THE DIURNAL PATTERN OF NITRATE UPTAKE AND REDUCTION BY SPINACH (SPINACIA-OLERACEA L)

Spinach plants were grown in bowls of aerated nutrient solution in a controlled environment chamber for 24 h, and harvested every 3·5-5 h to record their growth, nitrate and water uptake, and plant nitrate concentration. Twelve such experiments are described, either with a 14/10 h dark/light regime, or continuous light or darkness. The irradiance was either 110, 320, or 510 μmol m-2 s-1 (PPFD). All these regimes began at the end of the light period of a 14/10 h dark/light regime (510 μmol m-2 s-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almost unabated at about 17 μmol h-1 through the initial 14-h dark period, and then fell away sharply if the light was not restored, but increased slightly when it was. With continuous light at 510 μmol m-2 s-1, uptake rate rose steadily for the first 24 h of light, and then fell sharply for about 6 h. Shoot nitrate concentration increased about three-fold in the dark phase, and declined in the light at a rate which was positively related to the irradiance. Root nitrate concentration was several times higher than that of the shoot: its diurnal change was smaller (relative to the mean) than that of the shoot. Nitrate reduction occurred to a small extent in the dark, and increased rapidly as soon as the lights came on, to remain at a roughly constant rate (related to the irradiance) throughout the light phase. Dry matter increase in the light was related to irradiance, but with little increase above 320 μmol m-2 s-1. Respiratory weight loss in the dark was not detectable. Rate of fresh weight increase was approximately constant throughout light and dark periods. The results compare quite well with the predictions of a simple simulation model, based on the pump/leak principle.