Optimizing germination in oldman saltbush ( Atriplex nummularia ): understanding seed traits and identifying abiotic limits

Abstract Oldman saltbush ( Atriplex nummularia Lindl.) is an Australian woody shrub valued for livestock forage and shelter in saline, hot and semi-arid environments. Adoption is constrained by reliance on nursery-grown seedlings, as direct seeding remains unreliable. Despite its agricultural importance, quantitative data on optimal germination conditions are limited. We evaluated eight F₁ lines to quantify variation in propagule dry weight, seed fill percentage and seed weight over three seasons, and ascertained germination rate (GR), cardinal temperatures and low water potentials for germination. Seeds were germinated across a temperature range (5–50°C) and water potentials (0 to −1.5 MPa, generated using PEG6000 solutions), and GRs fitted to segmented linear or linear models. Collection year was the largest contributor to seed fill percentage, while maternal line was most important for propagule and seed weights. Ranking of most lines remained stable across years. All lines achieved high germination (74–98%) at cool temperatures. Germination declined rapidly above 25°C, with optimal temperatures of 18.1–23.8°C. At 40°C, germination percentage varied from 32% to 69% and was negligible at 50°C. Base water potential for germination differed significantly between lines (−1.16 to −1.69 MPa); water potential had minimal effect to −0.75 MPa, but both rate and proportion declined sharply at more negative potentials; at −1.5 MPa, germination was below 20% for all lines. Oldman saltbush’s preference for cool, moist germination conditions likely reflects adaptation to Mediterranean environments, enabling establishment before summer drought. Integrating these traits with nutritional value and agronomic performance will inform selection for nursery production and direct sowing.