown in Table 5, the growth traits of ‘Ao-Guan Pakchoi’ with the fertilization break were greater than the control, especially plant height, chlorophyll amount, root volume, and plant weight. The two treatments (3 d and 5 d) improved yield by 10.7% and 19.2%, respectively.
Table 5.
Growth traits in different fertilization-break treatments.
Table 5.View Table
Moreover, the growth traits of the 5-d treatment were slightly greater than the 3-d treatment. In ‘Ou-Luo’, the amount of chlorophyll, root volume, and plant weight increased significantly. Moreover, yield increased by 11.9% and 16.5%, respectively, with the 3-d and 5-d treatments. However, for ‘Da-Su-Sheng’, there was no significant difference between the control and the treatments in almost all traits. There was only a 2.0% and 6.4% increase in plant weight. These results confirm that a short-term fertilization break does not reduce plant development or yield, but may increase them instead.
Fertilization-break impact on edible qualities.
The results in Table 6 reveal that during the 3-d and 5-d treatments, the NO3 content of ‘Ao-Guan Pakchoi’ decreased by 20.9% and 26.0%, respectively; followed by vitamin C content decrease of 19.9% and19.6%, respectively; a total soluble solids content decrease of 2.6% and 8.7%, respectively; and soluble sugar decreased by 0.7% and 3.9%, respectively, compared with the control. The decreased NO3 content was detected in ‘American Da-Su-Sheng’ (15.9% and 47.8%). The fertilization break also improved other edible qualities of ‘American Da-Su-Sheng’, especially during the 5-d treatment. The soluble sugar content increased by 54.0%, vitamin C content increased by 82.8%, and the total soluble solid content increased by 27.9%. We also noticed the 5-d treatment in ‘Ou-Luo’ improved edible qualities as well. The NO3 content was reduced by 24.3%, the soluble sugar content increased by 88.6%, vitamin C content increased by 16.7%, and total soluble solid content increased by 20%. These result indicate that a fertilization break decreased the NO3 content and, at the same time, increased total soluble solids, soluble sugar, and vitamin C contents, especially during the 5-d treatment for the two lettuce cultivars, but not for oilseed rape.
Table 6.
The edible qualities in different fertilization-break treatments.
Table 6.View Table
During the fertilization break, the NO3 content in ‘Ao-Guan Pakchoi’ correlated positively with vitamin C, total soluble solids, and soluble sugar contents (r = 0.980, 0.846, and 0.769, respectively) (Table 7). This means the treatment reduced the NO3 content and other edible qualities synchronously in similar degrees. Contrary results were observed in lettuce; NO3 correlated negatively with soluble sugar, vitamin C, and total soluble solids contents (r = –0.943, –0.981, and –0.975, respectively for ‘American Da-Su-Sheng’; and r = –0.977, –0.930, and –0.858 for ‘Ou-Luo’, respectively). The fertilization break increased soluble sugar, vitamin C and total soluble solids contents while reducing NO3, especially in the 5-d treatment. In addition, there were no negative influences on yield.
Table 7.
Correlation of nitrate, vitamin C, soluble sugar, and total soluble solid contents after nutrient break.
Table 7.View Table
Discussion
Growth traits and edible qualities.
In our study, we describe NO3 content and total soluble solids (vitamin C and soluble sugar) as “edible qualities” to evaluate vegetable toxins and nutrition. It is worth noting that total soluble solids is a general term for all soluble substances (Kader, 2002). Because of limitations in labor and cost, we measured only two of them—vitamin C and soluble sugar—individually. Total soluble solids in our study is regarded as an index equaling vitamin C and soluble sugar used assess nutritional value.
