Tozer Seeds Ltd.

Prolonging seed longevity during storage is of major importance for gene banks and the horticultural industry. Slowing down biochemical deterioration, including oxygen-dependent deterioration caused by oxidative processes can boost longevity. This can be affected by the seed structure and the oxygen permeability of seed coat layers. Classical artificial seed ageing assays are used to estimate seed 'shelf-life' by mimicking seed ageing via incubating seeds at elevated temperature and elevated relative humidity (causing elevated equilibrium seed moisture content). In this study, we show that seed lots of vegetable Allium species are short-lived both during dry storage for several months and in seed ageing assays at elevated seed moisture levels. Micromorphological analysis of the Allium cepa x Allium fistulosum salad onion seed identified intact seed coat and endosperm layers. Allium seeds equilibrated at 70% relative humidity were used to investigate seed ageing at tenfold elevated partial pressure of oxygen (high pO2) at room temperature (22 ºC) in comparison to classical artificial ageing at elevated temperature (42 ºC). Our results reveal that 30 days high pO2 treatment causes a rapid loss of seed viability which quantitatively corresponded to the seed viability loss observed by ~ 7 days classical artificial ageing. A similar number of normal seedlings develop from the germinating (viable) proportion of seeds in the population. Many long-lived seeds first exhibit a seed vigour loss, evident from a reduced germination speed, preceding the loss in seed viability. In contrast to this, seed ageing of our short-lived Allium vegetable seems to be characterised by a rapid loss in seed viability.

Storage at an elevated partial pressure of oxygen and classical artificial ageing cause a rapid loss of seed viability of short-lived vegetable seeds.

Relative embryo size (embryo:seed length ratio) is a key trait in which the internal morphology of mature seeds differs. It has shaped the angiosperm history at major evolutionary and climatic events, but its adaptive significance and role in dormancy are unknown. We investigated Apium graveolens (celery) morphologically dormant (MD) fruits, which have underdeveloped (small) embryos embedded in abundant endosperm tissue, for their mechanisms in response to non-optimal colder and warmer temperatures. To germinate, the underdeveloped embryo must first grow inside the endosperm to reach a critical relative embryo size. Distinct hormone–temperature interactions and molecular mechanisms underpinned the reduced embryo growth in response to suboptimal and supraoptimal temperatures. Thermoinhibition (29 °C) inhibited germination by surpressing the initiation of embryo growth in a gibberellin (GA)–abscisic acid (ABA)-regulated manner. This included inhibited endo-β-1,4-mannanase, expansin, and auxin biosynthesis gene expression. In contrast to this, during chilling and across the entire suboptimal temperature range (6–20 °C), the initiation of embryo growth was delayed in a thermal time-compliant manner, as was the expression of GA-induced genes important for ABA-insensitive endosperm degradation and embryo growth. The thermal–hormonal control of germination in seeds with underdeveloped embryos (MD) constitutes a unique programme distinct from seeds with fully developed embryos.