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Tight control of growth media moisture content is needed when plant growth systems employ shallow root zones or for cultivating fast-growing plants (i.e., crops). Poor control of moisture can affect both growth rate and plant quality by either excessive watering (waterlogging) or drought events. We evaluated the performance of two types of moisture sensors: tensiometers and heat-pulse moisture sensors. The output from each sensor type was evaluated as a function of volumetric moisture content in 1 to 2 mm Turface. The tensiometers were more sensitive between 30% and 60% volumetric moisture content, and their output was nonlinear because they measure water potential directly. In contrast, both the sensitivity and the output of the heat-pulse moisture sensors, as a function of volumetric moisture content, were linear. The heat-pulse moisture sensors were used to control moisture content in a shallow root zone, whereby water was added or removed from the media through a porous tube using peristaltic pumps. Moisture content in the media could be maintained within ±2% of setpoint for moisture contents ranging from 20% to 100% volumetric moisture content. The heat-pulse sensors were better suited for controlling media moisture because of their linear output and because of their constant sensitivity as a function of volumetric moisture content.
The effects of planting density and short-term changes in photoperiod on the growth and photosynthesis of bean (Phaseolus vulgaris L.) was investigated. Two cultivars of bean (cv. Etna, a dry bean variety; cv. Hystyle, a snap bean variety) were grown using nutrient film technique hydroponics in a walk-in growth chamber with a 12 h/12 h (light/dark) photoperiod and a corresponding thermoperiod of 28/24 °C (light/dark) and constant 65% relative humidity. Lighting for the chamber consisted of VHO fluorescent lamps and irradiance at canopy level was 400 μmol·m-2·s-1 PPF. For each cultivar, plants were grown at densities of 16 or 32 plants/m2. Short-term photoperiod changes were imposed during vegetative growth (21-29 DAP) and pod-fill (42-57 DAP). From the base 12 h/12h (light/dark) photoperiod, lighting in the chamber was cycled to provide 18 h/06 h (light/dark) or 24 h/0 h(continuous light) for 48 h. Diurnal single leaf net photosynthetic rates (Pn) and net assimilation vs. internal CO2 (Aci) measurements were taken during the short-term photoperiod adjustments. Results showed that there was no difference between cultivars or planting density with regard to total biomass or single leaf photosynthetic rates, but cv. Etna produced 35% more edible biomass than cv. Hystyle. Additionally, there was no effect of short-term photoperiod adjustment on single leaf Pn or Aci.