Quantifying the thermal flowering rates of eighteen species of annual bedding plants

Abstract

The effect of mean daily air temperature (MDT) on flowering rate (the reciprocal of days to flower) was quantified for 18 species of annual bedding plants. Plants were grown in environmental growth chambers at constant air temperature set points of 5, 7.5, 10, 15, 25, or 30 °C and under an irradiance of 160–180 μmol m –2 s –1, with a 16-h photoperiod. Nonlinear mathematical equations were developed to predict the effect of MDT on flowering rate and to estimate the base, optimum, and maximum temperatures ( T min, T opt, and T max), which are the temperatures at which flowering rates are zero (low temperature), maximal, and zero once again (high temperature), respectively. The estimated T min varied among species and ranged from 1.1 °C in French marigold ( Tagetes patula L.) to 9.9 °C in angelonia ( Angelonia angustifolia Benth.). T opt and T max were only observed for 8–10 species with the temperature range tested. T opt ranged from 19.1 °C in dahlia ( Dahlia × hybrida Cav.) to 28.0 °C in blue salvia ( Salvia farinacea Benth.), whereas T max ranged from 30.3 °C in snapdragon ( Antirrhinum majus L.) to 31.7 °C in moss rose ( Portulaca grandiflora Hook.). Angelonia, browallia ( Browallia speciosa Hook.), cosmos ( Cosmos sulphureus Cav.), dahlia, and snapdragon grown at 25 or 30 °C developed a mean of two to seven more nodes before flowering compared with plants grown at ≤15 °C. The results indicate that in many species, flowering rate in response to MDT is asymmetrical around T opt and the temperature range between T min and T opt is wider than that between T opt and T max. This information could be used to improve the predictability of flowering time of these ornamental crops and to assist growers in determining energy-efficient production temperatures.