Abstract text: Soil acidification hinders root and root hair growth, affecting nutrient balance and shoot biomass. The broadly-accepted acid growth theory is that wall acidification leads to an increase in the cell wall extensibility, allowing turgor-driven growth. Recent studies in roots however, showed that auxin can induce both apoplastic acidification and alkalization via two opposing extranuclear pathways, promoting or repressing cell expansion, respectively (Pérez-Henríquez & Yang, 2022). The question of apoplastic pH effect on cell wall structure and biochemistry remains mostly unresolved. Here, microfluidics-mediated perturbation of extracellular pH shows that, in contrast to the acid growth theory, root hair growth is inhibited, not promoted, in response to the extracellular medium acidification to pH 4.5. Manipulation of the apoplastic pH using optogenetics and its monitoring with cell wall-localized, FLIM-based pH probes, confirmed this result. Interestingly, cell wall acidification in a mutant for Rapid ALkalinization Factor (RALF) 22, a peptide that is essential for root hair cell wall assembly and growth, caused their explosion. This could be reverted by applying a RALF22 version that is unable to bind its plasma membrane receptor (Schoenaers et al., 2024). Super-resolution microscopy offers insights into probable causes for the loss of cell wall integrity in acidic conditions.