Oxidative Stress, Ageing and Methods of Seed Invigoration: An Overview and Perspectives

The maintenance of seed quality during the long-term conservation of plant genetic resources is crucial for averting the projected food crises that are linked to the changing climate and rising world population. However, ageing-induced loss of seed vigour and viability during storage remains an inevitable process that compromises productivity in several orthodox-seeded crop species. Seed ageing under prolonged storage, which can occur even under optimal conditions, induces several modifications capable of causing loss of intrinsic physiological quality traits, including germination capacity and vigour, and stand establishment. The problems posed by seed ageing have motivated the development of various techniques for mitigating their detrimental effects. These invigoration techniques generally fall within one of two categories: (1) priming or pre-hydrating seeds in a solution for improved post-harvest performance, or (2) post-storage reinvigoration which often involves soaking seeds recovered from storage in a solution. Seed priming methods are generally divided into classical (hydropriming, osmopriming, redox priming, biostimulant priming, etc.) and advanced (nanopriming, magnetopriming and priming using other physical agents) techniques. With the increasing popularity of seed invigoration techniques to achieve the much-desired enhanced productivity and resilience in the face of a changing climate, there is an urgent need to explore these techniques effectively (in addition to other important practices such as plant breeding, fertilizer application, and the control of pests and diseases). This review aims to provide an overview of ageing in orthodox seeds and invigoration techniques that can enhance desirable agronomic and physiological characters.

Response of groundnut (Arachis hypogaea L.) genotypes to accelerated ageing treatment

Reduction in germination of crop seeds due to depletion of food reserves and decline in synthetic activity due to ageing has become a serious concern to groundnut growers who need adequate, high quality seeds to sustain groundnut production. Therefore, to stimulate farmers’ interest in groundnut production, an experiment was conducted to evaluate some groundnut varieties for their tolerance to seed ageing stress, with a view to recommending varieties that can be considered for production in tropical countries. Seeds of nine elite groundnut genotypes, sourced from The International Crops Research Institute of Semi-Arid Tropics (ICRISAT), Kano and three other genotypes sourced from local seed dealer in Ibadan, Nigeria were subjected to seed quality assessments in the seed testing laboratory of Institute of Agricultural Research and Training, Ibadan. The seed lots were subjected to accelerated ageing procedures of 42 °C temperature and 100% relative humidity for 24 hours. Twenty-five seeds of each genotype were drawn from each genotype in three replicates at 3, 6, 12 and 24 hours of ageing. The drawn samples were reassessed to determine their tolerance ability to ageing stress. Percentage germination was transformed using arc-sine before the data on preliminary seed germination and seedling vigour data and seed ageing data were subjected to analysis of variance (ANOVA) using SAS™ Means were separated using Duncan Multiple Range Test (DMRT) at 5% level of significance while k-means non-hierarchical clustering analysis was used to group the genotypes based on their response to the ageing. Result showed that seeds of the groundnut genotypes differ in their response to ageing stress factors. Seeds of ‘Samnut-24’, ‘Samnut-25’ and ‘Ex-Dakar’ (R) were found to be more tolerant to ageing stress while ‘Samnut 22’ and ‘Boro White’ were susceptible to ageing stress. Optimum ageing for 24 hours is recommended for testing seeds of groundnut varieties for storage tolerance

Early counts of radicle emergence, counted manually and by image analysis, can reveal differences in the production of normal seedlings and the vigour of seed lots of cauliflower

The ability of a single early count of radicle emergence (RE) to predict differences in the production of normal seedlings (NG) and seed vigour was examined in four seed lots from each of three varieties of cauliflower (Brassica oleracea var botrytis). Initial seed quality was assessed in germination and controlled deterioration (CD) vigour tests. RE (first appearance of the radicle) was counted manually after 48 hours in the germination test (RE 48h 20/30°C) and every two hours for 144 hours at 20°C using automated image analysis. Seed vigour was assessed in field and glasshouse trials. A large proportion of the variance in NG was explained by the RE count. The same RE counts at 20/30°C and 20°C predicted seed vigour in the field and glasshouse and revealed the same vigour differences as the validated CD test. Differences in vigour (field and glasshouse emergence) observed between varieties were related to the extent of previously sustained seed ageing revealed by the CD test, although genotypic differences in vigour could not be excluded. We propose that single counts of RE taken manually or by image analysis can be used to reveal differences in the NG and vigour of seed lots of cauliflower.

Seed treatment with 5-azacytidine reduces ageing-induced damage in onion seeds

The effect of treating aged onion seeds with 5-azacytidine (5-aza) on germination and vigour was evaluated. Seeds of two onion varieties, 'Bhima Raj' (BRJ) and 'Bhima Red' (BRD) were treated with 0, 10, 25 or 50 μg mL–1 5-azacytidine (a DNA demethylating agent). In comparison with the control treatment (0 μg mL–1 5-azacytidine), treatment with 5-azacytidine enhanced seed germination, seedling length, seedling dry weight and seed vigour indices. 5-azacytidine treatment also increased the activity of superoxide dismutase (SOD) and total antioxidant capacity (TAC). Seed treatment with 5-azacytidine has the potential to enhance the viability and vigour of aged onion seeds. This study provides phenotypic and biochemical data for further exploring the role of DNA methylation in understanding the process of seed ageing.

Cytoplasmic physical state governs the influence of oxygen on Pinus densiflora seed ageing

During desiccation, the cytoplasm of orthodox seeds solidifies in a glass with highly restricted diffusion and molecular mobility, which extend longevity. Temperature and moisture determine seed cellular physical state, and O2 can promote deteriorative reactions of seed ageing. However, whether seed physical state affects O2-mediated biochemical reactions during ageing remains unknown. Here, we answered this question using oil-rich Pinus densiflora seeds aged by controlled deterioration (CD) at 45 °C and distinct relative humidities (RHs), resulting in a glassy (9 and 33% RH) or fluid (64 and 85% RH) cytoplasm. Regardless of CD regimes, the cellular lipid domain remained always fluid. Hypoxia (0.4% O2) prevented seed deterioration only in the glassy state, limiting non-enzymatic lipid peroxidation, consumption of antioxidants (glutathione, tocopherols) and unsaturated fatty acids, accompanied by decreased lipid melt enthalpy and lower concentrations of aldehydes and reactive electrophile species (RES). In contrast, a fluid cytoplasm promoted faster seed deterioration and enabled the resumption of enzymatic activities implicated in glutathione metabolism and RES detoxification, regardless of O2 availability. Furthermore, seeds stored under dry/cold seed bank conditions showed biochemical profiles similar to those of CD-aged seeds with glassy cytoplasm under normoxia. These findings are discussed in the context of germplasm management.

Effect of Seed Ageing in Biochemical and Molecular Changes in Oilseeds: A Review

Seed deterioration is the loss of quality, viability and vigour either due to ageing or effect of adverse environmental factors. The process of deterioration is an irresistible physiological phenomenon. The seeds which are stored under suitable conditions may sometime undergo a series of changes which can ultimately result in the deterioration of the quality of the seed. Hence a thorough study should be done in these deteriorative changes so that proper measures can be adopted to control the variations. Therefore in this article the changes within cellular, biochemical and metabolic aspects of long term stored oilseeds are discussed.