Variation in the Photoperiodic Control of Flowering of Two Cultivars of Antirrhinum majus L.

The susceptibility to photoperiodic induction of an early (Pink Ice) and a late (Orchid Rocket) flowering variety of Antirrhinum majus was investigated. At various times during development plants of both varieties were subjected to 1, 2, 3 or 4 long-day cycles. The early variety became increasingly more responsive to long-days while the late variety showed no positive response throughout the course of the experiment. One hour light-breaks given midway through the 16-h dark period evoked a decrease in the leaf number of Pink Ice plants. The leaf areas of both varieties increased to levels equivalent to long-day grown plants. The net CO2 uptake of plants grown in light-breaks increased in proportion to the increase in leaf area. However the specific uptake of CO2 per unit area was similar for light-break and short-day grown plants but was substantially lower than the long-day plants. Suggestions are made to explain the differences in photoperiodic response of the two varieties in terms of variation in assimilation rate.