PhotoGEA (version 1.4.0)

error_function_c4_aci: Generate an error function for C4 A-Ci curve fitting

Description

Creates a function that returns an error value (the negative of the natural logarithm of the likelihood) representing the amount of agreement between modeled and measured An values. When this function is minimized, the likelihood is maximized.

Internally, this function uses apply_gm to calculate Cc, and then uses link{calculate_c4_assimilation} to calculate assimilation rate values that are compared to the measured ones.

Usage

error_function_c4_aci(
    replicate_exdf,
    fit_options = list(),
    sd_A = 1,
    x_etr = 0.4,
    a_column_name = 'A',
    ao_column_name = 'ao',
    ci_column_name = 'Ci',
    gamma_star_column_name = 'gamma_star',
    gmc_norm_column_name = 'gmc_norm',
    j_norm_column_name = 'J_norm',
    kc_column_name = 'Kc',
    ko_column_name = 'Ko',
    kp_column_name = 'Kp',
    oxygen_column_name = 'Oxygen',
    rl_norm_column_name = 'RL_norm',
    total_pressure_column_name = 'total_pressure',
    vcmax_norm_column_name = 'Vcmax_norm',
    vpmax_norm_column_name = 'Vpmax_norm',
    hard_constraints = 0,
    debug_mode = FALSE
  )

Value

A function with one input argument guess, which should be a numeric vector representing values of the parameters to be fitted (which are specified by the fit_options input argument.) Each element of guess is the value of one parameter (arranged in alphabetical order.) For example, with the default settings, guess should contain values of RL_at_25,

Vcmax_at_25, and Vpmax_at_25 (in that order).

Arguments

replicate_exdf

An exdf object representing one CO2 response curve.

fit_options

A list of named elements representing fit options to use for each parameter. Values supplied here override the default values (see details below). Each element must be 'fit', 'column', or a numeric value. A value of 'fit' means that the parameter will be fit; a value of 'column' means that the value of the parameter will be taken from a column in replicate_exdf of the same name; and a numeric value means that the parameter will be set to that value. For example, fit_options = list(RL_at_25 = 0, Vcmax_at_25 = 'fit', Vpmax_at_25 = 'column') means that RL_at_25 will be set to 0, Vcmax_at_25 will be fit, and Vpmax_at_25 will be set to the values in the Vpmax_at_25 column of replicate_exdf.

sd_A

The standard deviation of the measured values of the net CO2 assimilation rate, expressed in units of micromol m^(-2) s^(-1). If sd_A is not a number, then there must be a column in exdf_obj called sd_A with appropriate units. A numeric value supplied here will overwrite the values in the sd_A column of exdf_obj if it exists.

x_etr

The fraction of whole-chain electron transport occurring in the mesophyll (dimensionless). See Equation 29 from S. von Caemmerer (2021).

a_column_name

The name of the column in replicate_exdf that contains the net assimilation in micromol m^(-2) s^(-1).

ao_column_name

The name of the column in replicate_exdf that contains the dimensionless ratio of solubility and diffusivity of O2 to CO2.

ci_column_name

The name of the column in replicate_exdf that contains the intercellular CO2 concentration in micromol mol^(-1).

gamma_star_column_name

The name of the column in replicate_exdf that contains the dimensionless gamma_star values.

gmc_norm_column_name

The name of the column in replicate_exdf that contains the normalized mesophyll conductance values (with units of normalized to gmc at 25 degrees C).

j_norm_column_name

The name of the column in exdf_obj that contains the normalized J values (with units of normalized to J at 25 degrees C).

kc_column_name

The name of the column in replicate_exdf that contains the Michaelis-Menten constant for rubisco carboxylation in microbar.

ko_column_name

The name of the column in replicate_exdf that contains the Michaelis-Menten constant for rubisco oxygenation in mbar.

kp_column_name

The name of the column in replicate_exdf that contains the Michaelis-Menten constant for PEP carboxylase carboxylation in microbar.

oxygen_column_name

The name of the column in exdf_obj that contains the concentration of O2 in the ambient air, expressed as a percentage (commonly 21% or 2%); the units must be percent.

rl_norm_column_name

The name of the column in replicate_exdf that contains the normalized RL values (with units of normalized to RL at 25 degrees C).

total_pressure_column_name

The name of the column in exdf_obj that contains the total pressure in bar.

vcmax_norm_column_name

The name of the column in replicate_exdf that contains the normalized Vcmax values (with units of normalized to Vcmax at 25 degrees C).

vpmax_norm_column_name

The name of the column in replicate_exdf that contains the normalized Vpmax values (with units of normalized to Vpmax at 25 degrees C).

hard_constraints

To be passed to calculate_c4_assimilation; see that function for more details.

debug_mode

A logical (TRUE or FALSE) variable indicating whether to operate in debug mode. In debug mode, information about the guess is printed each time the error function is called; this can be helpful when troubleshooting issues with an optimizer.

Details

When fitting A-Ci curves, it is necessary to define a function that calculates the likelihood of a given set of alpha_psii, gbs, gmc_at_25, J_at_25, RL_at_25, Rm_frac, Vcmax_at_25, Vpmax_at_25, and Vpr values by comparing a model prediction to a measured curve. This function will be passed to an optimization algorithm which will determine the values that produce the smallest error.

The error_function_c4_aci returns such a function, which is based on a particular A-Ci curve and a set of fitting options. It is possible to just fit a subset of the available fitting parameters; by default, the fitting parameters are RL_at_25, Vcmax_at_25, and Vpmax_at_25. This behavior can be changed via the fit_options argument.

For practical reasons, the function actually returns values of -ln(L), where L is the likelihood. The logarithm of L is simpler to calculate than L itself, and the minus sign converts the problem from a maximization to a minimization, which is important because most optimizers are designed to minimize a value.

A penalty is added to the error value for any parameter combination where An is not a number, or where calculate_c4_assimilation produces an error.

Examples

# Read an example Licor file included in the PhotoGEA package
licor_file <- read_gasex_file(
  PhotoGEA_example_file_path('c4_aci_1.xlsx')
)

# Define a new column that uniquely identifies each curve
licor_file[, 'species_plot'] <-
  paste(licor_file[, 'species'], '-', licor_file[, 'plot'] )

# Organize the data
licor_file <- organize_response_curve_data(
    licor_file,
    'species_plot',
    c(9, 10, 16),
    'CO2_r_sp'
)

# Calculate temperature-dependent values of C4 photosynthetic parameters
licor_file <- calculate_temperature_response(licor_file, c4_temperature_param_vc)

# Calculate the total pressure in the Licor chamber
licor_file <- calculate_total_pressure(licor_file)

# Define an error function for one curve from the set
error_fcn <- error_function_c4_aci(
  licor_file[licor_file[, 'species_plot'] == 'maize - 5', , TRUE]
)

# Evaluate the error for RL_at_25 = 0, Vcmax_at_25 = 35, Vpmax_at_25 = 180
error_fcn(c(0, 35, 180))

# Make a plot of error vs. Vcmax_at_25 when the other parameters are fixed to
# the values above.
vcmax_error_fcn <- function(Vcmax_at_25) {error_fcn(c(0, Vcmax_at_25, 180))}
vcmax_seq <- seq(20, 50)

lattice::xyplot(
  sapply(vcmax_seq, vcmax_error_fcn) ~ vcmax_seq,
  type = 'b',
  xlab = 'Vcmax at 25 degrees C (micromol / m^2 / s)',
  ylab = 'Negative log likelihood (dimensionless)'
)