The Photosynthetic Performance of Red Leaf Lettuce under UV-A Irradiation

High Light Intensity from Blue-Red LEDs Enhance Photosynthetic Performance, Plant Growth, and Optical Properties of Red Lettuce in Controlled Environment

Plant factories using artificial light to produce vegetables have high energy costs due to the high demand for electricity for lighting. Compared to conventional light sources, light-emitting diodes (LEDs) offer the possibility of tailoring the light spectrum and regulating light intensity and are more energy-efficient in terms of energy conversion regardless of the levels of lighting intensity. Optimal light intensity and daily light integral (DLI) requirements are key factors for plant growth; however, their values vary among species and varieties. Our experiment aimed to identify the best light intensity to produce lettuce plants in controlled environment. Lettuce plants of the type Batavia cv ‘Blackhawk’ were grown in plastic pots filled with perlite and peat (20:80 v/v) for 33 days in a growth chamber under blue (B, 20%) and red (R, 80%) LED light at a photosynthetic flux density of 130 µmol m−2 s−1 (BR 130, DLI 7.49 mol m−2 d−1), 259 µmol m−2 s−1 (BR 259, DLI 14.92 mol m−2 d−1), and 389 µmol m−2 s−1 (BR 389, DLI 22.41 mol m−2 d−1). Our results showed that increasing light intensity and DLI promotes net photosynthesis, sustains the electron transport rate (ETR), and stimulates the synthesis of anthocyanins and carotenoids, with positive results for plant photoprotection. Furthermore, the decreases in vegetation indexes (photochemical reflectance index (PRI), greenness, and modified chlorophyll absorption in reflectance index (MCARI1)) also indicate changes in photosynthetic pigment content in response to plant acclimation to different DLIs. Among the three light intensities, 389 µmol m−2 s−1 (DLI 22.41 mol m−2 d−1) gave the best results for growing Batavia red lettuce cv ‘Blackhawk’, since it enhances both production and qualitative traits. These results highlight the importance of a proper light intensity to promote plant growth and qualitative traits and to reach high production targets. Hence, preliminary screening of plant performance under different light treatments is recommended to optimise plant response to artificial lighting.

Comparative Growth, Photosynthetic Pigments, and Osmolytes Analysis of Hemp (Cannabis sativa L.) Seedlings under an Aeroponics System with Different LED Light Sources

The performance of hemp seedlings was evaluated through morphological traits, photosynthetic pigments, and osmolytes under 11 light treatments (10 LED light compositions + natural light) in an aeroponics system. The seedlings were brought under treatment at 25 days of age, where the light intensity was 300 µmol m−2s−1 and duration was 20 days. A higher leaf number and node number were observed in L10 (R4:B2:W2:FR1:UV1) and L11 (R2:B2:G2:W2:FR1:UV1), and a higher leaf length and leaf width were recorded in the L2 (white), L3 (R8:B2), and L5 (R7:B2:FR1) treatments. Furthermore, a higher shoot length was recorded in L3 (R8:B2), L6 (R6:B2:G1:FR1), and L9 (R6:B2:FR1:UV1) while roots developed more in the L1 (natural light), L5 (R7:B2:FR1), and L9 (R6:B2:FR1:UV1) treatments. On the other hand, the L3 (R8:B2) treatment manifested higher chlorophyll a, chlorophyll b, and photosynthetic quantum yield (Fv/Fm). The hierarchical clustering and heatmap analysis revealed that higher leaf numbers and node numbers resulted in bushy plants with shorter shoots and longer roots. A negative correlation was also observed in photosynthetic traits (pigments and fluorescence) with osmolytes and root length. Importantly, the treatments L4 (R7:B2:G1), L6 (R6:B2:G1:FR1), L8 (R5:B2:G1:FR1:UV1), and L11 (R2:B2:G2:W2:FR1:UV1) manifested higher nodes with a higher osmolyte content, such as proline, ascorbic acid, total soluble carbohydrate, and sucrose, which may be a helpful indicator for higher branches and inflorescences, and ultimately higher cannabinoids accumulation in the plants. The approach and findings of this study could provide future research with the baseline information on optimizing the light composition to produce hemp plants with ideal phenotypes.

