The J. Derek Bewley Career Lectures presented at the triennial meetings of the International Society of Seed Science support early-career seed scientists by providing retrospective views, from those late in their careers, of lessons learned and future implications. Ambition, ability, inspiration, foresight, hard work and opportunity are obvious career requirements. The importance of mentoring and teamwork combined with the clear communication of results, understanding and ideas are emphasized. The role of illustration in research, and its dissemination, is outlined: illustration can support hypothesis development, testing and communication. Climate change may perturb the production of high-quality seed affecting conservation as well as agriculture, horticulture and forestry. An illustrative synthesis of the current understanding of temporal aspects of the effects of seed production environment on seed quality (assessed by subsequent seed storage longevity) is provided for wheat (Triticum aestivum L.) and rice (Oryza sativa L.). Seed science research can contribute to complex global challenges such as future food supplies from seed-propagated crops in our changing climate whilst conserving biological diversity (through seed ecology and technologies such as ex situ plant genetic resources conservation by long-term seed storage in genebanks), but only if that research can be – and then is – applied.

Oxygen is a major factor of seed germination since it allows resumption of respiration and subsequent metabolism reactivation during seed imbibition, thus leading to the production of reducing power and ATP. Most studies carried out in the 60s to 85s indicate that oxygen requirement depends on the species and is modulated by environmental factors. They have also demonstrated that the covering structures mainly inhibit germination by limiting oxygen supply to the embryo during imbibition through enzymatic oxidation of phenolic compounds by polyphenol oxidases (catechol oxidase and laccase) and peroxidases. Recent use of oxygen-sensitive microsensors has allowed to better characterize the oxygen diffusion in the seed and determine the oxygen content at the level of embryo below the covering structures. Here, I will also highlight the major data obtained over the last 30 years indicating the key role of oxygen in the molecular networks regulating seed germination and dormancy through (1) the hormonal balance (ethylene, ABA and GA), the hormone-signalling pathway and, in particular, the ABA sensitivity, (2) the emerging role of mitochondria in ROS production in hypoxia and (3) the involvement of the N-degron pathway in the turnover of proteins involved in seed tolerance to hypoxia.

Genetic and biochemical studies have greatly advanced our understanding of the biology of seeds in recent years. Another area of study, which could accelerate contemporary seed biology research, is phylogenomics that integrates the wealth of genome sequence data with evolutionary biology. The recent phylogenomic study of the DELAY OF GERMINATION1 family genes exemplifies how the molecular evolution of seed genes can be traced back through early diverging plants and what implications can be obtained from the analysis of seed gene diversification at ancient times. The identification of possible ancestors of seed genes in non-seed plants could illuminate the ancient roots of the molecular mechanisms driving seed maturation programmes. It is possible that the origins of molecular mechanisms associated with the induction of seed storage proteins and desiccation tolerance proteins date back to the time of, or even prior to, early diverging land plants. Abscisic acid-dependent growth arrest or dormancy mechanisms might date back to red algae, one of the oldest algal groups. Thus, understanding algal cell biology will also be an integral part of future seed biology research. Unravelling key events associated with the evolution of seed- and non-seed plants will not only advance basic research but could also contribute to applied aspects of seed science, potentially leading to technology development for agriculture.

Understanding the relative longevity of different seed lots, perhaps of different species or genotypes, but also following production under different environments or using different cultivation methods, or following different post-harvest treatments, is relevant to anyone concerned with the retention of seed lot viability and vigour during storage. However, different scientists over the years have used different conditions to assess seed lot longevity, as well as different variables as the measure of ‘longevity.’ Here, we give some of the backgrounds to how two standard protocols, with an open and closed system respectively, were derived, and explain why we consider p50, defined as the time during storage when seed lot viability, as measured through a germination test, has declined to 50%, is a suitable longevity trait parameter.

Cumulative oxidative damage from the unavoidable formation of reactive oxygen species (ROS) contributes to seed ageing. Low-molecular-weight (LMW) antioxidants, such as water-soluble glutathione (GSH) and lipid-soluble tocochromanols, can prevent ROS from causing damage, especially when antioxidant enzymes are inactive due to desiccation. However, loss of tocochromanols does not always accompany seed ageing, such as during accelerated ageing or controlled deterioration, despite the presence of oxygen and prevalent loss of GSH. To assess relationships between total germination (TG) and antioxidant changes under storage conditions with practical relevance, commercial seeds of Cucumis sativus, Daucus carota, Helianthus cucumerifolius, Latuca sativa, Lepidium sativum, Phaseolus vulgaris and Raphinus sativus of the same cultivar were obtained over multiple years and stored under ambient conditions (21.9 ± 2.1°C; 36.8 ± 6.6% relative humidity). Sigmoidal fitting of TG revealed time to when 50% of seeds had lost viability, which ranged from <5 years (D. carota) to >15 years (C. sativus). Cellular redox states were quantified via the half-cell reduction potential of LMW thiol/disulphide couples. These negatively correlated with TG (i.e. cell redox states were more oxidized in lots with lower TG), with an average R2 value of 0.62 for the most abundant thiol (GSH, or γ-glutamyl-cysteine in P. vulgaris). Concentrations of tocochromanols positively correlated with TG, with an average R2 value of 0.50 for the most abundant tocochromanol (γ or α in L. sativa and H. cucumerifolius). Therefore, during viability loss under ambient ageing conditions leading to the cytoplasm having a glassy state, the lipid domain in all species experienced oxidative damage.

