Changes in tocochromanols and glutathione reveal differences in the mechanisms of seed ageing under seedbank conditions and controlled deterioration in barley

Orthodox seeds are desiccation tolerant, enabling dry storage for agricultural and conservational purposes. Protocols of controlled deterioration (CD) using elevated temperatures and seed water contents (WCs) are frequently used to study mechanisms of seed ageing. However, the relevance of using CD to understand how seeds age at lower water contents and temperatures has been disputed. Here, comparisons in antioxidant changes are drawn between barley seed stored for up to 15 years in a seedbank with seeds aged using CD (45 degrees C, 75% RH; WC = 13% FW) under normoxia (21%) or elevated (75%) O-2. Regardless of ageing method, glutathione (GSH) levels decreased concomitantly with an oxidative shift in the glutathione redox state (E-GSSG/2GSH) as viability was lost. An earlier oxidative shift in E-GSSG/2GSH and more rapid loss of seed vigour was found in seeds aged by CD under 75% 02, compared to 21% 02, but longevity was not shortened by elevated O-2. Seedbank-aged seeds contained more glutathione disulphide (GSSG) levels, leaked more electrolytes upon imbibition and contained less tocochromanols than non-stored seeds. In contrast, in response to CD, levels of GSSG fell, electrolyte leakage did not increase and tocochromanol levels increased (21% 02), or remained stable (75% 02). In a further set of 24 barley genotypes stored for up to 15 years at 20 degrees C (WC = 11% FW) and 0 degrees C (WC = 8% FW), tocochromanol levels positively (r = 0.42) correlated with total germination, which ranged between 1-78% (20 degrees C) and 65-97% (0 degrees C). Of all tocochromanol species, alpha-tocopherol correlated most with total germination (r = 0.49) and gamma-tocotrienol least (r = 0.08). It is concluded that CD is not a perfect model for understanding mechanisms of ageing that occur under seedbank conditions.