The effect of artificially extended daylengths and of sowing times on Antirrhinum majus

The effect of artificially extended daylengths on the growth and flowering responses of 7 cultivars of Antirrhinum was assessed for 9 sowing-times at 2-week intervals. As the sowings progressed from Autumn to Winter, there was a reduction in the flowering-time and leaf number of the early cultivars after an initial increase; for the late cultivars there was a decrease from the first sowing. Only early cultivars showed a significant reduction for these parameters when artificially extended days were superimposed on plants from each sowing. The extended photoperiod had no effect on flower number, leaf area ratio (LAR) or the maximum growth rate (MGR). The LAR of early flowering cultivars decreased with sowing-time, whilst the late-flowering types showed the greatest change in MGR with sowing-time. These data are discussed in terms of selecting new Antirrhinum cultivars which, because of their sensitivity to daylength, can be easily manipulated by the grower.