Net ammonium and nitrate influx were independent of transpiration rate for intact seedlings of both a wild species of radish (Raphanus raphanistrum) and a wilty tomato mutant (Lycopersicon escwdkatum Mill. cv RR facca).Studies on the influence of water movement upon root ion fluxes have led to conflicting results (6, 10, 14). Jensen The night before the experiment, an intact plant was transferred to a measurement system in which the shoots and roots were enclosed in separate, but contiguous, cuvettes (3). Photosynthesis and transpiration were calculated from the changes in CO2 and H20 concentration of the gas passing through the shoot cuvette. Ion selective electrodes monitored the differences in ammonium and nitrate concentration of the nutrient solution entering and leaving the root cuvette and net influx estimated from these differences times the flow rate (2).All experiments were performed at 26C leaf temperature, 20C root temperature, 300 to 320 gbar ambient CO2 concentration, and 450 imol quantum m 2 s-' to the shoot. The incoming nutrient solution contained 1 mM Na2SO4 to adjust ionic strength, 0.1 mm CaSO4 to provide adequate amounts ofcalcium for membrane integrity, 0.5 mm KH2PO4 to avoid phosphate deficiencies, and varying amounts of NH4C1 and KNO3. The solution for radish contained 20 or 200 uM KNO3 and 20 Mm NH4Cl. The ammonium concentration was kept low because radish is susceptible to ammonium toxicity (9). The solution for tomato contained 200 or 1000 ,M of both NH4Cl and KN03.Humidity around the shoot was adjusted according to the following protocol: vapor pressure deficit was (a) started at a moderate level, (b) increased to a high level, (c) returned to the initial moderate level, (d) decreased to a very low level, and (e) time permitting, returned to the initial level. After each of these step changes in humidity, constant conditions were maintained for 1 to 2 h until steady state CO2, water, and nutrient fluxes were observed. The gas analyzer, humidity sensors, and electrodes were rezeroed after two or three consecutive measurements. All data were related to the leaf area and root dry weight obtained at the end of the experiment. RESULTSDaily changes in net ion influx did not correlate with changes of the water flux for either radish or tomato. While transpiration varied between 0.2 and 8.7 mmol m-2 s-', photosynthesis and net ammonium influx remained relatively constant; net nitrate influx increased steadily during the day for radish and remained relatively constant for tomato at a low supply or varied inconsistently with transpiration at a high supply (Fig. 1)

In response to injury, neurons activate a program of organized axon self-destruction initiated by the NAD+ hydrolase SARM1. In healthy neurons SARM1 is autoinhibited, but single amino acid changes can abolish autoinhibition leading to constitutively-active SARM1 enzymes that promote degeneration when expressed in cultured neurons. To investigate whether naturally-occurring human variants might similarly disrupt SARM1 autoinhibition and potentially contribute to risk for neurodegenerative disease, we assayed the enzymatic activity of 29 rare SARM1 alleles identified among 8,507 amyotrophic lateral sclerosis (ALS) patients. Ten missense variants or small in-frame deletions exhibit constitutive NADase activity, including more than half of those that are unique to the ALS patients or that occur in multiple patients. Expression of these constitutively active ALS-associated SARM1 alleles in cultured dorsal root ganglion (DRG) neurons is pro-degenerative and cytotoxic. Intrathecal injection of an AAV expressing the common SARM1 reference allele is innocuous to mice, but a construct harboring SARM1V184G, the constitutively active variant found most frequently in the ALS patients, causes axon loss, motor dysfunction, and sustained neuroinflammation. These results implicate rare hypermorphic SARM1 alleles as candidate genetic risk factors for ALS and other neurodegenerative conditions.