DOI: 10.5061/dryad.vc01dp6
title: Data from: Repeatable, continuous and real-time estimates of coupled nitrogenase activity and carbon exchange at the whole-plant scale

description: 1. Symbiotic nitrogen fixation (SNF) by higher plants and their bacterial
 symbionts is a globally important input of nitrogen. Our understanding of
 the mechanisms that control SNF and the timescales over which they operate
 has been constrained by limitations of the existing methods for measuring
 SNF. One method, Acetylene Reduction Assays by Cavity-ring down laser
 Absorption Spectroscopy (ARACAS), seems promising, as it is highly
 sensitive and gives rapid, continuous, repeatable and real-time
 measurements of nitrogenase activity. ARACAS has been used to study
 nitrogen fixation in lichens, mosses and asymbiotic bacteria, but adapting
 it to higher plants poses challenges because acetylene and ethylene can
 influence plant function. 2. Here we report modifications to ARACAS that
 allow it to be used on higher plants in an environmentally-controlled
 incubation chamber. The modifications include lower concentrations of
 acetylene (2%) and ethylene and concurrent measurements of whole chamber
 CO2 exchange, H2O exchange and nitrogenase activity, linking nitrogenase
 activity to whole-plant rates of photosynthesis and respiration. 3. After
 propagating the error terms from all sources, we establish the following
 parameters of the method: (A) The detection limit of our method was 2-3
 ppbv C2H4 hr-1, although it rose substantially when we used tank-derived
 acetylene, which has much higher ethylene contamination; (B) Repeated
 measures at a frequency of 3 days or longer did not diminish nitrogenase
 activity or photosynthesis, although daily measurements diminished
 nitrogenase activity; (C) This method can detect changes at timescales as
 short as seconds; (D) Continuous measurement of nitrogenase activity is
 maintained above 90% of the maximum rate for 7.0 ± 1.3 (mean ± SD) hours.
 4. This method has the potential to improve our understanding of the
 controls over SNF and therefore how SNF and global nitrogen and carbon
 cycling are likely to be affected by global change.
description: CO2_equilbration_dataData to determine e-folding time of NPE(t). Columns include the time, the CO2 concentration, and the plant #.CO2_exchange_continuous_dataContinuous measurements of CO2 exchange over 8-9 hours. Treatment includes NPE measurements with acetylene (Treatment = "C2H2") and without (Treatment = "control"). Columns include time, apparent photosynthesis, NPE, respiration, plant #, and treatment. Data displayed in Fig. S6.Fig3_codeR code to determine how the uncertainty in Vmax(t) and the detection limit changes as a function of ethylene and acetylene. Simulations are plotted in Fig. 3.FigS2_codeR code to determine how the uncertainty in Vmax(t) and the detection limit changes as a function of ethylene at 10% acetylene. Simulations are plotted in Fig. S2.FigS3_codeR code to determine how the uncertainty in Vmax(t) and the detection limit changes as a function of keff and sigma keff. Simulations are plotted in Fig. S3.FigS4_codeR code to determine how the uncertainty in Vmax(t) and the detection limit changes as a function of Km and sigma Km. Simulations are plotted in Fig. S4.keff_dataData for calculating the leak rate coefficient (keff) for our setup. Columns include the ethylene concentration, keff, and sigma keff.Km_dataData used to determine the Michaelis-Menten half saturation constant (Km) for how ethylene production changes as a function of acetylene. Columns include species identity, individual plant, acetylene concentration, uncertainty in the acetylene concentration, the change in ethylene over time, and the uncertainty in the change in ethylene over time. Species-specific Km values are displayed in Table 1.Nitrogenase_activity_continuous_dataContinuous measurements of nitrogenase activity over 8-9 hours. Columns include time, Vmax(t), and plant #. Data displayed in Fig. 4.Nitrogenase_activity_equilbration_dataData to determine e-folding time of Vmax(t). Columns include the time, the water-corrected ethylene concentration, and the plant #.repeat_measures_dataData to determine the effect of measuring nitrogenase activity by repeated measurements at different frequencies or acetylene concentrations. Columns include plant #, measurement day, acetylene concentration, and the mean and lower and upper bounds of the 95% CI for Vmax(t), apparent photosynthesis, NPE(t), and respiration. Data displayed in Figs. 5 and S7.