Low-dose ϒ-irradiation can stimulate seed performance in a biphasic (hormetic) manner, yet most studies remain descriptive and lack quantitative comparability across genotypes and traits. We analyzed three commercial varieties of Nicotiana tabacum L. (Burley, Dark, Virginia) exposed to eight doses (0–20 Gy) and evaluated four endpoint families: Day 9 categorical responses (germination, hypocotyl, cotyledon), kinetic summaries (AUC, T50), and seedling biomass (weeks 2 and 4). We fitted, per variety–endpoint, a quadratic model and the Brain–Cousens hormesis model via bounded nonlinear least squares, compared models by AIC/AICc/BIC, and derived radiosensitivity indices (optimal dose x*, maximal gain Hmax, initial sensitivity S=y’(0), ED50, and hormetic windows W(δ). Across endpoints, Brain–Cousens captured the asymmetry between low-dose stimulation and high-dose inhibition and was preferred by AICc when an interior maximum was evident; the quadratic provided a parsimonious local descriptor otherwise. Indices revealed variety-specific optima within the classical stimulatory range (≈5–15 Gy) and suggested a qualitative radiosensitivity pattern DK≳BY≳VA rather than a sharply separated ranking. A meta-regression yielded a concise predictive formula for x* from variety and endpoint, offering an exploratory tool that could inform future data-driven dose recommendations once validated on independent datasets. This parameterization is used here as a phenomenological link to physiology (for example, treating Brain–Cousens parameters as coarse proxies for antioxidant inducibility, tolerance threshold, and transition steepness), while recognizing that such mappings remain speculative in the absence of direct biochemical measurements.
