Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

Vertical farming systems (VFS) have been proposed as an engineering solution to increase productivity per unit area of cultivated land by extending crop production into the vertical dimension. To test whether this approach presents a viable alternative to horizontal crop production systems, a VFS (where plants were grown in upright cylindrical columns) was compared against a conventional horizontal hydroponic system (HHS) using lettuce (Lactuca sativa L. cv. “Little Gem”) as a model crop. Both systems had similar root zone volume and planting density. Half-strength Hoagland's solution was applied to plants grown in perlite in an indoor controlled environment room, with metal halide lamps providing artificial lighting. Light distribution (photosynthetic photon flux density, PPFD) and yield (shoot fresh weight) within each system were assessed. Although PPFD and shoot fresh weight decreased significantly in the VFS from top to base, the VFS produced more crop per unit of growing floor area when compared with the HHS. Our results clearly demonstrate that VFS presents an attractive alternative to horizontal hydroponic growth systems and suggest that further increases in yield could be achieved by incorporating artificial lighting in the VFS.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/fes3.83

Vertical farming increases lettuce yield per unit area compared to conventional horizontal hydroponics

Microgreens are an excellent source of health-maintaining compounds, and the accumulation of these compounds in plant tissues may be stimulated by exogenous stimuli. While light quality effects on green basil microgreens are known, the present paper aims at improving the quality of acyanic (green) and cyanic (red) basil microgreens with different ratios of LED blue and red illumination. Growth, assimilatory and anthocyanin pigments, chlorophyll fluorescence, total phenolic, flavonoids, selected phenolic acid contents and antioxidant activity were assessed in microgreens grown for 17 days. Growth of microgreens was enhanced with predominantly blue illumination, larger cotyledon area and higher fresh mass. The same treatment elevated chlorophyll a and anthocyanin pigments contents. Colored light treatments decreased chlorophyll fluorescence ΦPSII values significantly in the green cultivar. Stimulation of phenolic synthesis and free radical scavenging activity were improved by predominantly red light in the green cultivar (up to 1.87 fold) and by predominantly blue light in the red cultivar (up to 1.73 fold). Rosmarinic and gallic acid synthesis was higher (up to 15- and 4-fold, respectively, compared to white treatment) in predominantly blue illumination. Red and blue LED ratios can be tailored to induce superior growth and phenolic contents in both red and green basil microgreens, as a convenient tool for producing higher quality foods.

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Blue and Red LED Illumination Improves Growth and Bioactive Compounds Contents in Acyanic and Cyanic Ocimum basilicum L. Microgreens.

Plasticity of photosynthetic processes and the accumulation of secondary metabolites in plants in response to monochromatic light environments: A review

The primary issues regarding the lack of protocol reproducibility among laboratories are environmental factors. Light (quantity and particularly quality), is one of those main factors, and studies seldom present the spectral quality of the light sources used. With the advent of light-emitting diode (LED) technology, impressive progress has been made in environmental controls and morphogenetic responses, as directed by the light used in the culture shelves. A wide array of LED lights with different spectra are currently available and light is important in large-scale propagation, especially liquid bioreactor systems. LED technology continues to evolve rapidly and has created additional possibilities. This laboratory has dedicated extensive efforts to implement photoautotrophic propagation, and light is a key component of the system. This review presents relevant topics on the influence of light in various plant tissue culture-based techniques.

Light quality in plant tissue culture: does it matter?

Mesembryanthemum crystallinum is a succulent, facultative CAM plant. Plant growth and photosynthetic characteristics were studied when M. crystallinum plants were grown indoor under LED-lighting with adequate water supply. Plants were cultured aeroponically for a 16-h photoperiod at an equal photosynthetic photon flux density (PPFD) of 350 µmol m-2 s-1 under different red:blue LED ratios: 1) 100:0 (0B); 2) 90:10 (10B); 3) 80:20 (20B), 4) 70:30 (30B), 5) 50:50 (50B) and 6)100:0 (100B). M. crystallinum grown under 10B condition had the highest shoot and root biomass and shoot/root ratio while those grown under 0B condition exhibited the lowest values. Compared to plants grown under 0B condition, all other plants had similar but higher total chlorophyll (Chl) and carotenoids (Car) contents and higher Chl a/b ratios. However, there were no significant differences in Chl/Car ratio among all plants grown under different red- and blue-LEDs. Photosynthetic light use efficiency measured by photochemical quenching (qP) and non-photochemical quenching (NPQ) and electron transport rate (ETR), demonstrated that plants grown under high blue-LED utilized more light energy and had more effective heat dissipation mechanism compared to plants grown under 0B or lower blue-LED. Statistically, there were no differences in photosynthetic O2 evolution rate, light-saturated CO2 assimilation rate (Asat), and light-saturated stomatal conductance (gs sat) among plants grown under different combined red- and blue-LEDs but they were significantly higher than those of 0B plants. No statistically differences in total reduced nitrogen (TRN) content were found among all plants. For the total soluble protein, all plants grown under different combined red- and blue-LEDs had similar values but they were significantly higher than that of plants grown under 0B condition. However, plants grown under higher blue-LEDs had significant higher Rubisco protein than those plants grown under lower blue-LED. High Asat and gs sat but very low CAM acidity of all M. crystallinum plants during light period, imply that this facultative CAM plant performed C3 photosynthesis when supplied with adequate water. Results of this study suggest that compared to red or blue-LED alone, appropriate combination of red- and blue-LED lighting enhanced plant growth and photosynthetic capacities of M. crystallinum.

