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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 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.