Late-spring Frost Tolerance in Plants: Integrating Phenology, Organ Sensitivity and Conserved Mechanisms

Late-spring frost (LSF) is a phenology-dependent freezing hazard that occurs after spring growth has begun, affecting either deacclimated tissues in overwintering plants or newly emerged tissues with limited freezing tolerance in spring-sown crops. Unlike general cold stress, LSF disproportionately damages deacclimated young tissues and reproductive organs under rapidly fluctuating field temperatures. This review critically evaluates the phenological, physiological, and molecular determinants of LSF and distinguishes direct evidence from mechanisms inferred from conventional cold-stress studies. Rather than treating conserved cold-response pathways as established LSF mechanisms, we evaluate their relevance according to evidence strength, developmental stage, organ sensitivity, and deacclimation status. Across crops and woody perennials, injury severity is governed by the interaction among developmental stage, organ sensitivity, deacclimation status, minimum temperature, exposure duration, cooling rate, and post-frost recovery conditions. Major physiological constraints include membrane destabilization, osmotic imbalance, excessive ROS accumulation, and impaired photosynthesis and reproductive development. Conserved cold-response modules involving Ca²⁺ signaling, mitogen-activated protein kinase (MAPK) cascades, the inducer of CBF expression 1 (ICE1)-CBF/DREB pathway, antioxidant systems, lipid remodeling, and hormone crosstalk provide plausible mechanistic hypotheses for LSF resilience. However, direct evidence remains limited, and their relevance depends on tissue specificity, developmental timing, deacclimation status, and rapid reactivation after frost exposure. Hormonal regulation is further shaped by a growth-defence trade-off, because stress-protective responses can restrict spring growth and reproductive success, whereas growth-promoting programs accelerate the loss of cold hardiness. Future research should prioritize field-relevant frost simulations, organ- and stage-resolved phenotyping, validation of candidate genes in reproductive tissues under realistic LSF conditions, and integration of weather forecasting, phenology, cold-hardiness models, breeding, and practical frost protection. This review provides an evidence-constrained perspective on conserved cold-response mechanisms in the context of LSF and highlights priorities for future research.