The response of linseed to treatment with two plant growth regulators
(PGRs), chlormequat and
ethephon, applied at a range of growth stages (corresponding to average
mainstem lengths of 10, 23,
46, 55 and 64 cm) and in various combinations (single and repeated applications
of chlormequat and
ethephon, either alone or mixed) was studied in field experiments over
two seasons in 1992 and 1993
at the University College of Wales, Aberystwyth. Reduction in mainstem
length was dependent upon
the type of PGR and the timing of its application. Chlormequat alone was
consistently more effective
than ethephon alone, while a mixture of the two at half rates was similar
to chlormequat alone. The
largest reductions in mainstem length (16·6% with chlormequat and
6·1% with ethephon, averaged
over the two years) were achieved when the PGRs were applied when mainstems
averaged 22–23 cm
in length. Response to PGRs became progressively smaller with later applications.
However, the
reduction in mainstem length, when expressed as a percentage of that part
of the stem yet to extend
(i.e. final untreated stem length−stem length at the time of PGR
application), was shown to be
remarkably consistent across all (apart from the first) timings of application,
with average values of
28·4 and 7·4% for chlormequat and ethephon respectively.
Application of both PGRs increased tillering and increased significantly
the number of stems per
unit area at maturity. This response occurred irrespective of timing of
application and was greater
following treatment with ethephon than with chlormequat. In the first year,
the effects of PGRs on
dry matter production and seed yield were small and not statistically significant.
In the second year,
significant reductions in seed yield were associated with those PGR treatments
which promoted
tillering most, i.e. early application of ethephon. Smaller yields were
the result of fewer capsules per
plant. Furthermore, a significant correlation between mainstem length and
the number of seeds per
capsule indicated a reduction of 0·78 seeds per capsule for every
10 cm reduction in stem length. While
total oil content of seed remained unaffected by PGR applications, treatments
which included
chlormequat at an early growth stage (either alone or in combination with
ethephon) altered the
relative proportions of fatty acids, reducing the content of linolenic
acid while increasing that of oleic
acid.