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

Abstract

assimilation. While the purely photosynthetic assumption is convenient, a more realistic metabolism subA simple model is proposed to describe diurnal net model is needed. nitrate uptake rate patterns observed experimentally on young plants grown under constant non-limiting Key words: Light/dark cycles, day, night, water content, nutrition. It rests on two hypotheses: net uptake rate nitrogen, hydroponic. is under negative feedback control by internal plant nitrate content, and nitrogen metabolism occurs only during the light period. The model parameters were Introduction determined from the results of three independent experiments performed under non-disturbing condi- During plant growth, nitrate uptake is mainly determined tions in a growth room at constant air and solution by dry mass production in relation to ontogeny and temperatures. Net hourly nitrate uptake rate was environmental parameters (Imsande and Touraine, 1994). measured through a diurnal cycle and after an However, at an hourly time scale, under constant nutriextended 28 h period of darkness. It increased con- tion, uptake rate undergoes continuous variations tinuously during the light period and decreased during (Clement et al., 1978; Triboi-Blondel, 1979; Pearson et al., the dark period. Under prolonged darkness, net uptake 1981; Le Bot and Kirkby, 1992; Delhon et al., 1995a; declined to an asymptotic positive uptake rate of about Andriolo et al., 1996), with peak values during day time, 10’5 mol h’1 g’1 total plant dry weight. The measured and lowest values during the night. This eVect, clearly hourly nitrate uptake rate values were consistent with related to light interception, is based on still unclear independent determinations of long-term nitrate and mechanisms. On a large time scale, nitrate uptake correltotal N accumulations in the plant. Realistic simula- ates with transpiration, but it has been shown that in the tions of experimental data are achieved with the pro- diurnal cycle they are independent ( Triboi-Blondel, 1979; posed model. Furthermore, the maintenance of a Le Bot and Kirkby, 1992; Delhon et al., 1995b; Andriolo positive net uptake rate, measured in non-growing et al., 1996). As nitrate uptake also correlates with CO 2 plants subjected to prolonged darkness, is explained assimilation (Clement et al., 1978), an indirect control by in the model by the continuous increase of plant water photosynthesis has been suggested ( Wild et al., 1987). content. The importance of the diurnal variations of The flow of carbon compounds from the shoots might plant water content for nitrate uptake rate is emphas- regulate nitrate uptake, an ATP-dependent process, ized and gives consistency to the homeostasis hypo- through root respiration (Glass, 1989). Indeed, diurnal thesis of the model. The diurnal changes in nitrate root respiration seems closely related to nitrate uptake uptake predicted by the model are strongly dependent rate (Hansen, 1980). However, this hypothesis is still under discussion, as uptake regulation is expected to