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 CO2 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 CO2 concentrations. Finally, we contend that large diffusive resistance should lead to large CO2 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.

Blast, caused by Pyricularia oryzae, has become an economically important disease in wheat in Brazil, but little effort has been devoted to understanding the wheat-P. oryzae interaction. This study was intended to determine the effects of P. oryzae infection on the photosynthetic process in wheat plants using a susceptible (BR 18) and a partially resistant cultivar (BRS 229). It was found that the net carbon assimilation rate (A), stomatal conductance (gs), and transpiration rate were dramatically reduced in both cultivars due to P. oryzae infection but to a lesser degree in BRS 229. Photosynthesis was impaired in asymptomatic leaf tissues, indicating that blast severity is not an acceptable indicator for predicting P. oryzae-induced reductions in A. The proportionally larger decreases in A than in gs, in parallel with increases in internal CO2 concentration (Ci), suggest that the lower influx of CO2 into the diseased leaves caused by stomatal closure was not a prominent factor associated with the reduction in A. Additional support for this conclusion comes from the nonsignificant correlation between A and gs, the negative correlation between A and Ci and the positive correlation between blast severity and Ci. Both the maximum rate of carboxylation and the maximum rate of electron transport were dramatically depressed at advanced stages of P. oryzae infection, mainly in BR 18, although the reduction in A was not closely related to the decrease in the electron transport rate. In conclusion, biochemical limitations likely related to the reduced activity of Rubisco, rather than diffusive limitations, were the main factor associated with decreases in A during the infection process of P. oryzae on wheat leaves.

Silicon (Si) has been recognized as a beneficial element to improve rice (Oryza sativa L.) grain yield. Despite some evidence suggesting that this positive effect is observed when Si is supplied along the reproductive growth stage (from panicle initiation to heading), it remains unclear whether its supplementation during distinct growth phases can differentially impact physiological aspects of rice and its yield and the underlying mechanisms. Here, we investigated the effects of additions/removals of Si at different growth stages and their impacts on rice yield components, photosynthetic performance, and expression of genes (Lsi1, Lsi2 and Lsi6) involved in Si distribution within rice shoots. Positive effects of Si on rice production and photosynthesis were manifested when it was specifically supplied during the reproductive growth stage, as demonstrated by: (1) a high crop yield associated with higher grain number and higher 1000-grain weight, whereas the leaf area and whole-plant biomass remained unchanged; (2) an increased sink strength which, in turn, exerted a feed-forward effect on photosynthesis that was coupled with increases in both stomatal conductance and biochemical capacity to fix CO; (3) higher Si amounts in the developing panicles (and grain husks) in good agreement with a remarkable up-regulation of Lsi6 (and to a lesser extent Lsi1). We suggest that proper levels of Si in these reproductive structures seem to play an as yet unidentified role culminating with higher grain number and size.

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Key message This study highlights that wood density integrates the xylem structural changes and plays a key role in drought tolerance at the intraspecific level in clones of robusta coffee. Abstract Robusta coffee (Coffea canephora) is largely cropped in regions where drought stress is the major bottleneck limiting crop yields. We hypothesized that clonal differences in wood density (D w ) would be reflected in xylem anatomical differences with associated consequences for hydraulic functioning and ultimately drought tolerance. We assessed the major functional properties of water conduction systems at both the leaf and stem levels in 8-year-old clones of robusta coffee with varying degrees of drought tolerance. The plants were grown outdoors in 24-L pots and either irrigated or subjected to a 4-month water deficit. Upon drought imposition, increased D w , primarily associated with a rearrangement of the fiber matrix and secondarily associated with narrower vessels (although more numerous per cross-sectional area), was correlated with tolerance to hydraulic dysfunctions. Some coordination at the leaf level concerning hydraulic and stomatal anatomical patterns, with stem structural properties, was observed under ample irrigation, but this coordination was decoupled by the imposed drought stress. In conclusion, our data suggest a role for D w in drought tolerance in coffee; however, drought tolerance implies that clones that successfully thrive under low water supply might have compromised fitness under ample irrigation, suggesting a trade-off between D w and the conduction capacity in coffee.