What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

The commercial storage of fruits, vegetables, and florist and nursery stocks

Nitric oxide (NO) is a small highly diffusible gas and a ubiquitous bioactive molecule. Its chemical properties make NO a versatile signal molecule that functions through interactions with cellular targets via either redox or additive chemistry. In plants, NO plays a role in a broad spectrum of pathophysiological and developmental processes. Although nitric oxide synthase (NOS)-dependent NO production has been reported in plants, no gene, cDNA, or protein has been isolated to date. In parallel, precise and regulated NO production can be measured from the activity of the ubiquitous enzyme nitrate reductase (NR). In addition to endogenous NO formation, high NO emissions are observed from fertilized soils, but their effects on the physiology of plants are largely unknown. Many environmental and hormonal stimuli are transmitted either directly or indirectly by NO signaling cascades. The ability of NO to act simultaneously on several unrelated biochemical nodes and its redox homeostatic properties suggest that it might be a synchronizing molecule in plants.

Nitric oxide: the versatility of an extensive signal molecule.

A literature review discusses the possible direct and indirect effects of controlled atmospheres (CA's) on the biochemical and physiological properties of fruits and vegetables during transport and storage The effects specifically discussed are those associated with: respiratory metabolism; ethylene biosynthesis and action; chemical changes affecting product color, texture, flavor, and nutritional value; growth and development; physical injuries caused by bruising; water loss; and physiological disorders alleviated, aggravated, or induced by CA's The role of CA's on controlling pathogens that promote postharvest decay also is discussed(wz)

Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables

A Review of Evidence on the Control of Shoot: Root Ratio, in Relation to Models

Pure cytokinin standards and celery seed extracts containing cytokinin activity were bioassayed using a modified Amaranthus betacyanin bioassay The assay is very rapid and requires no special sterile precautions

A Modified Amaranthus Betacyanin Bioassay for the Rapid Determination of Cytokinins in Plant Extracts

Germination and seedling emergence studies were made on seeds harvested from four different umbel positions of three cultivars of celery (Apium graveolens L.). Although heavier seeds were produced from primary umbels than from other umbels, these were less viable as measured by the germination percentage at 18°C in the light. However, germination of viable seeds from quaternary umhels was lower than that of seeds from primary umbels at 18'^C in the dark when incubated with GA^^, (2 x 10""* M) and seeds from secondary and tertiary umbels tended to be intermediate in response. All viable seeds germinated when N^-benzyladenine (10"^ M) was used in combination with GA^^.,. Seeds from quaternary umbels of two of the cultivars had a lower high-temperature limit for germination in the dark than did seeds from other umbels. In glasshouse experiments the emergence of viable 'quaternary' seeds of these cultivars was higher than that of 'primary" seeds. Under these conditions the time to 50% of the final emergence as determined after 42 days was similar for seeds from all umbel positions within each cultivar. In two vaneties seedling weights were greater from seeds of primary as compared to quaternary umbels, and in general, the largest seedlings arose from the heaviest seeds and the smallest from the lightest seeds in all three cultivars.

Relationship between Position on the Parent Plant and Dormancy Characteristics of Seeds of Three Cultivars of Celery (Apium graveolens)

Seeds of five celery (Apium graveolens L.) cultivars germinated at 15°C in the light or dark but at 22°C only in the light. This light requirement was overcome by treatment with a mixture of the gibberellins GA4 and GA7 (GA4/7) but interactions of cytokinins, daminozide, ethephon, EDTA and N-phenyl-N′-4-pyridylurea (NC5392) with GA4/7 were observed. Varietal differences in response to GA4/7 concentration and the requirement for cytokinins were related to the upper temperature limits for germination of the different cultivars. Seeds of cultivars responding to low concentrations of GA4/7 appeared to contain less natural inhibitor than those requiring either high concentrations of GA4/7 or cytokinin in addition to low GA4/7. The cytokinin requirement for germination was partially removed by leaching the seeds with water. Interaction studies with applied hormones indicated that in seeds incubated in the light inhibition by abscisic acid was partially alleviated by N6-benzyladenine but not by GA4/7 application. The implications of these results are discussed in relation to the involvement of natural plant hormones in the dormancy mechanism of celery seeds.

Growth Regulators and the Phytochrome-Mediated Dormancy of Celery Seeds

SUMMARY
Carrot seeds which had been imbibed or germinated and were then sown in a gel extruded through standard coulters emerged five days earlier than dry seeds sown with a conventional drill and gave higher yields after 64 days although not after 77 days. There was no difference in mean time to emergence or in final plant stand between lettuces grown from dry seeds and those from seeds imbibed for 24 h and sown in a gel, but the duration of the emergence period of the latter was halved in one of the two experiments. Celery seeds, the light requirements of which had been satisfied during germination in optimum conditions in the laboratory before they were sown in the field in a gel, gave a 60% plant stand as compared with a stand of 2% from dry seeds.

The sowing of germinating vegetable seeds using a fluid drill

SUMMARY

Of seven systemic benzimidazole fungicides tested, four showed cytokinin activity in a modified Amaranthus betacyanin bioassay. In the presence of the gibberellins A4 and A7, in the dark at 25°C, the four active compounds stimulated the germination of seeds of two celery cultivars dependent on cytokinin for germination under these conditions. One cytokinin-active fungicide was methyl benzimidazol-2-yl carbamate (MBC) and the other three break down into this compound in solution. The three inactive fungicides do not. However, the hormonal properties of the cytokinin-active fungicides may not be due to the MBC moiety alone because sometimes they were more cytokinin-active than pure MBC.

Tudor H. Thomas

Papers

A Modified Amaranthus Betacyanin Bioassay for the Rapid Determination of Cytokinins in Plant Extracts

Relationship between Position on the Parent Plant and Dormancy Characteristics of Seeds of Three Cultivars of Celery (Apium graveolens)

Growth Regulators and the Phytochrome-Mediated Dormancy of Celery Seeds

The sowing of germinating vegetable seeds using a fluid drill

Investigations into the cytokinin-like properties of benzimidazole -derived fungicides