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.