Lawrence O. Copeland

Flowering and Seed Set

Plant reproduction is central to survival of the species. This process is accomplished asexually and/or sexually. Asexual reproduction often is the result of modification of vegetative structures such as stems or roots that possess sufficient fleshy tissue for energy storage. Such structures include rhizomes (bluegrass), corms (gladiolas), bulbs (onion), and tubers (potato). Another form of asexual reproduction is apomixis. Two types of apomixis result in the formation of seedlike structures: vivipary and agamospermy. Vivipary is often expressed by the conversion of the vegetative spikelet or leaf into a somatic structure with reproductive capability. Agamospermy occurs when nucellar (unfertilized) tissue in the embryo sac develops into a diploid egg cell that further differentiates into a “seed.” This form of asexual reproduction requires pollination even though fertilization does not occur. Most plants that reproduce primarily through apomixis also have a limited amount of sexual reproduction. This form of asexual reproduction is generally confined to certain grasses such as Kentucky bluegrass.

Annual legumes as green manure and forage crops in winter canola (Brassica napus L.) rotations

Cropping systems that reduce chemical fertilizer N requirements in winter canola (Brassica napus L.) need further study. Studies were conducted in 1994/1995 and 1995/1996 at two locations in Michigan i) to quantify the N accumulated at plowdown by berseem clover (Trifolium alexandrinum L.), alfalfa (Medicago sativa L.) and three annual medic species (Medicago truncatula Gaertn., M. polymorpha L., and M. scutellata L.) when they were managed as green manure or managed as forage; ii) to compare N response of the winter canola crop following the legumes; and iii) to estimate the N fertilizer replacement value (FRV) of the legumes. The legumes were seeded in early May in both years. Winter canola was planted after legume plowdown 90 d after planting (DAP). Four N rates (0, 50, 100, and 150 kg N ha−1) were applied to the winter canola in early spring. Herbage N yield at plowdown was highest (103.9 kg N ha−1) in berseem clover managed as green manure and this treatment resulted in the highest winter canola yiel...

Seed Longevity and Deterioration

Seeds are uniquely equipped to survive as viable regenerative organisms until the time and place are right for the beginning of a new generation; however, like any other form of life, they cannot retain their viability indefinitely and eventually deteriorate and die. Fortunately, neither nature nor agricultural practice ordinarily requires seeds to survive longer than the next growing season, though seeds of most species are able to survive much longer under the proper conditions.

Seed Viability Testing

Although the concept of seed viability is well known, there is considerable disagreement and confusion as to its precise meaning. To most seed technologists and commercial dealers, viability means that a seed is capable of germinating and producing a “normal” seedling. Therefore, it is used synonymously with germination capacity. In this sense, a given seed is either viable or nonviable, depending on its ability to germinate and produce a normal seedling; thus, only seed lots representing populations of seeds may exhibit levels of viability.

Seed Vigor and Vigor Testing

Seeds, as reproductive units, are expected to produce plants in the field. However, farmers and seed producers have long recognized that the labeled germination often overestimates the actual field emergence of seed lots. This occurs because by definition, germination is the “emergence and development from the seed embryo of those essential structures which, for the kind of seed in question, are indicative of the ability to produce a normal plant under favorable conditions (AOSA 2000).” As a result, the standard germination test may fail to provide accurate information concerning a seed lot&s field performance potential for at least four reasons. These include the following.

Seed Vigor and Vigor Tests

Seeds, as reproductive units, are expected to produce plants in the field. However, farmers and seed producers have long recognized that the labeled percent germination often overestimates the actual field emergence of seed lots. This observation is attributed to the objective of a standard germination test which states that germination is the emergence and development from the seed embryo of those essential structures which, for the kind of seed in question, are indicative of the ability to produce a normal plant under favorable conditions. (AOSA, 1991) As a result, the standard germination test fails to provide accurate information concerning a seed lot’s field performance potential for at least four reasons. These include the following.

Flowering Processes In Plants

Plant growth originates within the buds in regions known as meristems. In the meristems, cell division and elongation occur, and these processes produce tissues that soon develop into specific plant parts. Vegetative meristems give rise to parts such as stems, leaves, and roots, while reproductive meristems give rise to floral organs that ultimately produce fruits and seeds.

Seed Storage and Deterioration

Seeds are uniquely equipped to survive as viable regenerative organisms until the time and place are right for the beginning of a new generation; however, like any other form of life, they cannot retain their viability indefinitely and eventually deteriorate and die. Fortunately, neither nature nor agricultural practice ordinarily requires seeds to survive longer than the next growing season, though seeds of most species are able to survive much longer under the proper conditions.

Seed Formation and Development

Seed formation begins with the combination of a male and female gamete: a process known as fertilization. Fertilization, or syngamy, can occur when both male and female gametophytes are fully mature. This usually occurs in a dual fusion process known as double fertilization (Figure 2.1). When the pollen grain lands on the stigma, it germinates by sending out a pollen tube, which grows down the style, through the micropyle and into the embryo sac, with the tube nucleus closely following the tube apex downward. The tube nucleus soon degenerates, but the two pollen sperm cells enter the embryo sac, one fusing with the diploid (2N) polar nucleus to form a triploid (3N) endosperm nucleus and the other fusing with the egg cell to form a diploid (2N) zygote, or fertilized egg.

Seed Viability and Viability Testing

Although the concept of seed viability is well blown, there may sometimes be disagreement and confusion as to its precise meaning. To most seed technologists and most people in the seed industry, viability means that a seed is capable of germinating and producing a “normal” seedling. Therefore, it is used synonymously with germination capacity. In this sense, a given seed is either viable or nonviable, depending on its ability to germinate and produce a normal seedling; thus, only seed lots representing populations of seeds may exhibit levels of viability.