Artigo
Understanding the low photosynthetic rates of sun and shade coffee Leaves: bridging the gap on the relative roles of hydraulic, diffusive and biochemical constraints to photosynthesis
Autor
Martins, Samuel C. V.
Galmés, Jeroni
Cavatte, Paulo C.
Pereira, Lucas F.
Ventrella, Marília C.
DaMatta, Fábio M.
Institución
Resumen
It has long been held that the low photosynthetic rates (A) of coffee leaves are largely associated with diffusive constraints
to photosynthesis. However, the relative limitations of the stomata and mesophyll to the overall diffusional constraints to
photosynthesis, as well as the coordination of leaf hydraulics with photosynthetic limitations, remain to be fully elucidated
in coffee. Whether the low actual A under ambient CO 2 concentrations is associated with the kinetic properties of Rubisco and high (photo)respiration rates also remains elusive. Here, we provide a holistic analysis to understand the causes
associated with low A by measuring a variety of key anatomical/hydraulic and photosynthetic traits in sun- and shade- grown coffee plants. We demonstrate that leaf hydraulic architecture imposes a major constraint on the maximisation of the photosynthetic gas exchange of coffee leaves. Regardless of the light treatments, A was mainly limited by stomatal factors
followed by similar limitations associated with the mesophyll and biochemical constraints. No evidence of an inefficient
Rubisco was found; rather, we propose that coffee Rubisco is well tuned for operating at low chloroplastic CO 2 concentrations. Finally, we contend that large diffusive resistance should lead to large CO 2 drawdown from the intercellular airspaces to the sites of carboxylation, thus favouring the occurrence of relatively high photorespiration rates, which ultimately leads to further limitations to A.