The Uptake of Nitrate byLolium perennefrom Flowing Nutrient Solution

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Abstract
Experiments with simulated swards of perennial ryegrass (Lolium perenne L.) grown in flowing nutrient solution with NO3- held at 0.1 mg N I−1 show that the rate of NO3- uptake was related to diurnal, day-to-day, and seasonal changes in radiation. In summer the diurnal variation in NO3-uptake ranged from 25 to 50 mg N m−2 h−1 and the day-to-day variation ranged from 500 to 1500 mg N m−2 d−1. Mean daily rates of uptake over 12 d periods in summer and in winter averaged 908 and 44 mg N m−2, respectively. The pattern of NO3- uptake followed that of CO2 flux with the maximum rate of the former occurring 5 or 6 h after the maximum CO2 influx. After defoliation, NO3- uptake was severely curtailed for 2 d concomitant with a very small influx of CO2. Analysis of the changes that occurred in the rate of NO3- uptake immediately after the switching on or off of artificial light suggests that two reversible processes may be involved in the relation between NO3-uptake and radiation, one with a longer and the other with a shorter time constant.