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Plant Growth and Photosynthetic Characteristics of Mesembryanthemum crystallinum Grown Aeroponically under Different Blue- and Red-LEDs

Although tropical high ambient temperature and humidity
severely reduced the productivity of temperate plants, temperate vegetable crops
such as lettuce have been successfully grown in Singapore by only cooling its root-zone.
In this paper, a cool Meditteranean vegetable, Eruca sativa, was studied
to understand how different RZTs can impact its shoot productivity, photosynthesis
and nutritional quality. All plants were
cultivated using aeroponic systems in a tropical greenhouse under hot ambient conditions
where roots were subjected to four different root-zone temperatures (RZTs) of 20°C-RZT,
25°C-RZT, 30°C-RZT and fluctuating ambient temperatures ranged from
25°C to 38°C [25°C/38°C (ambient)]-RZT.
Parameters studied include shoot fresh weight (FW), photosynthetic gas exchange,
midday chlorophyll (Chl) fluorescence Fv/Fm ratio, Chl fluorescence
photochemical quenching (qP), non-photochemical quenching (qN) and electron transport
rate (ETR), total phenolic compounds and mineral content such as potassium (K),
calcium (Ca), magnesium (Mg) and iron (Fe). Among the 4 different RZT treatments, E. sativa plants grown
under ambient-RZT (25/38°C-RZT) had the lowest shoot and root FW while those
plants grown under 20°C-RZT had highest productivity
of shoot and root. However, there were no significant differences in shoot
and root FW in plants grown at 25°C- and 30°C-RZT. Compared to plants
grown under 25°C/38°C (ambient-RZT), light-saturated photosynthetic
CO2 assimilation rate (Asat)
and stomatal conductance (gssat)
were similarly higher in 20°C-, 25°C- and 30°C-RZT. All
plants had midday Chl fluorescence Fv/Fm ratio lower than
E. sativa plants protected their PS II from photoinactivation during midday in the
greenhouse. There were no significant differences observed in photochemical quenching
(qP), non-photochemical quenching (qN) and electron transport rate among plants
grown under 20°C-, 25°C- and 30°C-RZT. However, plants grown
under ambient-RZT had lower qP, qN and ETR compared to all other plants. E. sativa at 20°C-RZT with the best developed roots had the highest dietary mineral
(K, Mg, Ca and Fe) contents but lower total phenolics content. In contrast, ambient-RZT,
plants with poorly developed roots had the lowest mineral content but highest total
phenolic content. The results of this study suggest that cooling of roots is a feasible
method for the cultivation of E. sativa in the tropic, which enhances the
content of dietary minerals in shoots.

/pdf/effects-of-root-zone-temperature-on-photosynthesis-487ae1udlw.pdf

Effects of Root-Zone Temperature on Photosynthesis, Productivity and Nutritional Quality of Aeroponically Grown Salad Rocket ( Eruca sativa ) Vegetable

A major challenge for growing vegetables in an indoor vertical farming system will be supplying not only sufficient quantity but also quality of light. It has been reported that yield of crops is enhanced under appropriate combination of red and blue light compared with red light alone. This project aims to investigate the effects of different combinations of red and blue. Plants were cultured for a 12-h photoperiod at 210 μmol·m–2·s–1 photosynthetic photon flux density (PPFD) under different combinations of red (R) and blue (B) light-emitting diodes (LED). The R:B-LED ratios are: 1) 100:0 (0B); 2) 92:8 (8B); 3) 84:16 (16B) and; 4) 76:24 (24B). All combined RB-LEDs significantly increased light-saturated photosynthetic CO2 assimilation rate (Asat), stomatal conductance (gs sat) and productivity compared with those under 0B. Results suggested that 16B was the most suitable combination of LEDs to achieve the highest productivity for B. alboglabra. To further substantiate these results, comparative studies were conducted under equal photoperiod and PPFD among 16B (RB-LED), white LED (RBW-LED) and high-pressure sodium (HPS) lamps. Shoot, root biomass, leaf number, leaf mass per area and Asat were higher in plants under HPS lamps and RB-LED, than under RBW-LED. However, gs sat was lower under RB-LED and RBW-LED, than under HPS lamps. Plants under RB-LED had higher electron transport rate and photochemical quenching but lower non-photochemical quenching than those under RBW-LED and HPS lamps. Thus, these results more conclusively affirmed that 16B was the most suitable light source to achieve the highest photosynthetic capacities. The findings of this study could also be used in vertical farming to achieve the highest productivity of vegetable crops such as B. alboglabra within the shortest growth cycle with reduced energy consumption.

