Growth and nitrate consumption of sunflowers in the rhizostat, a device for continuous nutrient supply to plants

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Abstract
A hydroponic system was designed in which the replenishment of nutrients is proportional to the consumption of nitrate or ammonium by higher plants. Further characteristics of this ‘rhizostat’ are on‐line measurement of nitrate or ammonium, bubble‐free aeration of the culture solution with pure oxygen, and auxostatic pH control at 6.00±0.1. Young sunflowers were grown in the rhizostat for up to 16 days. Leaf area, total fresh weight, root fresh weight, and nutrient demand per day increased exponentially. The rate of nitrate consumption per plant followed a characteristic diurnal pattern with maximal values at the end of the 12‐h photoperiod. After two weeks of growth, about 95% of the inorganic bound nitrogen removed from the nutrient solution was recovered in the biomass of the plants. Growth of sunflower seedlings over two weeks led only to slight but tolerable deviations of macronutrient and trace element concentrations from the formula of the nutrient solution. As expected, nitrate‐grown plants lowered potassium concentration, whereas growth with ammonium resulted in an accumulation of this ion. Other plants successfully grown in the rhizostat have been corn, soybean, sugarcane, tobacco, and spinach.