Physiological studies on the growth and development of the carrot, Daucus carota L.
This thesis is concerned with a study of the external, environmental
conditions on the growth and development of the carrot Daucus carota L., as
well as the carotene content, morphogenetic and other effects of defoliation
or extirpation of the apex. Auxanometer records of root growth in different
environments were also obtained. Experiments have also been carried
out to simulate environmental and other effects by means of external application
of known plant growth substances, in an attempt to explain morphogenetic
changes in terms of internal plant regulation. Briefly, the results
of daylength/temperature experiments on tops and roots have indicated that
maximum total plant growth results at relatively warm temperatures, but
under these conditions the leaf fraction is favoured; lone-day treatment
results in similar effects. Root and top temperatures had striking differential
effects. Both can be simulated by gibberellic acid application
which is not dependent on the site of treatment. At lower temperatures
and in short—day, root growth is favoured, although total plant size may
be less. Treatment with the dwarfing compound CCC which is known to have
anti—gibberellic properties modified growth in a similar way to low temperature,
a foliar spray being more beneficial for root growth.
In contrast to gibberellic acid, application of cytokinin increases
the growth (probably cell division as well as cell expansion) at or near
the site of application which may be young leaves, hypocotyl or root.
Only if cytokinins have direct access to the open vascular system is the
effect noticeably away from the site of application.
Root aeration is of vital importance to the carrot plant and lack
of oxygen in the root medium affects both root and top growth detrimentally,
although some degree of adaptation seems possible. Soil compaction greatly
affects yield and maybe related to root aeration. High carotene content
is promoted by relatively low temperature (15°0 and well aerated root
The inter-relation of environmental effects in terms of internal
carbohydrate and hormone balances have been discussed. Some conclusions
are also drawn as to the practical application of some of these results.