Seed size is an important physical indicator of seed quality that affects vegetative growth and is frequently related to yield, market grade factors and harvest efficiency. The use of high-quality seeds is essential for successful crop production and food security. Crop yield and resource use efficiency depend on the successful plant establishment in the field, and seed vigor is what defines the ability to germinate and establish seedlings rapidly, uniformly and robustly, across diverse environmental conditions.
Distinct seed sizes have different levels of starch and other energy reserves which may be an important factor to improve the expression of germination and initial growth of seedlings. Germination depends on the ability of the seed to use reserves more efficiently, by mobilization of seed reserves for the germination traits. A wide array of different effects of seed size in non-stressful and stressful conditions has been reported for seed germination, seedling emergence and establishment in many crop species. However, these results vary widely between the crop species and the germination and growth environment. In general, large seeds have a higher seedling survival rate, higher growth and better field performance than small seeds, under non-stressful environments.
Genetic variation is the cause for variation in the size of seed between varieties. Based on the size, the seeds are classified as very large, large, medium, small, and very small. The variation is due to flow of nutrients into the seed at the mother plant. Seed size is considered as the component of seed quality which affects the performance of crops ( Ojo, 2000; Adebasi et al., 2011).
The size of the seed is known to affect the fitness of the plant growing from it, larger seeds often have higher fitness. Generally, large seed has better field performance than small seed. Seedling vigour and germination rate increases with the increase in size of seed in rice and wheat. Variation in seed size has been observed in many species among populations, among plants within populations and within single plants. While seeds of particular sizes appear to be favored in particular environments, seed size has also been observed to affect the time and probability of germination, seedling survival and adult reproductive output within the same environment.
Many scientists have shown that use of large seed size and increased seeding rates can improve wheat competitiveness and provide an effective means to reduce wild oat biomass and seed production. The seed size often controls the germination and initial seedling growth in many species. Different seed sizes having different levels of starch and other food storage may be one factor that influences the expression of germination and growth of the plants. Seed grading based upon their size and weight is a common practice to regulate germination and subsequent seedling growth.
For a successful crop production, the use of good quality seed is very essential which increases the yield by 15-20%. The extent of this increase is directly proportional to the quality of seed that is being sown. The seeds of the seed lot may differ by size, weight and density due to production environment and cultivation practices. Size is a widely accepted measure of seed quality and large seeds have high seedling survival growth and establishment. A wide array of different effects of seed size has been reported for seed germination, emergence and related agronomic aspects in many crop species.
Effect of large seed on seedling vigor:
In wheat, seed size is positively correlated with seed vigor, larger seeds tend to produce more vigorous seedlings. Germination rate and seedling vigor index values increased with the increase of seed size suggesting that selection of larger seeds for good stand establishment in rice. In wheat, seed size is positively correlated with seed vigor: larger seeds tend to produce more vigorous seedlings. Nagaraju (2001) observed higher plant height (97.83 g), number of leaves (7.58) and stem girth (6.98 cm) in plants raised by large size (3.0 mm oblong hole screen) followed by medium (seeds passed through 3.00 mm screen) compared to small seeds (passed through 2.8 mm) (90.50, 6.53 and 6.30 cm, respectively), in sunflower. Nagaraju (2001) noticed higher germination percentage, seedling length, seedling vigor index, dry weight and field emergence in large size seeds compared to small seeds (87.16%, 22.33 cm, 1941,0.22 mg and 77.83%), respectively, in sunflower. Nerson (2002) showed that small muskmelon seeds had the lowest percentage germination, emergence and the lowest seedling growth demonstrating that there is an association between seed physical parameters and seed quality.
Effect of large seed on seed yield:
Larger seeds of spring wheat produce a higher yield than smaller seeds under late shown conditions but not under optimum management conditions. It is reported that in wheat, seed size not only influences emergence and establishment but also affects yield components and ultimately grain yield.
Effect of small seeds on seedling vigor:
Small to medium-sized ones produced better germination and seedling vigor than those of bigger ones. Small seeds germinate faster and grow higher under saline conditions and that they could be preferred for use in saline soils to achieve better stands.
