Cytorrhysis under drought, osmotic and freezing stress.

Cytorrhysis describes the shrinkage of plant cells caused by water loss, during which the protoplast and cell wall contract while remaining connected. Cytorrhysis can occur naturally during drought and freezing and can be experimentally induced by exposure to osmotic substances. Although traditionally studied separately, the cytorrhysis process itself shares the same underlying mechanisms: water efflux along chemical potential gradients and the mechanical response of the cell wall-plasma membrane continuum. Cytorrhysis has been documented across diverse taxa - including algae, bryophytes, ferns and seed plants - indicating that it represents a fundamental response of walled cells to dehydration. Besides osmotic effects, the extent of cytorrhysis is also strongly affected by biomechanical cell structure. Cell size, wall thickness, elasticity and tissue architecture influence the likelihood of cellular collapse. Small, thick-walled cells withstand larger pressure differences and develop negative turgor pressure whereas large, thin-walled cells collapse more readily. During freezing, extracellular ice imposes temperature-dependent dehydration forces, and biomechanical constraints may generate negative turgor pressure. Recent advances, including cryo-microscopy, differential scanning calorimetry and psychrometric water-potential measurements, allow more precise quantification of freeze dehydration and structural responses. Functionally, cytorrhysis can be protective or damaging. Moderate, reversible cytorrhysis helps maintain membrane integrity e.g. in desiccation-tolerant species. In freezing environments, cells undergoing cytorrhysis survive temperatures far below those tolerated by supercooling cells. However, excessive dehydration may cause cell death. Integrating classical observations with modern biophysical approaches provides a renewed perspective on cytorrhysis and highlights structural traits that could be exploited to enhance plant resilience to drought and freezing stress in a changing climate.