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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 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.