A Low Red/Far-Red Light Ratio Promotes a Reduction in Time from Sowing to Flowering in Wheat Under Speed Breeding Conditions

Conventional methods for obtaining pure durum wheat lines are time-consuming and low-throughput, making speed breeding (SB) a promising alternative. This study investigated SB optimization using far-red (FR) light. Plants were grown under three red/far-red (R/FR) ratios (6.6, 1.0, 0.4) and on three substrates (peat, soil mixture, mineral wool). Reducing the R/FR ratio significantly accelerated flowering, with the most substantial reduction (R/FR = 0.4) shortening the time to flowering by 4.1–4.2 days. The extent of this acceleration and a concurrent negative impact on spike productivity (vegetative weight of dried spikes, the number of spikelets, and the number of grains per spike) were both dependent on the substrate type. Furthermore, a positive correlation was found between the duration of the sowing-to-flowering period and spike productivity components (spike length and number of grains per spike). Increasing the proportion of FR light enhanced the 1000-grain weight and did not affect the germination rate or regenerative capacity. Modifying the SB for durum wheat by adding FR light (R/FR = 0.4) is a useful strategy for increasing its efficiency, and the negative impact of FR light can be mitigated by adjusting mineral nutrition.