Seed vigour may be defined as 'the sum total of the properties of seed which determine its potential activity and performance during germination and seedling emergence' (adapted from Perry, 1978). Seeds are subjected to varying degrees of water and temperature stress during their development, maturation, harvest and storage during seed production, and then again during rehydration and germination following sowing. Those with high vigour are capable of rapid germination and the establishment of healthy seedlings, leading to efficient crop production.
Plants respond to environmental stresses by forming a series of specific proteins. These include heat shock proteins (HSPs) and late embryogenesis abundant proteins (LEA proteins) (Bettey and Finch-Savage, 1998). These proteins also form in the absence of external stress as part of the normal developmental processes. They are found at specific stages in the growth cycle, especially during dehydration such as in pollen and seed formation. The HSPs prevent protein aggregation and, by acting as molecular chaperones, ensure that protein folding is correctly completed. Late embryogenesis abundant proteins have conserved elements existing as amphiphilic a-helices that may be associated with desiccation protection.
Both types of protein increase during the later stages of seed development; this is when seed vigour also increases. Hence there can be a positive direct correlation between the occurrence of these two characteristics. It has yet to be established whether stress protein content contributes directly to seed vigour. This question was investigated with batches of B. oleracea L. var. capitata cv. Bartolo (Fx hybrid white cabbage) using seed lots grown in different years, representing the product of interactions with a range of environmental conditions and possessing high levels of viability. Hence any differences between them would relate to vigour characters and the ability to withstand stressful conditions.
It is thought that HSP17.6 contributes to the performance of cabbage seed under stressful conditions and, therefore, is an important component of vigour. The seed was subjected to 'rapid ageing' processes (42 °C, 10 days, 10% moisture content). The HSP17.6 was unaffected, and seed with the highest concentrations were better able to withstand subsequent stresses.
Since vigour results from a combination of characters, seed batches with high vigour relative to others as determined by one set of tests may have lower vigour when this is tested using a different set of environmental conditions. Thus measurement of a single characteristic does not provide a reliable estimate of seed vigour. A combination of tests is essential for the reliable prediction of seedling vigour.
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