Elevated atmospheric CO 2 consistently boosts photosynthetic carbon fixation, but these increases rarely lead to proportional gains in biomass, yield, or long-term productivity. This ongoing mismatch is often related to acclimation, downregulation, or sink limitation, yet the exact mechanisms remain only partially understood. Here, I argue that this paradox mainly stems from confusion between carbon input and metabolic flux. Photosynthesis shows how quickly carbon enters the system, while growth depends on continuous carbon flow through downstream metabolic and developmental sinks. Elevated CO 2 primarily increases carbon pressure within plant metabolic networks without necessarily boosting flux, because flux is limited primarily by sink development rather than by enzyme activity.

Seed water imbibition is critical to the seedling establishment in tropical forests. The neotropical tree Hymenaea ( Hymenaea courbaril) is a model system to study seed storage xyloglucan (XyG) mobilization after germination. Typical of many legumes, seed coats of Hymenaea are formed by a palisade of lignified cells, conferring imperviousness to water and the need for scarification to germinate. Below this fortified cell layer, a parenchyma layer, composed mainly of pectins, is juxtaposed on the surface of the cotyledon, whose cells contain thick cell walls containing mostly XyG. Here we used scanning electron and fluorescence microscopies, and NMRi spectroscopy to visualize water uptake and distribution in seeds of Hymenaea. An experiment with in vitro Hymenaea pectin or XyG composites with cellulose from the Whatman paper demonstrated the cell wall polymers’ functions during imbibition. We observed that water follows distinct pathways containing different cell wall compositions to varying speeds through the seed coat and cotyledons until embryo metabolism is activated synchronically with storage mobilization. We conclude that the dynamic interactions between water and wall polysaccharides (pectins and hemicellulose) of different seed tissues are central to determining water distribution and preparing the seedling for establishment.