Abscisic acid (ABA) regulates plant stress adaptation, growth, and reproduction. Despite extensive ABA-Ca 2+ signaling links, imaging ABA-induced Ca 2+ concentration increase was challenging, except in guard cells. Here, we visualize ABA-triggered [Ca 2+ ] dynamics in diverse organs and cell types using a genetically-encoded Ca 2+ ratiometric sensor (CRS) with nanomolar affinity and large dynamic range. Subcellular-targeted CRS reveals time-resolved and unique spatiotemporal Ca 2+ signatures from the initial plasma-membrane nanodomain, cytosol, to nuclear oscillation. Via receptors and sucrose-non-fermenting1-related protein kinases (SNRK2.2,2.3,2.6), ABA activates nanomolar Ca 2+ waves and Ca 2+ -sensor protein kinase (CPK10,30,32) signalling in the root cap cycle from stem cells to cell detachment. Surprisingly, unlike the prevailing NaCl-stimulated micromolar Ca 2+ spike, salt stress induces a nanomolar Ca 2+ wave through ABA signaling, repressing key transcription factors dictating cell fate and enzymes crucial to root cap maturation and slough. Our findings uncover ultrasensitive ABA-Ca 2+ -CPK signalling in modulating root cap cycle plasticity in adaptation to adverse environments.