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- Author or Editor: Cary A. Mitchell x
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.
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.
Photoperiod and harvest scenario of cowpea (Vigna unguiculata L. Walp) canopies were manipulated to optimize productivity for use in future controlled ecological life-support systems. Productivity was measured by edible yield rate (EYR: g·m-2·day-1), shoot harvest index (SHI: g edible biomass·[g total shoot dry weight]), and yield-efficiency rate (YER: g edible biomass·m-2·day-1per [g nonedible shoot dry weight]). Breeding lines `IT84S-2246' (S-2246) and `IT82D-889' (D-889) were grown in a greenhouse under 8-, 12-, or 24-h photoperiods. S-2246 was short-day and D-889 was day-neutral for flowering. Under each photoperiod, cowpeas were harvested either for leaves only, seeds only, or leaves plus seeds (mixed harvest). Photoperiod did not affect EYR of either breeding line for any harvest scenario tested. Averaged over both breeding lines, seed harvest gave the highest EYR at 6.7 g·m-2·day-1. The highest SHI (65%) and YER (94 mg·m-2·day-1·g-1) were achieved for leaf-only harvest of D-889 under an 8-h photoperiod. For leaf-only harvest of S-2246, both SHI and YER increased with increasing photoperiod, but declined for seed-only and mixed harvests. However, photoperiod had no effect on SHI or YER for D-889 for any harvest scenario. A second experiment utilized the short-day cowpea breeding line `IT89KD-288' (D-288) and the day-neutral breeding line `IT87D-941-1' (D-941) to compare yield parameters using photoperiod extension under differing lamp types. This experiment confirmed the photoperiod responses of D-889 and S-2246 to a mixed-harvest scenario and indicated that daylength extension with higher irradiance from high pressure sodium lamps further suppressed EYR, SHI, and YER of the short-day breeding line D-288.
Physiological characteristics of cowpea suggest it as a candidate species for CELSS. Improving productivity will be essential for CELSS. The leaves of cowpea provide an added source of edible biomass and may extend harvest index (HI=edible/total biomass). A greenhouse study evaluated the interaction of photoperiod and cultivar with 3 harvest scenarios: 1) seed only, 2) seed and leaves (mixed), and 3) leaves only. Overall, seed-only HI was 25% less than leaf-only HI. For IT84S-2246, a short-day cultivar for flowering, both mixed and seed only HI dropped 20% as photoperiod increased from 8 to 24 hours. While edible yield rate remained constant, shoot dry weight increased as photoperiod increased for both harvest scenarios, thus decreasing HI. For the same reason, the leaf-only HI of S-2246 increased as photoperiod increased. For IT82D-889, a day-neutral cultivar for flowering, HI remained constant regardless of photoperiod. Leaf-only HI was 65%, whereas seed and mixed HIs were 40 and 45% respectively. For all harvest scenarios, D-889 produced 25 grams less edible biomass per plant than that of S-2246. Due to the interaction of harvest scenario and cultivar, cultivar selection must be based on yield performance in each type of harvest scenario. Further investigations will answer the extent of sacrificing seed yield for leaf harvest. NASA Grant NAGW-2329.
The vigorous growth habit and tolerances to heat, water, and acid stresses suggest cowpea as a candidate species for Controlled Ecological Life-Support Systems (CELSS). The low fat, high protein, moderate carbohydrate content of the edible leaves and seeds complement cereal grains in the vegetarian diets planned for CELSS. Cowpea canopy densities of 3.6, 7.2, 10.7, and 14.3 plants·m-2 were grown under CO2 levels of 400 or 1200 μl·l-1. Plants were grown in a deep-batch recirculating hydroponic system. pH was maintained at 5.5 by a pH controller with an in-line electrode. The nutrient solution was replaced as needed and sampled weekly for analysis by inductively coupled plasmaatomic emission spectrometry. Fluorescent lights provided 674±147 μmol·m-2s-1 PAR for an 8-hour photoperiod. Day/night temperature was maintained at 27/25°C. CO2 draw-down within the growth chamber was measured to calculate net photosynthesis. Power consumption was metered and canopy quantum efficiency was calculated. Crop yield rate (g·m-2·d-1). harvest index (% edible biomass), and yield efficiency (edible g·m-2·d-1·(nonedible g)-1) were determined to evaluate the productivity of cowpea for a CELSS. This study was supported by NASA Grant NAGW-2329.