/pdf/photosynthetic-capacities-and-productivity-of-indoor-i6qp0v4obk.pdf

Photosynthetic Capacities and Productivity of Indoor Hydroponically Grown Brassica alboglabra Bailey under Different Light Sources

LED lamps with various combinations of red (R) and blue (B) wavelengths were used to supplement sunlight for the growth of a heat-resistant (HR) and heat-sensitive (HS) recombinant inbred lines (RIL) of lettuce. The RB-LED ratios were 100R:0B (0B), 92R:8B (8B), 84R:16B (16B), and 76R:24B (24B) with an equal PPFD of 100 μmol m−2 s−1. The greatest leaf expansion rates were observed at 8B for both genotypes. All HR-RILs had similar values of growth parameters and specific leaf area (SLA). However, higher values of growth parameters were observed in HS-RIL with 0B, 8B, and 16B than that under 24B and sunlight. Furthermore, HS-RIL had higher SLA under 0B compared to other conditions. Photosynthetic light-use efficiency and maximal oxygen evolution rate were the lowest under 8B for both genotypes. The quality of LED lighting, if provided, seemed to implicate genotype dependence, probably as a result of their different sensitivities to heat stress.

/pdf/quality-of-supplementary-led-lighting-effects-on-growth-and-zpvshiflqb.pdf

Quality of supplementary LED lighting effects on growth and photosynthesis of two different Lactuca recombinant inbred lines (RILs) grown in a tropical greenhouse

Plant propagation via in vitro culture is a very laborious and time consuming process. The growth cycle of some of the crop species is slow even in the field and the consistent commercial production is hard to maintain. Enhanced methods of reduced cost, materials and labour significantly impact the research and commercial production of field crops. In our studies, stem-segment explants of Brassica species were found to regenerate adventitious roots (AR) in aeroponic systems in less than a week. As such, the efficiency of root regeneration from stem explants of six cultivar varieties of Brassica spp were tested without the use of any plant hormones. New roots and shoots were regenerated from Brassica alboglabra (Kai Lan), Brassica oleracea var. acephala (purple kale), Brassica rapa L. ssp. Chinensis L (Pai Tsai, Nai Bai C and Nai Bai T) explants after 3 to 5 days of growing under 20±2°C cool root zone temperature (C-RZT) and 4 to 7 days in 30±2°C ambient root zone temperature (A-RZT). However, Brassica rapa nipposinica (Mizuna) explants were only able to regenerate in C-RZT. All regenerated explants were able to develop into whole plants, with higher biomass obtained from plants that grown in C-RZT. Moreover, explants from both RZTs produced higher biomass than plants grown from seeds (control plants). Regeneration efficiency was affected by RZTs and explant cuttings of donor plants. Photosynthetic CO2 assimilation rate (Asat) and stomatal conductance (gs sat) were not significantly differentiated between plants derived from seeds and explants at both RZTs. As such, this method of regeneration using aeroponics is suitable for all commercial cultivars studied. The findings of this study could be applicable in the mass propagation of vegetable crops, shortening the growth cycle as seed germination and seedling development periods can be eliminated.

/pdf/an-efficient-method-for-adventitious-root-induction-from-2hsi0n7c4n.pdf

Tsui Wei Choong

Papers

Plant Growth and Photosynthetic Characteristics of Mesembryanthemum crystallinum Grown Aeroponically under Different Blue- and Red-LEDs

Effects of Root-Zone Temperature on Photosynthesis, Productivity and Nutritional Quality of Aeroponically Grown Salad Rocket ( Eruca sativa ) Vegetable

Photosynthetic Capacities and Productivity of Indoor Hydroponically Grown Brassica alboglabra Bailey under Different Light Sources

Quality of supplementary LED lighting effects on growth and photosynthesis of two different Lactuca recombinant inbred lines (RILs) grown in a tropical greenhouse

An Efficient Method for Adventitious Root Induction from Stem Segments of Brassica Species