EFFECTS OF LOW-TEMPERATURE ON NITRATE UPTAKE, AND XYLEM AND PHLOEM FLOWS OF NITROGEN, IN SECALE-CEREALE L AND BRASSICA-NAPUS L

The effect of low temperature on nitrate uptake and subsequent N trans location and cycling was investigated in Secale cereale L. and Brassic a napus L. transferred to 7 degrees C for 9 d or kept at a warm temper ature (20 degrees C). Some plants were grown with a split root system and (NO3-)-N-15 labelled to measure NO3- uptake, root-to-shoot and sho ot-to-root translocation of N from NO3- taken up. Other plants with si ngle-root system were subjected to N-15 pulse-chase labelling to quant ify endogenous N remobilization. Lowering growth temperature from 20 t o 7 degrees C reduced nitrate uptake more strongly in rye (-59%) than in winter rape (-28%). A very large proportion of the nitrate taken up uas further translocated to shoots in both species. However, lowering the temperature decreased xylem N translocation by about 60 and 30% i n S. cereale and B. napus, respectively. Most root N in the latter spe cies came directly from root absorption, while in rye roots the propor tions ascribable to direct root allocation and phloem flow were well b alanced. Cold treatment did not significantly modify the imbalance bet ween the two origins. Mobilization of endogenous N from roots to shoot , estimated by pulse-chase labelling, was limited in B. napus and sign ificant in S. cereale despite a depressive effect of low temperature. In general, low temperature led to an increase in root N concentration in both species. It is suggested that low temperature may directly af fect the nitrate uptake system, and also that N accumulation in the ro ots (resulting from greater inhibition of N xylem flow than of NO3- up take) might increase the inhibition of uptake by higher cellular NO3- and/or amino acid content. The usefulness (for spring growth) of root N accumulation triggered by low temperature is discussed.