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Comprehensive Notes on Vertical Farming Systems

Review Article: Current Status and Future Challenges in Implementing and Upscaling Vertical Farming Systems

This review article, authored by S. H. van Delden and others, published in Nature Food in December 2021, discusses the state of the art of vertical farming and its future challenges across various fields.

Introduction and Overview

  • Global Challenges to Food Supply Chain: Rapid urbanization, climate change, land degradation, pandemics, biodiversity loss, and extensive use of pesticides and fertilizers. These factors threaten food security.
  • Consumer Demands: Increasing demand for healthy, tasty, locally produced, plant-based food with low environmental impact.
  • Food Waste: 24\% of all food currently produced never reaches consumers, partly due to low quality and long supply chains.
  • Vertical Farming Potential: Vertical farming (VF) offers a solution to these challenges by improving the production of high-quality products like fresh herbs, fruits, vegetables, and flowers. It can also boost the production of plant-based cosmetic and medicinal products.
  • Definition of Vertical Farming Systems (VFS) in this Review: Multi-layer indoor crop production systems without solar light, where growth conditions are precisely controlled. These systems enable year-round guarantees on product quantity and quality, independent of outdoor conditions, allowing for location-independent production (e.g., from tundra to desert, outer space to urban regions).
    • Ambiguity in Terminology (Box 1):
      • VFS can refer to multi-layer indoor crop production systems with artificial lights and precisely controlled growth conditions, where plants can grow vertically or horizontally.
      • Other terms include Plant Factories with Artificial Light (PFAL), Vertical Farms with Artificial Light (VFAL), and Fully Contained Cultivation Systems.
      • Smaller systems might be called Vertical Farming Units (VFU) or Container Farms.
      • When direct sunlight is used (e.g., high-tech multi-layer greenhouses), terms like Closed Plant Production System (CPPS), Plant Factory with Solar Lighting (PFSL), or greenhouse–VFU hybrid might be used.
      • Significant system differences (e.g., hydroponic vs. aeroponic fertigation, wall-mounted vs. shelf-mounted orientation) may not be captured by current terminology.
  • Production Structures: VFS can be implemented in repurposed high-rise buildings, cellars, growth chambers, and shipping containers.
  • Resource Efficiency: Extremely low use of land area, water, pesticides, and nutrients compared to open-field and greenhouse production.
  • Control and Quality: Full control over the production process, including hygiene, reduces pathogen contaminations and improves uniformity, nutritional value, taste, and shelf life.
  • Resilience: VFS offer better resilience to catastrophic events such as extreme weather, pandemics, supply chain disruptions, and even nuclear fall-out (e.g., Fukushima in 2011).
  • Research Application: Scientific findings from lab conditions can be applied more readily in VFS than in field or greenhouse settings.
  • Historical Roots: Techniques in modern vertical farming are inspired by:
    • Hydroculture along the Nile in ancient Egypt.
    • Chinese floating gardens (4th century).
    • Aztec floating rafts (chinampas, 12th-14th century).
  • Modern Development: Soilless cultivation techniques, artificial lighting, and modernized greenhouses in the 19th and 20th centuries advanced indoor plant cultivation. First modern VFS appeared in the US, Japan, and the Netherlands by the late 20th century.
  • Recent Expansion: Fueled by advances in Light-Emitting Diode (LED) lighting technology and promotion by industry icons like Dickson Despommier and Toyoki Kozai.
  • Key Differences (Fig. 1):
    • Open-field farming: Uncontrolled sunlight, temperature, [CO_2], water, relative humidity. Low and unpredictable productivity, unguaranteed and non-uniform quality, high water and pesticide use, low energy use, substantial food miles.
    • Vertical farming: Controlled light, temperature, [CO_2], water, relative humidity. High and predictable productivity, guaranteed and uniform quality, low water and no/low pesticide use, high energy use, potential for minimal food miles.
    • Greenhouse horticulture is considered an intermediate system.

