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Since the phytometric system, the new system of plant-based radiation measurement, allows for the expression of radiation measurement in terms of the standard radiation measures of flux, irradiance, exitance, intensity and radiance, one of its immediate advantages is that it makes possible the direct use of the equations of point radiation for plant-lighting applications, including those for bioregeneratve space advanced life support.

The normalized quantum efficiency (RQE) curve that shows the relative photosynthetic response to light of the average photosynthesizing plant – originally published by Mcree (1972a, 1972b), replicated by Inada (1976, 1978a, 1978b), and subsequently refined by Sager et al. (1982, 1988) — was used as the basis in developing the phytometric system, a new and more flexible concept of light measurement for plants. Based on the convolution of the RQE curve with the spectral power distribution (SPD) of a given light source, the phytometric measurement would yield units of phytoWm−2. The unit phytoW easily provides conversion factors to the radiometric, photometric, and photon flux (quantum) systems within the photosynthetically active radiation (PAR) of 400 to 700 nm or within the extended PAR of 300 to 800 nm. Indeed, a calculated phytometric value would provide a more accurate measure of photosynthetic photon flux (PPF) or yield photon flux (YPF).

The long-term hypothesis of this study is that the patterns in uptake of certain nutrient species in the hydroponic nutrient solution can serve as an early-warning stress detector for specific hydroponically grown crops. This is a two-part hypothesis: first, it posits that the time variation in the uptake of specific nutrient species under a given nutrient regime shows fairly reasonable regularity; and, second, it posits that deviations from such regularity actually correlate with the occurrence of certain plant stress. Addressing the first part of the hypothesis, the objective of the current study was to determine the temporal variations in the concentrations of nitrate, potassium, and manganese under the following four nutrient regimes used for sweetpotato hydroponics: standard or control, elevated nitrogen by ammonium, elevated nitrogen by nitrate, and elevated potassium conditions.

The next-generation of plant hydroponic systems for advanced life support will most likely require a dynamic monitoring capability for their nutrient species in solution for two reasons: (1) to be able to optimize nutrient use, which would help to reduce the mass and volume of stored inorganic chemicals; and (2) to be able to dynamically correlate the fluctuations in uptake of individual nutrient species with the plant’s physiological state (e.g., stress) over time under microgravity conditions. The latter in turn will provide advanced physiological diagnoses for the crops and could help reduce the astronaut man-hours for crop maintenance. The results of this study suggested that a combination of inductively coupled plasma (ICP) spectroscopy and ion selective electrodes (ISEs) could be a competent strategy for designing a dynamic nutrient-monitoring capability for hydroponic systems.

Hybrid solar and electric lighting (HYSEL) systems constitute the latest generation of lighting systems for advanced life support, exhibiting continued potential for reducing the significant electrical power demand of current bioregenerative life support systems (BLSS). Two experimental HYSEL systems were developed: one employing xenon-metal halide (XMH) lamps and the other adopting light-emitting diodes (LEDs) as the electric-lighting components, and both using a mirror-based, fiberoptic-based solar collection system. The results showed that both the XMH and LED HYSEL systems effected reduced effective plant growing volume, indicating potential for a compact plant hardware design. The apparent electrical conversion efficiency of the LED HYSEL system exceeded that of the XMH HYSEL system by five-fold. Both the XMH and LED HYSEL systems provided reasonably acceptable spectral quality and lighting uniformity.

The Hybrid Solar and Artificial Lighting (HYSAL) system used in this study consisted of a mirror-based Optical Waveguide (OW) Solar Lighting System as the solar component and four 60-W xenon-metal halide illuminators as the artificial-light component. A reference (or control) system consisted of a conventional 250-W high pressure sodium (HPS) lamp. Solar irradiance was harnessed whenever available for the HYSAL treatment. During the course of the 30-day growth period for lettuce (Lactuca sativa), the HYSAL's solar PPF varied with the natural fluctuations of terrestrial solar irradiance, which changed dramatically within each day and between days. When averaged over the entire growth period, the average instantaneous solar PPF for the HYSAL treatment turned out to be 322 μmol m−2 s−1 for an average daily photoperiod of only 3.86 hours owing to numerous cloudy days.

Significant reductions in electrical-power demand as well as in related mass and physical volume might be achieved if available extraterrestrial solar irradiance could be utilized for plant production in a Bioregenerative Life Support System (BLSS) on Mars. Working estimates of the available photosynthetic photon flux (PPF) at Chryse Planitia (22.3° N, 47.9° W), landing site for the Viking Lander 1 (VL-1) on Mars and geographically near the Mars Pathfinder's landing site, were simulated based on the year-long actual irradiance measurements and downward spectral characteristics made by VL-1 in the 1970's. The results showed that the Wm−2 to µmol m2 s−1 conversion factors for Earth and Mars are essentially equal, being approximately 4.6 µmol m−2 s−1/Wm−2. For half of the total sunshine hours at Chryse Planitia for a whole Martian year, the incident PPF level is at least 400 µmol m−2 s.

The long-term supplemental terrestrial solar lighting made available to the Biomass Production Chamber (BPC) located in the Subterranean Plant Growth Facility (SPGF) at The University of Arizona was determined for two cases where two types of Solar Irradiance Collection, Transmission and Distribution System (SICTDS) were used for the facility. Databases for hourly solar irradiance incident upon Tucson, AZ compiled over a 12-year period from 1987 through 1998 were used to calculate the projected average instantaneous PPF within the BPC per hour and per day throughout the year. The results showed that replacing the available solar irradiance within the BPC as delivered by the Himawari SICTDS in June would require either 97.7 W m−2 of HPS lighting or 185.9 W m−2 of CWF lighting supplied continuously for 450 hrs. In energy terms, these would be equivalent to 44.0 kW-hr m−2 for the HPS lamp and 83.7 kW-hr m−2 for the CWF lamp.

The evolution of lighting systems for Bioregenerative Space Life Support (BLSS) has been brought about by two major challenges confronting current BLSS models: (1) the extensive use of highly energy-intensive artificial lamps; and (2) the substantial energy wastes incurred through heat dissipations by these lamps, frequently dictating unnecessarily large, and costly, physical volumes for the plant growing structures. The results of our studies showed that Solar Irradiance Collection, Transmission and Distribution Systems (SICTDS) should be used to augment artificial lighting for growing plants in a BLSS to constitute a reliable, energy-efficient and mass-optimized Hybrid Solar and Artificial Lighting (HYSAL) system for a BLSS.