Light-emitting diodes (LEDs) have been widely applied in the controlled environment agriculture, which are characterized by relatively narrow-band spectra and energetical efficiency.  Most recently, the spectrum of Sunlike LEDs has been engineered and it closely resembles solar spectrum in the range of photosynthetic active radiation (PAR, 400–700 nm).  To investigate how plant growth responses to the spectrum of Sunlike LEDs, cucumber and lettuce plants were cultivated and their responses were compared with the conventional white LEDs as well as composite of red and blue LEDs (RB, R/B ratio was 9:1).  We observed that although Sunlike LEDs resulted in a longer stem in cucumber, dry weight and leaf area were similar as those under RB LEDs, and significantly higher than those under white LEDs.  Moreover, cucumber leaves grown under Sunlike and white LEDs showed higher photosynthetic capacity than those grown under RB LEDs.  For lettuce, plants grown under Sunlike LEDs showed larger leaf area and higher dry weight than the other two treatments.  However, the leaf photosynthetic capacity of lettuce grown under Sunlike LEDs was the lowest.  In this context, the spectrum induced plant functions are species-dependent.  Furthermore, the three types of LEDs show distinct light spectra and they are different in many aspects.  Therefore, it is difficult to attribute the different plant responses to certain specific light spectra.  We conclude that plants grown under Sunlike LEDs exhibit larger leaf area, which may be due to some specific spectrum distributions (such as more far-red radiation), and consequently are favorable for light interception and therefore result in greater production.

 

A deficiency in selenium (Se) in the human diet is a worldwide problem.  The intake of Se-rich vegetables can be a safe way to combat Se deficiency for humans.  However, most leafy vegetables can accumulate a high content of nitrates, which poses a potential threat to human health.  Light is an important environmental factor that regulates the uptake and distribution of mineral elements and nitrogen metabolism in plants.  However, the effects of Se forms and light conditions, especially light spectra, on the uptake and translocation of Se and on nitrate reduction are poorly understood.  In this study, lettuce (Lactuca sativa L.) was treated with exogenous Se applied as selenate (10 mmol L^–1) and selenite (0.5 mmol L^–1) and grown under five different light spectra: fluorescent light (FL), monochromatic red LED light (R), monochromatic blue LED light (B), and mixed red and blue LED light with a red to blue light ratio at 4 (R/B=4), 8 (R/B=8), and 12 (R/B=12), respectively.  The effects of light spectra and Se forms on plant growth, photosynthetic performance, Se accumulation and nitrate reduction were investigated.  The results showed that the light spectra and Se forms had significant interactions for plant growth, foliar Se accumulation and nitrate reduction.  The Se concentration and nitrate content in the leaves were negatively correlated with the percentage of red light from the light sources.  Compared to Se applied as selenite, exogenous Se applied as selenate was more effective in reducing nitrate via promoting nitrate reductase and glutamate synthase activities.  The lowest nitrate content and highest plant biomass were observed under R/B=8 for both the selenate and selenite treatments.  The significant effect of the light spectra on the root concentration factor and translocation factor of Se resulted in marked variations in the Se concentrations in the roots and leaves.  Compared with FL, red and blue LED light led to significant decreases in the foliar Se concentration.  The results from this study suggest that the light spectra can contribute to Se distribution and accumulation to produce vegetables with better food quality. 

The effect of external roof shading on the spatial distribution of air temperature and relative humidity in a greenhouse (T_in and RH_in) was evaluated under the arid climatic conditions of Riyadh City, Saudi Arabia.  Two identical, evaporatively-cooled, single-span greenhouses were used in the experiment.  One greenhouse was externally shaded (G_s) using a movable black plastic net (30% transmissivity), and the other greenhouse was kept without shading (G_c).  Strawberry plants were cultivated in both greenhouses.  The results showed that the spatial distribution of the T_in and RH_in was significantly affected by the outside solar radiation and evaporative cooling operation.  The regression analysis showed that when the outside solar radiation intensity increased from 200 to 800 W m^–2, the T_in increased by 4.5°C in the G_c and 2°C in the G_s, while the RH_in decreased by 15% in the G_c and 5% in the G_s, respectively.  Compared with those in the G_c, more uniformity in the spatial distribution of the T_in and RH_in was observed in the G_s.  The difference between the maximum and minimum T_in of 6.4°C and the RH_in of 10% was lower in the G_s than those in the G_c during the early morning.  Around 2°C difference in the T_in was shown between the area closed to the exhausted fans and the area closed to the cooling pad with the external shading.  In an evaporatively-cooled greenhouse in arid regions, the variation of the T_in and RH_in in the vertical direction and along the sidewalls was much higher than that in the horizontal direction.  The average variation of the T_in and RH_in in the vertical direction was 5.2°C and 10% in the G_c and 5.5°C and 13% in the G_s, respectively.  The external shading improved the spatial distribution of the T_in and RH_in and improved the cooling efficiency of the evaporative cooling system by 12%, since the transmitted solar radiation and accumulated thermal energy in the greenhouse were significantly reduced. 

Far-red (FR) light regulates phytochrome-mediated morphological and physiological plant responses.  This study aims to investigate how greenhouse tomato morphology and production response to different durations of FR light during daytime and at the end of day (EOD).  High-wire tomato plants were grown under intra-canopy lighting consisting of red (peak wavelength at 640 nm) and blue (peak wavelength at 450 nm) light-emitting diodes (LEDs) with photosynthetic photon flux density (PPFD) of 144 μmol m^–2 s^–1 at 10 cm away from the lamps, and combined with overhead supplemental FR light (peak wavelength at 735 nm) with PPFD of 43 μmol m^–2 s^–1 at 20 cm below the lamps.  Plants were exposed to three durations of FR supplemental lighting including: 06:00–18:00 (FR12), 18:00–19:30 (EOD-FR1.5), 18:00–18:30 (EOD-FR0.5), and control that without supplemental FR light.  The results showed that supplemental FR light significantly stimulated stem elongation thereby resulting in longer plants compared with the control.  Moreover, FR light altered leaf morphology toward higher leaf length/width ratio and larger leaf area.  The altered plant architecture in FR supplemented plants led to a more homogeneous light distribution inside the canopy.  Total plant biomass was increased by 9–16% under supplemental FR light in comparison with control, which led to 7–12% increase in ripe fruit yield.  Soluble sugar content of the ripe tomato fruit was slightly decreased by longer exposure of the plants to FR light.  Dry matter partitioning to different plant organs were not substantially affected by the FR light treatments.  No significant differences were observed among the three FR light treatments in plant morphology as well as yield and biomass production.  We conclude that under intra-canopy lighting, overhead supplemental FR light stimulates tomato growth and production.  And supplementary of EOD-FR0.5 is more favorable, as it consumes less electricity but induces similar effects on plant morphology and yield.