Crop Growth

  • Key Challenge: Iterative identification and optimization of the most limiting growth factor, following Liebig's law of the minimum, to improve yields and Resource Use Efficiency (RUE).
  • Environmental Control: VFS allows precise control over many environmental variables: light quantity and spectrum, water and nutrient availability, temperature, relative humidity, airflow, and CO_2 concentration. This leads to predictable plant composition and growth rates.
  • Precision Control through AI: Achieved by combining advanced decision-making software and sensor data (from plant and environment). Sensor-informed AI updates self-learning dynamic growth prediction models (partially process-based, partially data-driven). These models control illumination, fertigation, and HVAC systems for real-time integrated environmental regulation.
  • Increased RUE: The combination of sensing, AI, production-systems operation, and plant physiological knowledge in near-airtight VFS can substantially increase RUE by dynamically adjusting variables to meet crop-specific requirements.
  • Lower Labor Costs: AI can reduce the need for expert farmers to determine optimal growing conditions.
  • Energy Use and Light Use Efficiency (LUE): VFS energy use is heavily determined by the need to deliver photosynthetically active radiation (PAR). Maximizing LUE is crucial.
  • Four Strategies to Maximize LUE:
    1. Increasing the fraction of light intercepted by the crop:
      • Minimize light lost to walls, aisles, or floor.
      • Achieve continuous canopy closure through variable plant density, intercropping, or optimized lighting strategies.
      • Dynamically managed plant density allows gradual density decrease as plants grow.
      • Potential for laser diodes to shoot photons onto specific leaves.
      • Maximum light interception often at Leaf Area Index (m^2 leaf area per m^2 floor) of 3-4. Further gains at higher LAI are minimal.
      • Far-red light can trigger fast leaf outgrowth and stem elongation, increasing whole-canopy light capture in early growth but may reduce leaf thickness.
      • Intercropping (simultaneous growing of multiple crops) is underexplored but promising for improving light interception.
    2. Improving light distribution within the crop:
      • Plant architecture (e.g., open canopy with long internodes and narrow leaves) is critical for uniform light distribution.
      • Challenge: Combine compact plants (desired in VFS) with uniform light distribution.
      • Functional-structural plant models can simulate 3D architecture to identify ideal plant ideotypes for VFS, considering light positioning and reflection.
      • Bottom lighting (underneath lettuce leaves) can preserve photosynthetic capacity of lower leaves, delaying senescence and increasing yield, but effects on LUE and quality are unknown.
    3. Optimizing leaf photosynthesis:
      • Maximize photosynthetic quantum yield (\mu \text{mol CO}_2 fixed per \mu \text{mol} photons absorbed).
      • High environmental control allows integral tweaking of photoperiod, light spectrum, Photosynthetic Photon Flux Density (PPFD), CO_2 concentration, relative humidity, and leaf temperature.
      • Diurnal changes in quantum yield occur due to photosynthetic induction, incomplete carbon utilization, photodamage, and circadian rhythms (e.g., Photosystem II repair, stomatal conductance).
      • Dynamic adjustment of environmental factors (e.g., PPFD) based on continuous sensing and prediction of quantum yield is a promising, underexplored area.
      • Effect of CO2 Concentration: High CO2 concentration can significantly increase LUE in C3 plants by suppressing photorespiration, boosting yields up to 40\%. VFS can maintain high CO_2 with low input as most is absorbed by plants.
      • Air Uniformity Challenge: Maintaining uniform temperature, relative humidity, and CO_2 profiles is challenging due to incomplete air mixing, especially in vertically stacked layers with limited headspace.
        • Stagnant areas lead to non-uniform growth, high humidity/temperature, and a large leaf boundary layer reducing gas exchange.
        • Can cause reduced growth and physiological disorders like tip-burn.
        • Solutions: Localized air distribution systems, computational fluid dynamics (CFD) for design optimization, conveyor belts, or dynamic spacing.
    4. Directing photosynthates into harvestable products:
      • VFS require short plants with small root systems and a high harvest index (partitioning of assimilates to marketable organs).
      • For leafy greens, high leaf partitioning is desired; for other crops, flowers, fruits, or underground parts are targets.
      • Achieved through plant breeding and environmental manipulation (e.g., far-red light supplementation increased fruit assimilate partitioning in tomato from 33\% to 40\%).
  • Root Environment Control: Soilless cultivation (Fig. 2) allows high control over the rhizosphere (root environment), optimizing root function.
    • Mineral salt concentration in VFS rhizospheres is typically ten times higher than in open-field crops.
    • Regulation of nutrient concentration, pH, water availability, oxygenation, and temperature ensures high plant nutrient content and growth rates.
    • Growth-stage-specific or diurnally timed nutrient application can improve product quality and quantity.
    • Benefits: Clean products (no soil residue, fewer pathogens), high-quality below- and aboveground organs.
  • Overlooked Growth Factors:
    • Volatiles: Release of volatile organic compounds (VOCs) by plants and materials (e.g., plastics emitting phthalates and formaldehydes) can cause chlorosis, growth retardation, and allergic reactions in workers. Impurified air can also affect pathogen resistance.
    • Microplastics: Small plastic particles in the rhizosphere may be taken up by roots, influencing yield and product quality.

Product Quality

  • Definition of Quality: An umbrella term covering morpho-physiological properties (water/mineral content, tissue texture) and phytochemical levels.
  • Phytochemicals: Affect taste/aroma, appearance, shelf life, nutritional value, and pharmaceutical value.
  • Environmental Control for Quality Enhancement: Can increase beneficial phytochemicals and reduce undesirable ones (e.g., phenolics in browning, anti-nutrients like glucosinolates, nitrite, lectin, or phytate).
  • Challenges in Application: Translating findings into specific