Off-season seed multiplication in spring barley: a scale-up step for speed breeding.

Speed breeding (SB) accelerates generation turnover, but the small seed quantities produced during rapid generation advance remain a bottleneck. Here, we quantify intensive off-season seed multiplication as a scale-up stage of the SB pipeline. Using spring barley, we tested two lighting systems, high-pressure sodium (HP S) and light-emitting diode (LED), and two cultivars at three pot configurations in Moscow (55.75°N) over winter at 24°C and a 22-h photoperiod. Fertile tillers reached 31-105 per plant, and the mean seed number ranged from 553 to 1,562 per plant, giving a seed multiplication ratio ( SMR; seeds harvested per seed sown) of up to ~1,560, against typical field values of ~30-120 seeds per plant. The response was reproduced in both cultivars. Per lamp-supplied photon, HP S matched or exceeded LED efficiency in all three pot configurations, indicating that the higher LED yields reflect a greater photon dose rather than spectral superiority. Per unit area, 2 L pots were the most efficient (~20,000-24,500 seeds m-2), so a single winter cycle in ~1.3-3.3 m2 of chamber space can supply enough seed for a competitive yield trial and save up to two breeding years. This is a proof of concept based on two cultivars, one winter season, one site, and two replicates; within those limits, the approach requires no lighting beyond that already installed in controlled-environment facilities.