GROWTH REQUIREMENT FOR N AS A CRITERION TO ASSESS THE EFFECTS OF PHYSICAL MANIPULATION ON NITRATE UPTAKE FLUXES IN SPINACH

The effects of physical manipulation of hydroponically grown plants of spinach (Spinacia oleracea L., cvs Subito and Glares) on nitrate upta ke fluxes were studied in a long-term experiment (3 days), and in shor t-term label experiments (2 h) with N-13-nitrate and N-15-nitrate. In the long-term experiment, net nitrate uptake rate (NNUR) was measured by following the nitrate depletion in the uptake solution, which was r eplaced at regular intervals. In the short-term experiments, NNUR and nitrate influx were measured by simultaneous application of N-13-nitra te and N-15-nitrate. Plants were gently transferred into the labelled uptake solution, as is usually done in nutrient uptake studies. In add ition, a more severe physical manipulation was carried out, including blotting of the roots, to mimic pretreatments which involve more handl ing of the plants prior to uptake measurements. Nitrate influx was mea sured immediately after physical manipulation and after 2 h of recover y. To assess the impact of the physical manipulation the experimentall y determined nitrate uptake fluxes were compared with the N demand for growth, defined as relative growth rate (RGR) times plant nitrogen co ncentration (PNC) of parallel plants, which were left undisturbed. Nit rate influx and efflux were both subject to changes after physical man ipulation of the plants. Physical handling, however, did not always re sult in an alteration of NNUR, which complicates the determination of the length of the recovery period. The impact of the handling and the time course of the recovery depended on the severity of the disturbanc e and were independent of the light conditions during the experiments. Even after a gentle transfer of the plants, recovery, in most cases, was not complete within 2 h. The data emphasise the need for minimal d isturbance of plants during the last hours prior to nutrient uptake me asurements.