Rapid Whole-Plant Signalling Under Compound Environmental Stresses: Systemic Integration, Stomatal Coordination and Stress Memory.

Plants rarely encounter environmental stress as a single, steady stimulus. Heat, drought, high evaporative demand, excess light, nutrient limitation, mechanical damage and infection often overlap, creating compound conditions that require rapid coordination among leaves, roots, vascular tissues and meristems. This review develops a mechanistic framework for rapid whole-plant stress signalling under compound environmental stresses. We propose that acclimation depends on a multi-speed signalling architecture in which reactive oxygen species, Ca2+, electrical and hydraulic waves transmit fast systemic information, while hormones, metabolites, redox networks, organelle signals, transcriptional circuits and chromatin states shape specificity, duration and memory. Particular emphasis is placed on stomatal coordination because guard cells integrate the central heat-drought trade-off between water conservation, evaporative cooling, photosynthesis and defence. The review identifies unresolved questions concerning signal specificity, interaction among fast waves during simultaneous stresses, cell-type-specific decoding and the separation of beneficial memory from growth penalty. Finally, it outlines an experimental pipeline combining live biosensors, electrophysiology, vascular imaging, spatial omics, perturbation genetics and crop phenotyping. Linking dynamic signals to physiological outcomes will help convert systemic stress biology into mechanistic principles for climate-resilient crop design.