NITRATE CONCENTRATION IN GREENHOUSE LETTUCE: A MODELING STUDY

Abstract

A simple two-state-variable model was developed to describe the nitrate concentration in lettuce when nitrate supply is unlimited. A central element of the model is a negative correlation between the concentrations of soluble carbohydrates and nitrate in the cell sap, a correlation which reflects the equivalent roles of nitrate and organic solutes in the maintenance of cell turgor. A conventional carbon balance model was used to predict the fluctuations of soluble carbohydrates as a function of time. The carbohydrate-to-nitrate correlation was then used to compensate for these fluctuations by adjusting the nitrate concentration. The concentration of carbohydrates in the non-structural buffer (vacuoles) is determined by the balance between source activity (supply of carbohydrates by photosynthesis, controlled by light and CO2) and sink activity (demand for carbohydrates by growth and maintenance, controlled by temperature). As a result, winter conditions, characterized by source-limited growth, result in a depletion of carbon from the cell sap and, via the negative correlation, in a high nitrate concentration. Conversely, summer conditions result in low nitrate levels. A fair agreement was obtained when the model was tested with an available experimental data set for all-year-round production of greenhouse lettuce in Northern Germany.