Spectral composition of LED light differentially affects biomass, photosynthesis, nutrient profile, and foliar nitrate accumulation of lettuce grown under various replacement methods of nutrient solution

Abstract

To enhance crop yield and quality, plant cultivation in controlled‐growing systems is an alternative to traditional open‐field farming. The use of light‐emitting diode (LED) as an adjustable light source represents a promising approach to improve plant growth, metabolism, and function. The objective of this study was to assess the impact of different light spectra (red, red/blue (3:1), blue, and white) with an emission peak of around 656, 656, 450, and 449 nm, respectively, under various replacement methods of nutrient solution (complete replacement (CR), EC‐based replacement (ECBR), and replacing based on plant needs (RBPN)), on biomass, physiological traits, and macro‐ and micronutrient contents of two best‐known lettuce varieties, Lollo Rossa (LR) and Lollo Bionda (LB), in the nutrient film technique (NFT) hydroponic system. The results indicated that mix of red and blue LED spectra under RBPN method is the most effective treatment to enhance fresh and dry weights of lettuce plants. In addition, red LED spectrum under RBPN, and red and blue light under ECBR nutrient solution significantly increased leaf stomatal conductance, net photosynthesis and transpiration rate, and intercellular CO2 concentration of LR variety. Phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mn) content in LR variety, and iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) content in both varieties increased upon exposure to blue and red LED light spectrum with RBPN method. Our results suggest that exposure to combination of red and blue light along with feeding plants using RBPN and ECBR methods can increase absorption of macro‐ and micronutrient elements and improve photosynthetic properties, and eventually increase lettuce yield with lower nitrate accumulation.

Dynamics of micro and macronutrients in a hydroponic nutrient film technique system under lettuce cultivation

“…Precisely controlled environmental and lighting conditions in indoor grow rooms versus greenhouses may also lower nutrient losses [ 48 ]. Lighting conditions may also influence N uptake, and lettuce tissue N contents were found to increase 16–17 % under red/blue LED lights compared to white lights [ 49 ]. Furthermore, N overapplication during the early plant stages and heterotrophic microbial communities may contribute to denitrification and nitrous oxide (N 2 O) emissions [ 50 ].…”

“…This suggests that hydroponic growers could reduce P inputs into continuously NFT recirculating system without compromising yield [ 55 ]. Nutrient management methods based on maintaining a high EC through the addition of pre-mixed nutrient blends and weekly nutrient solution replacement resulted in similar tissue N and P contents [ 49 ]. Early stages of lettuce production in NFT systems were characterized by high nutrient concentrations remaining in the nutrient solution but increases in lettuce growth over time led to increased nutrient and water demand, reducing nutrient and water losses [ 49 ].…”

“…Nutrient management methods based on maintaining a high EC through the addition of pre-mixed nutrient blends and weekly nutrient solution replacement resulted in similar tissue N and P contents [ 49 ]. Early stages of lettuce production in NFT systems were characterized by high nutrient concentrations remaining in the nutrient solution but increases in lettuce growth over time led to increased nutrient and water demand, reducing nutrient and water losses [ 49 ]. Sensors-based approaches may provide a tool for managing N concentrations in recirculating nutrient solutions; however, lower EC setpoints at early lettuce stages and incremental EC adjustments based on biomass increases and corresponding nutrient uptake increases may improve hydroponic NFT efficiency.…”