Limitations of p50 as a measure of seed longevity in comparative studies and the way forward

Premise: Studies comparing seed longevity between species or genotypes commonly measure seed longevity as the time it takes for seed lot viability to drop to 50% (p50). However, p50 is influenced by the initial viability. Although standard protocols for comparative studies thus recommend using seed lots with similar and high initial viability (>85%), variation in viability strongly influences p50. Methods: We simulated seed viability decline across a range of initial viabilities and rates of probit viability loss (σ) to illustrate how variation in initial viability affects p50 estimates and to test approaches for reducing this bias. Results: For hypothetical seed lots with identical rates of seed viability loss, variation in initial viability leads to a threefold variation in p50 estimates. Narrowing the initial viability (e.g., to 85–95%) reduced this bias. Alternatively, p50 can be recalculated to a standardized value of initial viability (e.g., 90%), which makes it proportional to the rate of probit viability loss. The most straightforward measure of seed longevity for comparative studies is the probit rate of viability loss itself, represented by σ from the viability equation. Conclusions: p50 is confounded by variation in initial seed lot viability and is suboptimal for comparative studies of seed longevity among species or genotypes. Robust measures of seed longevity include the rate of probit viability loss (σ) or p50 standardized to a certain initial seed viability.