A custom oxygen analyzer in conjunction with an infrared carbon dioxide analyzer and humidity sensors permitted simultaneous measurements of oxygen, carbon dioxide, and water vapor fluxes from the shoots of intact barley plants (Hordeum vulgare L. cv Steptoe). The oxygen analyzer is based on a calciazirconium sensor and can resolve concentration differences to within 2 microliters per liter against the normal background of 210,000 microliters per liter. In wild-type plants receiving ammonium as their sole nitrogen source or in nitrate reductase-deficient mutants, photosynthetic and respiratory fluxes of oxygen equaled those of carbon dioxide. By contrast, wild-type plants exposed to nitrate had unequal oxygen and carbon dioxide fluxes: oxygen evolution at high light exceeded carbon dioxide consumption by 26% and carbon dioxide evolution in the dark exceeded oxygen consumption by 25%. These results indicate that a substantial portion of photosynthetic electron transport or respiration generates reductant for nitrate assimilation rather than for carbon fixation or mitochondrial electron transport.The influence of NO3-assimilation upon photosynthesis and respiration has been the subject of much speculation (9,17,20,25). To provide energy for NO3-assimilation, a portion of the electrons that are usually transferred to CO2 during photosynthesis or to 02 during respiration may be instead transferred to 19 (8, 26). Plant material is exposed to an atmosphere highly enriched with the heavy isotope 180. Changes in the levels of 160 and 180 indicate rates of oxygen production and uptake, respectively. These systems cannot monitor water fluxes and, therefore, cannot estimate intercellular CO2 concentrations. (c) Paramagnetic analyzers can resolve small 02 concentration differences (1 1), but demonstrate a high sensitivity to gas flow rate and vibration ( 16). (d) The signal drift and noise of polarographic 02 sensors require that relatively large (_300 ML L-') 02 depletions be obtained (13,14). (e) Commercial instruments based on ceramic electrolytic cells have had adequate sensitivity and stability only at very low ambient 02 concentrations (_2000 AL L-') (4,15

Net uptakes of K' and NO3-were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K and N03-uptake in the absence of NH4R or Cl1. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH4', K', Cl1, or NO3-affected the cultivars very differently: (a) the presence of NI4-as NH4CI stimulated net NO3-uptake in Prato barley but inhibited net N03-uptake in Olli barley; (b) Cl inhibited net NO3-uptake in Prato but had little effect in 011; and (c) NH4I as (NH42SO4 inhibited net K uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl-initially inhibited net K' uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NW4+ and C1-uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.Plant roots normally absorb nutrients from an environment in which many ions are present. The interactions among these ions are often complex. For example, the presence of NH4' in a nutrient medium may either inhibit the absorption of K+ (15,18,19) and NO3 (8,11,12,14,15,18) Two to three weeks after germination, when the third leaf was emerging, a plant was transferred to a measurement system in which the root and the shoot were enclosed by separate, but contiguous, cuvettes (4, 21). To allow for recovery from any transplant shock, this plant was maintained for at least 8 h under the initial nutrient conditions before experimental data were taken. Ion-selective electrodes simultaneously monitored changes in the concentrations of H+, K+, NO3-, NH4', and C1 as a nutrient solution flowed through the root cuvette (3). This solution contained (a) 1 mM Na2SO4 to adjust ionic strength, (b) 0.1 mM CaSO4 to maintain membrane integrity, (c) 0. Three to six replicate plants were monitored on consecutive days.Net Uptake as a Function of Ion Concentration. Each plant was used for a series of measurements at sequentially increasing concentrations (5, 10, 20, 50, 100, and 200 AM) of (a) KNO3 alone, (b) KNO3 and NH4Cl, or (c) KNO3 and 50 AM CaCl2. The plant was held at a given concentration for 2 to 3 h until the net uptake of all ions reached a steady rate. This protocolthe use of plants grown at low nutrient levels (i.e. low-salt plants) and the sequence of measurements from low to high concentrations-minimizes the influence of internal storage pools (9); thus, these measurements should reflect the nature of the transport systems.Net Uptake as a Function of Exposure Time. During longer exposures to ions, the effects of internal storage pools and feedback regulation are more likely to become evident. To examine these phenomena experimental plants were held at 100 AM KNO3 until net u...