Severe nickel stress constrains phenotypic plasticity and induces developmental canalisation in Catharanthus roseus

Phenotypic plasticity is a key strategy for plant acclimatisation; however, its limits under lethal stress remain poorly understood. This study investigated the morphophysiological responses and changes in the developmental architecture of Catharanthus roseus subjected to acute nickel stress (70 and 130 mg kg⁻¹). While the intermediate dose induced a succulence-by-dilution mechanism and an avoidance syndrome (etiolation), severe stress caused metabolic collapse, evidenced by a 28% reduction in chlorophyll and stagnation of the absolute growth rate. Nevertheless, allometric analysis revealed an unexpected phenomenon of developmental canalisation under maximum toxicity. In contrast to the control plants, which exhibited high phenotypic plasticity (high residual variability), plants under 130 mg kg⁻¹ Ni adopted a strictly coordinated and mathematically predictable growth pattern (R²= 0,93; MAPE = 3.97%). These results suggest that, faced with the energy scarcity imposed by severe toxicity, C. roseus sacrifices its developmental plasticity, channeling resources toward a rigid survival phenotype. This study provides new perspectives on plant allometry under stress, demonstrating that extreme toxicity can act as a canalising force that reduces developmental "noise".