Environmental Factors, Developmental Genes and Oxidative Stress Determine Inter-Species Variability in Seed Longevity in Salicaceae

Dry seed longevity varies considerably among species, but little is known about its relation with the climate and the molecular mechanisms that determine seed lifespan. Salicaceae species, with more than 620 species worldwide, are known to produce short-lived seeds, making them particularly good models to explore ageing processes in the glassy state rather than under accelerated ageing. We compared seed lifespan for 13 species of <i>Salix</i> and <i>Populus</i> across a broad geographical range (up to 2200 m a.s.l.). High-quality seeds were obtained by optimizing collection time (just before capsule dehiscence) and post-harvest handling (i.e., the use of negative pressure to remove seed hairs). At optimal moisture contents (MCs) between 6 and 9%, most species seeds demonstrated minimal decreases in viability after storage at -20 °C or in liquid nitrogen for 3 years. Dry room (15% RH, 15 °C) storage differentiated between species' seed lifespans (P₅₀s) of c. 150 to >1200 d. Unlike <i>Salix</i>, <i>Populus</i> species from warm wet environments tended to produce longer-lived seeds in dry storage. Based on transcriptome data on <i>Populus davidiana</i> (longer-lived) and <i>Populus euphratica</i> (shorter-lived), we revealed high correlations between late seed maturation genes, such as 60% of <i>HSP</i> and 67% of <i>LEA</i> genes showed higher expression in <i>P. davidiana</i> seeds, while 70% of <i>WRKY</i> transcription factors showed significantly higher expression in <i>P. euphratica</i> seeds. For these two species, genes related to oxidative stress might be the most important contributor to different seed longevity in the dry glassy state.