Spingerin C-4

C4 photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C3 trees can establish, their height advantage enables them to outcompete low stature C4 vegetation. Had C4 photosynthesis been able to evolve in an arborescent life form, forest cover (by C4 trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C4 trees. Physiological explanations associated with low light performance of C4 photosynthesis are not supported, because C4 shade-tolerant species exhibit similar performance as shade-tolerant C3 species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C4 trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C4 lineages are more plausible. Most C4 species are grasses and sedges, which lack meristems needed for arborescence, while most C4 eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C4 eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C4 evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C4 lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.

C(4) photosynthesis under optimal conditions enables higher-efficiency use of light, water, and nitrogen than the C(3) form used by many crops. It is associated with the most productive terrestrial plants and crops but is largely limited to the tropics and subtropics. It has been argued that the C(4) photosynthetic apparatus is inherently limited to warm environments. A small group of C(4) species appear to have overcome this, and in contrast to the major C(4) crop, maize, these species are able to acclimate their photosynthetic apparatus to chilling conditions. This review explores the mechanisms underlying this difference as well as the potential of introducing these changes into maize and other warm-climate C(4) crops.

Single-photon, photoelectron-photoion coincidence spectroscopy is used to record the mass-selected ion spectra and slow photoelectron spectra of C4H5 radicals produced by the abstraction of hydrogen atoms from three C4H6 precursors by fluorine atoms generated by a microwave discharge. Three different C4H5 isomers are identified, with the relative abundances depending on the nature of the precursor (1-butyne, 1,2-butadiene, and 1,3-butadiene). The results are compared with our previous work using 2-butyne as a precursor [Hrodmarsson, H. R. J. Phys. Chem. A 2019, 123, 1521-1528]. The slow photoelectron spectra provide new information on the three radical isomers that is in good agreement with previous experimental and theoretical results [Lang, M. J. Phys. Chem. A 2015, 119, 3995-4000; Hansen, N. J. Phys. Chem. A 2006, 110, 3670-3678]. The energy scans of the C4H5 photoionization signal are recorded with substantially better resolution and signal-to-noise ratio than those in earlier work, allowing the observation of autoionizing resonances based on excited states of the C4H5 cation. Photoelectron images recorded at several energies are also reported, providing insight into the decay processes of these excited states. Finally, in contrast to the earlier work using 2-butyne as a precursor, where H-atom abstraction was the only observed process, F- and H-atom additions to the present precursors are also observed through the detection of C4H6F, C4H5F, and C4H7.