Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: A Bayesian modelling approach

Abstract

Field measurements of photosynthetic carbon isotope discrimination (13Δ) of Fagus sylvatica, conducted with branch bags and laser spectrometry, revealed a high variability of 13Δ, both on diurnal and day-to-day timescales. We tested the prediction capability of three versions of a commonly used model for 13Δ [called here comprehensive (13Δcomp), simplified (13Δsimple) and revised (13Δrevised) versions]. A Bayesian approach was used to calibrate major model parameters. Constrained estimates were found for the fractionation during CO2 fixation in 13Δcomp, but not in 13Δsimple, and partially for the mesophyll conductance for CO2 (gi). No constrained estimates were found for fractionations during mitochondrial and photorespiration, and for a diurnally variable apparent fractionation between current assimilates and mitochondrial respiration, specific to 13Δrevised. A quantification of parameter estimation uncertainties and interdependencies further helped explore model structure and behaviour. We found that 13Δcomp usually outperformed 13Δsimple because of the explicit consideration of gi and the photorespiratory fractionation in 13Δcomp that enabled a better description of the large observed diurnal variation (≈9‰) of 13Δ. Flux-weighted daily means of 13Δ were also better predicted with 13Δcomp than with 13Δsimple. Molecular interaction between NRT2.1 transcript levels and the ethylene signaling pathway under nitrate deficiency is still elusive. Here, we report a low nitrate (LN) treatment-induced rapid burst of ethylene production and regulated expression of ethylene signaling components CTR1, EIN3 and EIL1 in wild-type Arabidopsis thaliana (Col-0) seedlings, and enhanced ethylene response reporter EBS:GUS activity in both Col-0 and the ethylene mutants ein3-1eil1-1 and ctr1-1. Comparison of ethylene production and EBS:GUS activity between nrt1.1, nrt2.1 mutants and Col-0 indicated that this up-regulation of NRT2.1 expression caused a positive effect on ethylene biosynthesis/signaling under LN treatment, and ethylene down-regulated NRT2.1 expression and reduced the high-affinity nitrate uptake. Together, these findings uncover a negative feedback loop between NRT2.1 expression and ethylene biosynthesis/signaling under nitrate deficiency, which may contribute to finely tuning of plant nitrate acquisition during exploring dynamic soil conditions. Commentary: Advances in measurements and models of photosynthetic carbon isotope discrimination in C3 plants © 2013 John Wiley & Sons Ltd.