Hypocotyl–apical hook coordination determines cotton emergence under low-temperature and salinity stress

Seedling emergence is a major determinant of stand establishment and subsequent yield formation in cotton (Gossypium hirsutum L.), yet it is highly vulnerable to early-season low temperature and soil salinity. This review synthesizes morphological, physiological, genetic, and mechanobiological advances to propose that hypocotyl elongation and apical hook development operate as a coordinated “elongation–protection” unit during cotton emergence. Hypocotyl cell expansion and cell-wall loosening generate the driving force needed for soil penetration, whereas hook curvature, produced by differential growth, buffers mechanical stress and protects the shoot apical meristem. We show that emergence failure under low temperature and salinity often occurs after germination, at the soil-penetration stage, when reduced elongation capacity, unstable hook curvature, or premature hook opening disrupt this integrated unit. Low temperature and salinity constrain emergence through convergent, multi-level mechanisms, including perturbed membrane/redox homeostasis, impaired energy metabolism, reduced cell-wall plasticity, and extensive rewiring of hormone biosynthesis, transport, and signaling. Recent advances further indicate that cell-wall integrity sensing, pectin remodeling, and energy–redox coordination are integral to the regulation of elongation and hook maintenance under stress. We propose a mechanistic framework linking hormone homeostasis, cell-wall mechanics, and morphogenetic output, in which auxin–ethylene coordination directs differential growth and hook curvature, gibberellins and brassinosteroids support elongation competence, and abscisic acid prioritizes stress responses. Cotton-specific evidence, including GhHLS1, GhSAL1, GhBRX family genes, and recent transcriptomic/network analyses, suggest that conserved regulatory modules are retained but rewired within a polyploid-specific regulatory architecture. Finally, we advance a unifying mechano-physiological concept of “resistance–driving force matching” and outline research priorities in quantitative mechanobiology, cotton-specific genetic regulatory network dissection, genetic variation mining, and integration with agronomic management and field phenotyping to improve stress-resilient cotton emergence.