THE EFFECTS OF TEMPERATURE, PHOTOPERIOD, AND PHOTOSYNTHETIC PHOTON FLUX ON THE TIME TO FLOWERING OF PETUNIA EXPRESS BLUSH PINK

The effects of temperature and sowing date on the time to first flower ing were investigated in Petunia xhybrida Vilm 'Express Blush Pink' so wn on three separate dates (8 Feb., 1 Mar. and 22 Mar. 1993) and grown in glasshouse compartments set to provide six air temperature regimes (minimum temperatures of 4, 10, 14, 18, 22, and 26 degrees C). Flower ing was hastened under high temperatures and sowing later in the seaso n (22 Mar.). To determine the extent to which this seasonal effect was due to photoperiod, a second experiment was conducted where plants we re grown under controlled daylengths (8, 11, 14, and 17 h.d(-1)) withi n six temperature-controlled glasshouse compartments (set to provide m inimum temperatures of 6, 10, 14, 18, 22, and 26 degrees C). The rate of progress to first flowering increased linearly with lengthening pho toperiod up to a critical photoperiod of 14.4 h.d(-1), while further i ncreases in daylength had no further affect in hastening flowering. Th e rate of progress to flowering increased linearly with increasing tem perature, however, the optimum temperature, at which the rate of progr ess to flowering was maximal, was lower under short days compared to l ong days. Furthermore, the rate of progress to flowering increased lin early with increasing photosynthetic photon flux (PPF). Data from both experiments were analyzed to construct a model to predict the effects of temperature, photoperiod, and PPF on time of flowering in petunia. This model accurately (r(2) = 0.88) predicted the flowering times of a different set of plants sown on three dates and grown under six temp erature regimes (6, 10, 14, 18, 22, and 26 degrees C).