The effects of temperature, photoperiod and light integral on the time to flowering of pansy cv. universal violet (Viola x wittrockiana Gams.)

Abstract

The effects of temperature, photoperiod and light integral on the time to first flowering of pansy (Viola x wittrockiana Gams) were investigated. Plants were grown at six temperatures (means between 14.8 and 26.1 °C), combined with four photoperiods (8, 11, 14 and 17 h). The rate of progress to flowering increased linearly with temperature (up to an optimum of 21.7 °C) and with increase in photoperiod (r2 = 0.91, 19 d.f.), the latter indicating that pansies are quantitative long day plants (LDPs). In a second experiment, plants were sown on five dates between July and December 1992 and grown in glasshouse compartments under natural day lengths at six temperatures (means between 9.4 and 26.3 °C). The optimum temperature for time to flowering decreased linearly (from 21.3 °C) with declining light integral from 3.4 MJ m-2 d-1 (total solar radiation). Data from both experiments were used to construct a photothermal model of flowering in pansy. This assumed that the rate of progress to flowering increased as an additive linear function of light integral, temperature and photoperiod. Independent data from plants sown on three dates, and grown at five temperatures (means between 9.8 and 23.6 °C) were used to validate this model which gave a good fit to the data (r2 = 0.88, 15 d.f.). Possible confounding of the effects of photoperiod and light integral are discussed.