We investigated the thermal thresholds to seed germination and the variations in abscisic acid (ABA) levels and oxidative metabolism during seed dormancy-breaking and germination in two palm species with differences in desiccation tolerance. We used Mauritia flexuosa (buriti palm, desiccation-sensitive seeds) from swampy habitats (Veredas) and Attalea speciosa (babassu, desiccation-tolerant seeds) from the transition zone between the forest and semi-arid region (drained soils). Germination was evaluated at 15–40°C after dormancy-breaking (operculum removal). At optimal temperature for both species (30°C), embryos were sampled in distinct germination phases – dry, imbibed, after operculum removal and at early germination – and used for quantifying ABA and hydrogen peroxide (H2O2) content, antioxidant enzyme activities and for histolocalization of superoxide anion (O2−). Seeds of M. flexuosa germinated only in a narrow temperature range (25–35°C), while A. speciosa seeds germinated between 15 and 40°C. After operculum removal, reduced ABA levels in embryos of M. flexuosa were accompanied by constant H2O2 levels, while in A. speciosa, similar levels of ABA and H2O2 were maintained throughout all germination phases. The presence of O2− was restricted to the haustorium, and an increase in O2− accumulation was observed in both species after operculum removal. Similarities were noted between both species regarding enzyme activities; however, the activities were higher in embryos from M. flexuosa. The presence of O2− only in the haustorium indicates that this region of the embryo is an active structure following imbibition and is involved in germination itself, not just functioning in reserve mobilization.

In small seeds, light often promotes germination and longer-term exposure to darkness reduces light sensitivity. In cacti inhabiting harsh environments, a rapid response to light exposure is potentially advantageous for seedling establishment. We exposed dark-imbibed seeds of the cactus Cereus repandus to doses of red (RED) light and far-red (FR) light. The seeds exhibited positive photoblastism to RED light. Although the initial levels of germination varied between seed lots, the sensitivity to increasing the RED dose did not. As little as 5 min per day for 4 d was sufficient to saturate the light requirement for germination. The effects of RED light were reversed by FR exposure as long as the interval between RED and FR did not extend to 2 d, by which time the seeds had ‘committed’ to germinate. Dark incubation for 1–2 weeks prior to RED exposure reduced light sensitivity in two seed lots, such that RED only promoted around 20% germination. Phytochrome is assumed to mediate the reversibility of the RED:FR response. High sensitivity to light spectral quality suggests that seeds of C. repandus are able to germinate quickly in high-quality microsites, but seed burial or shading may commit the seeds to form a soil seed bank. The light characteristics of the germination trait in this species are typical of many small seeded species of the drylands.

Generally, orchids produce dust-like seeds in which endosperm reduction and embryo undifferentiation represent a derived state shared with species in about 11 other plant families. Orchid seeds are proposed to have a special kind of morphological or morphophysiological dormancy. We test this proposition, overcoming several design limitations of earlier studies, specifically that the in vitro germination method for orchid seeds uses pro-oxidants for disinfection and incorporates nitrate in the medium; both ‘treatments’ might contribute to dormancy breaking, potentially confounding judgement on the depth and nature of the dormant state. Seeds of the tropical orchid Dendrobium cruentum Rchb. f., were sown both in vitro, on a nutrient medium, and ex vitro, on plain agar omitting prior disinfection with sodium hypochlorite. Seeds previously stored and fresh seeds were incubated under combinations of vitro conditions, light treatments, constant or alternating temperatures and nitrate concentration. Seeds of D. cruentum are very small but have a large embryo that occupies most of the seed. Over a range of constant temperature seeds germinated to the spherical protocorm stage just as well ex vitro as in vitro. Neither light nor nitrate were prerequisites for ex vitro germination. The ability of D. cruentum seed to germinate in the absence of environmental or chemical stimuli suggests that mature seed can be non-dormant. Our results support the proposition that neither all DUST seed fit a dormancy class nor all orchids produce morphological or morphophysiological seeds. Finally, embryo/seed volume determinations in orchids may prove as valuable in studies on the evolution and ecology of germination and dormancy as embryo:seed ratios in other angiosperm species.