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Cary A. Mitchell

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Cary A. Mitchell

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Cary A. Mitchell

Following is the invited perspective of an academic researcher and director of a multi-institutional research and education project tasked to test the feasibility of adopting light-emitting diode (LED) technology for application by the commercial horticulture industry. Academics researching basic specialty-crop responses to spectra, intensities, and durations of lighting with LEDs often find technical queries from growers, vendors, and entrepreneurs to go beyond the capabilities and scope of systematic research to answer definitively. Differences between commercial and academic research approaches to LED technology development are noted, including legal obstacles to open collaboration. Early generation commercial LED technology for horticultural applications is based on research begun >20 years ago. The basis for selection of various LED wavebands for inclusion in LED plant growth arrays is presented for both commercial as well as research applications. Advantages of light distribution from LED sources for different crop applications are presented, especially including close-canopy and intracanopy lighting, both of which contribute substantially to energy savings. Challenges to providing accurate LED light prescriptions for different crops are discussed, including those for supplemental lighting as well as for sole-source lighting applications. Anticipated trends are projected for horticultural applications of LED technology, including multispectral, individually adjustable, high-intensity arrays; increasing electrical efficacy of future LEDs; and reduced costs of mass production for particular applications.

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Cary A. Mitchell

The most recent platform for protected horticultural crop production, with the shortest history to date, is located entirely indoors, lacking even the benefit of free, natural sunlight. Although this may not sound offhand like a good idea for commercial specialty-crop production, the concept of indoor controlled-environment plant growth started originally for the benefit of researchers—to systematically investigate effects of specific environmental factors on plant growth and development in isolation from environmental factors varying in uncontrolled ways that would confound or change experimental findings. In addition to its value for basic and applied research, it soon was discovered that providing nonlimiting plant-growth environments greatly enhanced crop yield and enabled manipulation of plant development in ways that were never previously possible. As supporting technology for indoor crop production has improved in capability and efficiency, energy requirements have declined substantially for growing crops through entire production cycles in completely controlled environments, and this combination has spawned a new sector of the controlled-environment crop-production industry. This article chronicles the evolution of events, enabling technologies, and entrepreneurial efforts that have brought local, year-round indoor crop production to the forefront of public visibility and the threshold of profitability for a growing number of specialty crops in locations with seasonal climates.

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Changhoo Chun and Cary A. Mitchell

`Waldmann's Green' leaf lettuce (Lactuca sativa L.) is being used as a model leafy vegetable crop to develop a protocol for variable control of photosynthetic photon flux (PPF) during crop production. Feedback from real-time photosynthetic gas exchange rates by lettuce canopies is used to modulate electronic dimming ballasts of lamp banks. Algorithms within process-control software are being fine tuned to maximize increments of photosynthetic output relative to increments of photon input. Dynamic optimization of PPF was 21% more efficient than constant high PPF saturating photosynthesis with respect to biomass accumulated per photons absorbed. Dynamic optimization also is being combined with principles of phasic control, in which environmental resources such as photosynthetically active radiation (PAR) and carbon dioxide (CO2) are deliberatively limited in input during specific phases of crop development when plants are less sensitive to inputs (e.g., lag, plateau, and senescence phases) but optimized for the responsive exponential phase. Preliminary results indicate that leaf lettuce growth benefits from optimizing environments for no more than 4 or 5 days during a 20-day production cycle. Dynamic optimization of CO2 level is achieved by controlling the injection of CO2 into the inlet air stream of Minitron II crop canopy cuvette/growth chambers. Algorithms are being modified to simultaneously vary PPF and CO2 for optimum photosynthesis.

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Manette Schönfeld and Cary A. Mitchell

CowPea (Vigna unguiculata (L.) Walp.) is a candidate species for inclusion in a space-deployed Controlled Ecological Life Support System (CELSS) because it contributes to a balanced diet with its moderate protein content, high complex carbohydrate content, and low fat content, and because leaves and unripe pods as well as dry seeds are edible. Pour harvest scenarios were compared in the experimental line IT84S-2246 under controlled conditions with and without CO2 enrichment. Plants kept vegetative by removal of flowers and periodically stripped of fully expanded leaves yielded as much as either mixed-harvest scenario in which leaves were stripped at either 1- or 2-week intervals until pods started forming. The 2-week harvest scenario outyielded the 1-week scenario by 15 to 25%. The seed-only control produced the same amount of seeds as the 2-week leaf harvest scenario, but had lower total edible biomass because leaves were not harvested. Under 1000 ppm CO2, all treatments yielded from 30 to 70% more edible biomass than under non-CO2-enriched conditions. Research sponsored by NASA Cooperative Agreement NCC 2-100.

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Celina Gómez and Cary A. Mitchell

The relative coolness-to-touch of light-emitting diodes (LEDs) has enabled commercial implementation of intracanopy lighting (ICL) in the greenhouse. Intracanopy lighting, which refers to the strategy of lighting along the side or from within the foliar canopy, can increase canopy photosynthetic activity, but physiological and productivity responses of high-wire greenhouse tomato (Solanum lycopersicum) to intracanopy supplemental lighting (SL) still are not yet fully understood. Two consecutive production experiments were conducted across seasons in a glass-glazed greenhouse located in a midnorthern, continental climate [lat. 40°N (West Lafayette, IN)]. Plants were grown from winter-to-summer [increasing solar daily light integral (DLI)] and from summer-to-winter (decreasing solar DLI) to compare three SL strategies for high-wire tomato production across changing solar DLIs: top lighting with high-pressure sodium lamps (HPS) vs. intracanopy LED vertical towers vs. hybrid SL (HPS + horizontal ICL-LEDs). A control treatment also was included for which no SL was provided. Supplemental DLI for each experimental period was adjusted monthly, to complement seasonal changes in sunlight, aiming to approach a target total DLI of 25 mol·m‒2·d‒1 during fruit set. Harvest parameters (total fruit fresh weight, number of fruit harvested, and average cluster fresh weight), tissue temperature, chlorophyll fluorescence, and stomatal conductance (g S) were unaffected by SL treatment in both experiments. Among the physiological parameters evaluated, CO2 assimilation measured under light-saturating conditions, light-limited quantum-use efficiency, and maximum gross CO2 assimilation (A max) proved to be good indicators of how ICL reduces the top-to-bottom decline in leaf photosynthetic activity otherwise measured with top lighting only (HPS-SL or solar). Although SL generally increased fruit yield relative to control, lack of SL treatment differences among harvest parameters indicates that higher crop photosynthetic activity did not increase fruit yield. Compared with control, intracanopy SL increased yield to the same extent as top SL, but the remaining photoassimilate from ICL most likely was partitioned to maintain nonharvested, vegetative plant parts as well.