Lettuce growth limited by nitrate supply

Limiting the supply of nitrate to winter-lettuce in greenhouses is one appr oach to assuring an acceptably low nitrate level in the marketed produce. T o optimize growth under such conditions, a two-state-variable lettuce model , originally developed to predict the nitrate concentration when the supply of nutrients is unlimited (Seginer et al., 1998), has been modified to tak e account of growth limited by nitrate supply. Nitrate in the plant serves two functions: it is a vital raw material for p rotein synthesis and also an important osmoticum in the cell sap. The plant model responds to limited nitrate supply by placing a higher priority on i ts role as raw material. Nitrate in the cell sap is replaced, as much as ne eded, by assimilates (sugar), freeing the recovered nitrate for protein syn thesis. The modified model includes a balance equation for nitrate in the plant dS(Nv)/dt = F-Nrv - rF(Cvs) , where S-Nv is nitrate-N content, F-Nrv is nitrate-N uptake by the plant, F- Cvs is the rate of structural growth and r is the nitrogen content of the s tructural material, dS(Nv)/dt is determined by the rate of change of the as similate content of the cell sap, as described by the original model. If ni trate supply is abundant, its rate of uptake, namely F-Nrv, is determined a s a remainder. If nitrate supply is limiting, and therefore is all taken up by the plant, it is the nitrate used for growth, rF(Cvs), that is determin ed by the balance. Comparison with limited experimental results produced a good agreement.