Diurnal nitrate uptake in young tomato (Lycopersicon esculentum Mill.) plants: test of a feedback-based model

A simple model is proposed to describe diurnal net nitrate uptake ratepatterns observed experimentally on young plants grown under constantnon-limiting nutrition. It rests on two hypotheses: net uptake rate isunder negative feedback control by internal plant nitrate content, andnitrogen metabolism occurs only during the light period. The modelparameters were determined from the results of three independentexperiments performed under non-disturbing conditions in a growth room atconstant air and solution temperatures. Net hourly nitrate uptake rate wasmeasured through a diurnal cycle and after an extended 28 h period ofdarkness. It increased continuously during the light period and decreasedduring the dark period. Under prolonged darkness, net uptake declined to anasymptotic positive uptake rate of about 10^-5 molh^-1 g^-1 total plant dryweight. The measured hourly nitrate uptake rate values were consistent withindependent determinations of long-term nitrate and total N accumulationsin the plant. Realistic simulations of experimental data are achieved withthe proposed model. Furthermore, the maintenance of a positive net uptakerate, measured in non-growing plants subjected to prolonged darkness, isexplained in the model by the continuous increase of plant water content.The importance of the diurnal variations of plant water content for nitrateuptake rate is emphasized and gives consistency to the homeostasishypothesis of the model. The diurnal changes in nitrate uptake predicted bythe model are strongly dependent on the assumption made for diurnal changesin nitrate assimilation. While the purely photosynthetic assumption isconvenient, a more realistic metabolism sub-model isneeded.