Determination of the Length of the Vegetative and Pre-floral Stages in the Day-Neutral PlantHelianthus annuusby Chilling Pulses

Abstract

Helianthus annuus seedlings grown in an 18 h day at 28 °C were exposed to one 6 d chilling pulse of 12 °C, at spaced times during the first 21 d from sowing. At 2 d intervals, the terminal buds of 5 plants were dissected to determine leaf number and to score the vegetative or flowering state of the shoot apex. It was found that, while the rate of leaf initiation was reduced equally by each chilling pulse, pulses commencing on days 9 or 12 reduced the total leaf number from 30 to 26, while pulses applied earlier had little effect. This variation is interpreted in terms of the time available for leaf production. The apices of control plants commenced the visible transition to flowering 16 d after sowing. Chilling pulses applied from days 3 or 6 delayed this transition by about 5 d, whereas later pulses caused only a 1⋅5 d delay. In a second experiment, where the chilling was reduced to 2 d duration, it was again found that chilling delayed flowering during the first 8 d and was progressively less effective when applied later. From this variation in temperature sensitivity it is proposed that chilling sunflower plants immediately after sowing delays flowering by extending the vegetative phase of growth and so delaying the attainment of a ‘ripeness to flower’ state that appears to coincide with the expansion of the first pair of leaves. From day 8 onwards processes leading to flowering that are relatively temperature insensitive apparently become dominant in the apex and result in visible signs of flowering 8 d later, although during this transitional stage leaf primordia continue to be initiated on the flanks of the apex.