Cannabinoids Accumulation in Hemp (Cannabis sativa L.) Plants under LED Light Spectra and Their Discrete Role as a Stress Marker

Hemp adaptability through physiological and biochemical changes was studied under 10 LED light spectra and natural light in a controlled aeroponic system. Light treatments were imposed on 25 days aged seedlings for 16 h daily (300 µmol m-2 s-1) for 20 days. Plant accumulated highest Cannabidiol (CBD) in R7:B2:G1 light treatment, with relatively higher photosynthetic rate and lower reactive oxygen species, total phenol content, total flavonoid content, DPPH radical scavenging capacity, and antioxidant enzymatic activities. Tetrahydrocannabinol (THC) also accumulated at a higher level in white, R8:B2, and R7:B2:G1 light with less evidence of stress-modulated substances. These results indicated that CBD and THC have no or little relation with light-mediated abiotic stress in hemp plants. On the contrary, Tetrahydrocannabinolic acid (THCA) was accumulated higher in R6:B2:G1:FR1 and R5:B2:W2:FR1 light treatment along with lower photosynthetic rate and higher reactive oxygen species, total phenol content, total flavonoid content, DPPH radical scavenging capacity, and antioxidant enzymatic activities. However, Cannabidiolic acid (CBDA) was accumulated higher in R6:B2:G1:FR1 light treatment with higher stress-modulated substances and lower physiological traits. CBDA was also accumulated higher in R8:B2 and R7:B2:G1 light treatments with less evidence of stress-modulated substances. Besides, Greenlight influenced CBD and CBDA synthesis where FR and UV-A (along with green) play a positive and negative role in this process. Overall, the results indicated that the treatment R7:B2:G1 enhanced the medicinal cannabinoids most, and the role of THCA as a stress marker is more decisive in the hemp plant than in other cannabinoids under attributed light-mediated stress.

Plant biology for space exploration - Building on the past, preparing for the future

A review of past insights of space experiments with plants outlines basic space and gravity effects as well as gene expression. Efforts to grow plants in space gradually incorporated basic question on plant productivity, stress response and cultivation. The prospect of extended space missions as well as colonization of the Moon and Mars require better understanding and therefore research efforts on biomass productivity, substrate and water relations, atmospheric composition, pressure and temperature and substrate and volume (growth space) requirements. The essential combination of using plants not only for food production but also for regeneration of waste, and recycling of carbon and oxygen production requires integration of complex biological and engineering aspects. We combine a historical account of plant space research with considerations for future research on plant cultivation, selection, and productivity based on space-related environmental conditions.

Regulation of the Photon Spectrum on Growth and Nutritional Attributes of Baby-Leaf Lettuce at Harvest and during Postharvest Storage

The photon flux density (PFD) and spectrum regulate the growth, quality attributes, and postharvest physiology of leafy vegetables grown indoors. However, limited information is available on how a photon spectrum enriched with a broad range of different wavebands regulates these factors. To determine this, we grew baby-leaf lettuce ‘Rouxai’ under a PFD of 200 µmol m⁻² s⁻¹ provided by warm-white (WW; control) light-emitting diodes (LEDs) supplemented with either 30 µmol m⁻² s⁻¹ of ultraviolet-A (+UV30) or 50 µmol m⁻² s⁻¹ of blue (+B50), green (+G50), red (+R50), or WW (+WW50) light. We then quantified growth attributes and accumulated secondary metabolites at harvest and during storage in darkness at 5 °C. Additional +G50 light increased shoot fresh and dry weight by 53% and 59% compared to the control. Relative chlorophyll concentration increased under +UV30, +G50, and especially +B50. At harvest, +B50 increased total phenolic content (TPC) by 25% and anthocyanin content (TAC) by 2.0-fold. Additionally, +G50 increased antiradical activity (DPPH) by 29%. After each day of storage, TPC decreased by 2.9 to 7.1% and DPPH by 3.0 to 6.2%, while TAC degradation was less pronounced. Principal component analysis indicated a distinct effect of +G50 on the lettuce at harvest. However, concentrations of metabolites before and during storage were usually greatest under the +B50 and +R50 treatments.