Mechanically-induced stress (MIS) occurs naturally in plants as the aerial parts are moved, usually by wind, but also by such agents as rain and animals . It can be induced indoors by various actions such as rubbing or bending the stem or shaking or brushing the entire shoot . The most noticeable effect of MIS is a reduction in stem, leaf or petiole length invariably resulting in plants which are smaller and more compact than unstressed controls. However, the response of other variables can often differ between species and there may be either increases or decreases in stem or petiole diameter, root : shoot weight ratio, chlorophyll content or drought resistance . Why species should differ in this way, and what is the endogenous control mechanism for MIS responses, are unanswered questions . Ethylene, which increases as a result of MIS in several species may cause some MIS responses such as increased stem diameter, epinasty or a change in sex expression . However, evidence suggests that MIS retardation of extension growth may equally be due to lower or supraoptimal auxin levels or lower gibberellin levels .The uses in the field of the growth promoter gibberellin or the growth retardant chlormequat chloride (CCC) appear to be examples of respectively reversing or stimulating MIS growth response . MIS may be applied indoors if short compact plants are needed, either for aesthetic purposes as with floral crops, or if hardier and more manageable plants are needed, such as seedlings for transplanting in the field . Much more research is needed to estimate the importance of MIS in the field and to assess how such knowledge may be used to improve crop yield .

Ethylene produced by plant tissues grown in vitro may accumulate in large quantities in the culture vessels, particularly from rapidly growing non-differentiated callus or suspension cultures, and hence is likely to influence growth and development in such systems. Research into this aspect of tissue culture has been sparse, although it has grown recently with the increasing importance of in vitro regeneration. This review deals with the measurement and relevance of the accumulated ethylene, and the influence of both exogenous and endogenous ethylene in the different types of tissue culture systems. The relationships between ethylene and other growth regulators in tissue culture growth and development are also discussed. Although in some cases its influence seems negligible, in many types of tissue culture ethylene may act either as a promoter or inhibitor depending on the species used. Thus ethylene has an important influence on many aspects of in vitro regeneration, but it is also clear that we cannot at present describe a specific role or roles for ethylene in tissue culture which can be applied at a general, species-wide level. If its effects are to be enhanced or diminished in order to improve the efficiency and range of plant tissue culture, then more research is needed to clarify what its fundamental role might be in in vitro growth and development.

No abstract

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.

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'-4pyridylurea (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 N^-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.

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 I8°C in the light. However, germination of viable seeds from quaternary umbels was lower than that of seeds from primary umbels at 18°C in the dark when incubated with GA4, (2 × 10 −4M) and seed from secondary and tertiary umbels tended to be intermediate in response. All viable seeds germinated when N6‐benzyladenine (10−2M) was used in combination with GA4. 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 varieties 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.

No abstract

The naturally occurring cytokinins, zeatin, zeatin riboside and dihydrozeatin did not promote the germination of celery (Apium graveolens L.) seeds and 6‐Δ2‐isopentenyladenine (2iPA) and its riboside were only moderately active. Of the synthetic cytokinins, kinetin, kinetin riboside, and the disubstituted urea, N‐phenyl‐N′‐pyridyl urea (NC5392) were moderately active, and 6‐benzyl‐aminopurine (BA) and its derivatives BA riboside and 6‐benzyl‐amino‐9(tetrahydropyran‐2yl)purine (SD8339) were the most active cytokinins tested. 6‐(o‐hydroxybenzyl)aminopurine (hyd‐BA) and its naturally occurring riboside inhibited germination under normally inductive conditions. All the cytokinins examined were more active in promoting germination of lettuce (Lactuca sativa L.) than celery seeds. BA, BA riboside and SD8339 were again the most active cytokinins. In contrast to the results with celery, zeatin and zeatin riboside were highly active. The other cytokinins also showed high activity with the exception of dihydrozeatin, hyd‐BA and hyd‐BA riboside which were less active. Cytokinin ribosides were less active than the corresponding free bases during the early period of the lettuce seed incubation but total germination after 90 h was similar.