For two management strategies (supplementary light and fertilization), the growth traits we selected in the experiments were somewhat different. Often, insufficient lighting stimulates petiole development and elongation, which make leaves growing lengthwise; on the contrary, leaves elongate sideways under adequate light (Muramoto et al., 1965; Pepper et al., 1994; van der Graaff et al., 2000). It has been reported that the shortage of nutrients in hydroponic solution could stimulate root development (Hodge et al., 2009; Trejo-Téllez and Gómez-Merino, 2012). Therefore, in our study, we tested leaf growth and root features of the plants to verify the effects of lighting and fertilization breaks. We found that leaf and root features are in accordance with growth in insufficient lighting and fertilization-free conditions. These two representative morphology changes can help researchers ensure rationality of the results quickly. We also measured chlorophyll content to evaluate the health of the plants, because abundant chlorophyll usually implies vigorous growth and development of plants (Chaerle and Van Der Straeten, 2001).
Supplementary lighting.
Light is one of the most critical factors during plant growth. Despite photosynthesis, most plant characteristics are also influenced by the mode of light, including intensity, rhythm, period, and type (Kami et al., 2010; Takemiya et al., 2005). There are many theories which can explain the correlation between light and NO3 content, and the lighting drove activity changes of NO3 reductase can be one of the causes (Konstantopoulou et al., 2010). As others have reported, a reduction in light intensity is accompanied by a decrease in NO3 reductase activity, which induces fast NO3 accumulation in several important leafy vegetables (Fallovo et al., 2009; Pilgrim et al., 1993). Our research confirmed toxicity reduction with gradients in light intensity. Another explanation of NO3 reduction in our experiment is the variation of NO3 among different parts of every plant. The order of NO3 content has been listed by former researches as petiole > leaf > stem > root > inflorescence > tuber > bulb > fruit > seed (Santamaria et al., 1999). In general, the NO3 concentration in the petiole is about two to five times greater than in leaf, depending on the vegetable species (Elia et al., 2000; Koh et al., 2012; Umar et al., 2007). During the development of the plant, the petiole is the basic part to form the leaf, root, and other storage organs, where NO3 tends to accumulate, compared with other parts of the vegetable (Maynard et al., 1976; Santamaria et al., 1999). The greater growth of the petiole under insufficient lighting was recognized in our study, although it was not surprising to find high-level NO3 accumulation in these plants.
Despite our encouraging results, we found some discrepancies with other work. With supplemental lighting, vitamin C decreased slightly with increasing light intensity, which does not reflect results from previous studies (Li and Kubota, 2009). Those studies claimed that, with a rapid increase in biomass under supplemental lighting, vitamin C should increase synchronously with the growth surge (Sørensen et al., 1994). This phenomenon may be caused by inconsistent NO3 accumulation and vitamin C production during the whole growth period, which is affected easily by many factors (Chen et al., 2003; Lee and Kader, 2000).
Fertilization break.
N is the necessary element during plant growth and development, but it is also the source of the NO3 hazard (Mantelin and Touraine, 2004). An appropriate nutrient formula and management would minimize this harm without losses in yield or nutritional value (Bar-Yosef et al., 2009). Short-term fertilization break has been considered to be a reliable method for reducing the NO3 hazard (Borgognone et al., 2016). Our results showed that the yield of all tested plants increased, and NO3 contents declined, after the fertilization break treatment, which is consistent with the reports. As plants reach maturity, their requirements for N decrease (Blom-Zandstra and Lampe, 1983). Because the fertilization break removed all excess N supplies, plants demonstrated a dramatically reduced NO3 content (Borgognone et al., 2016; Malagoli et al., 2004). In another aspect, the absence of fertilization could stimulate root development, which could consume a large amount of NO3 stores in petioles. Because N mainly helps to form storage organs (roots, rhizomes, and tubers) (Alexander et al., 2008), the edible part (yield) of the vegetable would not be influenced by the treatments. The mechanism by which yield increased during the two fertilization-break treatments is still not clear yet.
Moreover, contrary to the lettuce cultivars used in our study, total soluble solids, soluble sugar, and vitamin C contents decreased with NO3 decline in oilseed rape as reported by Oh et al. (2009). Although this result can be explained by content variations among species, according to Bell (1993), further research is still needed to elucidate why fertilizer-break treatment reduced the NO3 content but increased yield, especially for those regional preference vegetables like oilseed rape.