The nicolet lettuce model : a theme with variations

The NICOLET model has been developed to predict the growth and nitrate content of greenhouse lettuce. Four single-organ versions have been developed: [1] abundant supply of nitrogen (1998), [2] mild N-stress (1999), [3] severe N-stress (2003), and [4] ontogenetic changes of organic-N and water content (in preparation). The `abundant-N¿ model [1] and the `mild-stress¿ model [2] have two compartments: `structure¿ and `vacuole¿, while the `severe-stress¿ model [3] requires a third compartment: `excess-carbon¿, and the `ontogenetic¿ model [4] has separate `metabolic¿ and `support¿ structural sub-compartments. The main special features of the NICOLET model are [1] the osmotica balance of the `vacuole¿, where nitrate and hydrocarbons play a complementary role in maintaining a constant osmotic potential, [2] the excess-carbon compartment, where `dry¿ carbon compounds are stored, and [3] the sub-division of the `structure¿ into sub-compartments of different compositions. Loosely speaking, the first feature controls the nitrate concentration, the second controls the organic-N and water contents, and the third controls the ontogenetic changes. The NICOLET model has been able to mimic `normal¿ seasonal variations of nitrate content, as well as the effects of drastic N-stress treatments. These results are illustrated by comparing measured data with model-simulations. Accurate prediction of nitrate concentration is difficult, due to its sensitivity to changes in the environment. Exact control of nitrate under commercial conditions may require transient corrective measures, such as N-interruption, in conjunction with a good plant-nitrate monitoring system.