Daily macronutrient uptake patterns in relation to plant age in hydroponic lettuce

A greenhouse experiment was conducted to study and quantify the daily uptake rate of nitrate, ammonium, phosphoric acid, potassium, calcium, magnesium, and sulfate (NO₃^?, NH₄⁺, H₂PO₄^?, K⁺, Ca²⁺, Mg²⁺ and SO₄^2?) and to characterize the uptake daily pattern at different plant ages in a lettuce crop. The uptake rates per gram of plant fresh weight were calculated at 2-hour intervals through sampling the nutrient solution and analyzing it by NO₃^?/NH₄⁺ conductivity and inductively coupled plasma atomic emission spectrometry (ICP-AES). The uptake rate of nitrogen, phosphorus and potassium (N, P and K) per unit mass of plant decreased with plant age following a reduction in plant relative growth rate. No significant differences were found in the absorption of Ca, Mg and sulfur (S) between the different weeks of growth. The daily absorption patterns showed no preference for the absorption of any of the ions during the daytime. A significant reduction in the absorption peaks of all the ions with increasing plant age was observed.