Seed vigour is a term encompassing the sum total of those properties of the seed that determine the potential performance of the seed or seed lot during germination and seedling emergence (Perry, 1978). Rapid and uniform germination are among the properties of vigorous seeds (Argerish & Bradford, 1989). Low vigour adversely affects such factors as optimal emergence, stress resistance and uniform growth of emergent seedlings (Patrick et al, 2000) and is thus of great economic importance. Seeds, which grow well under the ideal conditions of the germination test, often do not perform so well when planted under the more stressful conditions of the field. Some seedlots perform particularly badly under stress, and are said to be of low vigour. A considerable amount of research has been carried out to discover the causes of low vigour, since if it could be prevented, fewer low-vigour seedlots would need to be wasted (pers.comm. Dr. A. Goldsworthy, 2003). Unfortunately, there does not seems to be a single underlying cause.
Seeds which are either smaller or larger than normal, are often of low vigour. Sawan et al (1999) found that both seed size and seed density are correlated with seedling vigour in cotton. Small seeds may lack food reserves, and large seeds may be more easily damaged by harvesting or have metabolic abnormality. These can be removed from an otherwise healthy seedlot by seiving. The seedling characteristics most closely associated with lower seedling vigour are a small embryo, small primary leaves on the main shoot, a low leaf area to leaf weight ratio, and a low frequency of coleoptile tillers (Richards & Lukacs, 2002).
Sometimes low vigour seeds have a low protein content. This may be due to inadequate nitrogen fertilisation. Application of more nitrogen will remedy this problem (Sawan et al, 1999).
Most cases of low vigour are due to seed damage (pers.comm. Dr. A.Goldsworthy, 2003). Seeds can be damaged in a multitude of ways and the actual cause of the trouble is often difficult to ascertain. Damage from harvest can often be diagnosed by visual inspection. Such damage can be caused if the harvesting and threshing machinery is run too fast, especially if the seeds are at the wrong moisture content. If the seeds were too dry and brittle, there may be a large number of split or broken seeds in the sample. Conversely, if the seeds were too wet and soft, the embryos are more likely to be partially pulled-out or missing. Grass and Tourkmani (1999) looked at rejected durum wheat seed lots in Morocco. All the samples had a low moisture content, with an average moisture content of 9.3%. The most prevalent form of damage was mechanical damage from harvesting and threshing machines. The types of mechanical damage comprised damaged embryos, damaged scutellum and damaged integuments covering the embryos. They suggested that main reason behind mechanical damage was the threshing machine being run at too high a cylinder speed. Seedlings from mechanically damaged seeds showed a high proportion of abnormalities; however, this was related to the type of seed damage, with embryo damage resulting in the highest proportion of abnormalities.
In regions characterised by periods of air temperature and relative humidity higher than 25OC and 65-70%, respectively, as in the humid tropics, a storage of more than 3-4 months may be harmful to maize seed viability and vigour unless seed is stored in suitably dry, preferably air-conditioned conditions. In these tropical environmental conditions the seed is highly susceptible to fungal attack resulting in low seed viability and vigour. Fungicide treatment has been shown to lower seed equilibrium moisture in stored maize and thus improve germination and vigour (Abba & Lovato, 1999). Insect attack on stored seed and subsequent insecticide treatment generally have a deleterious effects on seed vigour. When infestations of Green Gram seed...