effect of root temperature on growth and uptake of ammonium and nitrate by Brassica napus L. cv. Bien venu in flowing solution culture. II. Uptake from solutions containing NH4NO3

The effects of root temperature on uptake and assimilation of $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and $\mathrm{N}{\mathrm{O}}_{3}^{-}$ by oilseed rape (Brassica napus L. cv. Bien venu) were examined. Plants were grown for 49 d in flowing nutrient solution at pH 6.0 with root temperature decrementally reduced from 20 °C to 5 °C; and then exposed to different root temperatures (3, 5, 7, 9, 11, 13, 17 or 25 °C) held constant for 14 d. The air temperature was 20/15 °C day/night and nitrogen was supplied automatically to maintain 10 mmol m—3 NH4NO3 in solution. Total uptake of nitrogen over 14 d increased threefold between 3—13 °C but was constant above 13 °C. Net uptake of $\mathrm{N}{\mathrm{H}}_{4}^{+}$ exceeded that of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ at all temperatures except 17 °C, and represented 47—65% of the total uptake of nitrogen. Unit absorption rates of $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and $\mathrm{N}{\mathrm{O}}_{3}^{-}$ had a 5-fold range and changed with temperature and time; the Q10 values of 0.9—2.0 for $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and of 1.5—2.7 for $\mathrm{N}{\mathrm{O}}_{3}^{-}$ suggested that $\mathrm{N}{\mathrm{O}}_{3}^{-}$ absorption was more sensitive than $\mathrm{N}{\mathrm{H}}_{4}^{+}$ absorption to temperature. Rates of absorption were relatively stable at 3 °C and 5 °C compared with those at 17 °C and 25 °C which increased sharply after 10 d. Tissue concentration of N in the shoot, expressed on a fresh weight basis, was independent of root temperature throughout, but doubled between 3—25 °C when expressed on a dry weight basis. The apparent proportion of net uptake of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ that was assimilated was inversely related to root temperature. The results are used to examine the relation between unit absorption rate and shoot : root ratio in the context of short and long term responses to change of root temperature.