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.

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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.