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 rate patterns observed experimentally on young plants grown under constant non-limiting nutrition. It rests on two hypotheses: net uptake rate i s under negative feedback control by internal plant nitrate content, a nd nitrogen metabolism occurs only during the light period. The model parameters were determined from the results of three independent exper iments performed under non-disturbing conditions in a growth room at c onstant air and solution temperatures. Net hourly nitrate uptake rate was measured through a diurnal cycle and after an extended 28 h period of darkness. It increased continuously during the light period and de creased during the dark period. Under prolonged darkness, net uptake d eclined to an asymptotic positive uptake rate of about 10(-5) mol h(-1 ) g(-1) total plant dry weight. The measured hourly nitrate uptake rat e values were consistent with independent determinations of long-term nitrate and total N accumulations in the plant. Realistic simulations of experimental data are achieved with the proposed model. Furthermore , the maintenance of a positive net uptake rate, measured in non-growi ng plants subjected to prolonged darkness, is explained in the model b y the continuous increase of pliant water content. The importance of t he diurnal variations of plant water content for nitrate uptake rate i s emphasized and gives consistency to the homeostasis hypothesis of th e model. The diurnal changes in nitrate uptake predicted by the model are strongly dependent on the assumption made for diurnal changes in n itrate assimilation. While the purely photosynthetic assumption is con venient, a more realistic metabolism submodel is needed.