Leaf day respiration involves multiple carbon sources and depends on
previous dark metabolism
Abstract
Day respiration ( R d) is the metabolic,
non-photorespiratory process by which illuminated leaves liberate CO
2 during photosynthesis. R d is
used routinely in photosynthetic models and is thus critical for
calculations. However, metabolic details associated with R
d are poorly known, and this can be problematic to
predict how R d changes with environmental
conditions and relates to night respiration. It is often assumed that
day respiratory CO 2 release just reflects ‘ordinary’
catabolism (glycolysis and Krebs ‘cycle’). Here, we carried out a
pulse-chase experiment, whereby a 13CO
2 pulse in the light was followed by a chase period in
darkness and then in the light. We took advantage of non-targeted,
isotope-assisted metabolomics to determine non-‘ordinary’ metabolism,
detect carbon remobilisation, and compare light and dark
13C utilisation. We found that several concurrent
metabolic pathways (‘ordinary’ catabolism, oxidative pentose phosphates
pathway, amino acid production, nucleotide biosynthesis, and secondary
metabolism) took place in the light and participate in net CO
2 efflux associated with day respiration. Flux
reconstruction from metabolomics leads to an underestimation of R
d, further suggesting the contribution of a variety of
CO 2-evolving processes. Also, the cornerstone of the
Krebs ‘cycle’, citrate, is synthetised de novo from photosynthates
mostly in darkness, and remobilised or synthesised from stored material
in the light. Collectively, our data provides direct evidence that leaf
day respiration ( i) involves several CO
2-producing reactions and ( ii) is fed by
different carbon sources, including stored carbon disconnected from
current photosynthates.