Carbon fixation and water use efficiency (WUE)
Our data clearly demonstrate that the constitutive overexpression ofFbβCA3 in the cytoplasm of the transgenics resulted in higher
rates of photosynthetic carbon assimilation, per unit leaf area, than in
the VC plants (Fig. 6a ); this corresponds to the higher
electron transport rates of PSI and PSII that provided sufficient NADPH
and ATP for increased CO2 assimilation. Higher protein
content must have been the reason not only for the elevation of the
rates of photosynthesis, but for the increased metabolic processes
including Chl biosynthesis (Table1 ), and photosynthesis. In the
transgenics, as compared to VC, the quantum yield of CO2assimilation measured in limiting light intensities increased by 17% to
20%. These observations suggest that the transgenics had inherently
higher photosynthetic efficiency and that they utilized the absorbed
light energy much more efficiently.
The FbβCA3 overexpressors had lower stomatal conductance,
decreased transpiration rate per unit leaf area, and a better WUE
(Figs. 6d,e,f ). Overexpression of βCA1 or βCA4 in
guard cells of Arabidopsis thaliana has shown a reduced stomatal
conductance and a substantial increase in the WUE (cf. Hu et al., 2010);
however, there was no increase in the rate of photosynthesis in that
work.
Owing to the overexpression of FbβCA3 , having a higher affinity
for CO2 than that of endogenous CA in the
cytosol, the production of HCO3- is
definitely expected to be higher diminishing the cytoplasmic
[CO2]. It would provide a higher concentration
gradient for a better diffusion of CO2 from the
intercellular space to the cytoplasm of the mesophyll cells to keep a
balance between CO2 uptake and
HCO3- assimilation by cytoplasmic
endogenous PEP carboxylase of Arabidopsis thaliana . The
overexpression of FbβCA3 could, indeed, produce enough
HCO3- under ambient
CO2 to partially close the stomata to reduce the
stomatal conductance and transpiration rate and increase the WUE
(Figs. 6d,e,f ). Conversely, Kolbe et al. (2018) found that theβca1βca2 maize mutants, with 3% of wild type CA activity, had
higher stomatal conductance, a slower stomatal closure in response to an
increase in CO2 concentration. Further, in transgenic
lines of Setaria viridis (C4 grass), with less
than 13% CA activity, Osborn et al. (2017) observed that the
photosynthetic rates decreased at lower CO2concentrations. Further, in CA-deficient (less than 20% of the wild
type) F. bidentis little effect was found on carbon assimilation,
and on g m (Cousins et al., 2006; Ogee et al.,
2018). However, a knockout of chloroplastic βCA1 in A. thalianahas been shown to have poor seedling establishment due to decreased
photosynthetic performance under ambient CO2 (Ferreira
et al., 2008). For efficient photosynthetic performance by plants,
relatively high CA activity is one of the important requirements inA. thaliana . On the basis of the above results, we conclude that
the CAx plants had higher photosynthetic efficiency with higher water
use efficiency.