LEAF UNFOLDING RATE IN BEGONIA-XHIEMALIS

The rate of leaf unfolding as a function of temperature was determined for Begonia x hiemalis Fotsch under long-day (16 hours of light) conditions before flower initiation. Irradiance was maintained at 280 +/- 20-mu-mol.m-2.s-1 (16.1 mol.m-2.day-1). The two cultivars Hilda and Ballet had similar rates of leaf unfolding in the range from 13 to 28C. The rate increased to a maximum of 0.116 leaves/day at 21C and then decreased at higher temperature. The following quadratic function (where T is the temperature in degrees-C) was selected to describe initial long-day leaf unfolding rate in B. xhiemalis: leaves/day = -0.2083 + 0.03145 x T -0.0007631 x T2, (r2 = 0.97). The leaf unfolding response to temperature varied for plants of 'Hilda' and 'Ballet' during short days (10 hours of light) following the initial long-day period. Plants of 'Ballet' continued to unfold leaves at a similar rate as under initial long photoperiods, while the leaf unfolding rate for 'Hilda' decreased to half the rate observed under long days.