Prolonged High Temperature Exposure and Daily Light Integral Impact Growth and Flowering of Five Herbaceous Ornamental Species

Abstract

Flowering of many herbaceous ornamentals is reduced or eliminated under high temperatures. On warm, sunny days, greenhouse growers often cover crops with light-reducing screening materials to reduce air and plant temperature. However, low irradiance can also reduce flowering on many species. To examine the impacts of temperature and irradiance on herbaceous ornamental flowering and to select a model to study high temperature-reduced flowering, Antirrhinum majus L. (snapdragon) `Rocket Rose', Calendula officinalis L. (calendula) `Calypso Orange', Impatiens wallerana Hook.f. (impatiens) `Super Elfin White', Mimulus ×hybridus Hort. ex Siebert & Voss (mimulus) `Mystic Yellow', and Torenia fournieri Linden ex E. Fourn (torenia) `Clown Burgundy' were grown at constant 32 ± 1 °C or 20 ± 1.5 °C under a 16-hour photoperiod with daily light integrals (DLI) of 10.5, 17.5, or 21.8 mol·m-2·d-1. Flower bud number per plant (all flower buds ≥1 mm in length when the first flower opened) of all species was lower at 32 than 20 °C. Reduction in flower bud number per plant at 32 compared to 20 °C varied from 30% (impatiens) to 95% (torenia) under a DLI of 10.5 mol·m-2·d-1. Flower diameter of all species except snapdragon was less at 32 than 20 °C. Decreasing DLI from 21.8 to 10.5 mol·m-2·d-1 decreased flower diameter of all species except snapdragon. Calendula, impatiens, and torenia leaf number below the first flower was greater at 32 than 20 °C, regardless of DLI. Increasing DLI from 10.5 to 17.5 mol·m-2·d-1 increased shoot dry mass gain rate of all species, regardless of temperature. Further increasing DLI from 17.5 to 21.8 mol·m-2·d-1 at 20 °C increased shoot dry mass gain rate of all species except snapdragon and mimulus, indicating that these species may be light saturated below 21.8 mol·m-2·d-1. Under DLIs of 17.5 and 21.8 mol·m-2·d-1 shoot dry mass gain rate was lower at 32 than 20 °C for all species except torenia. Torenia shoot dry mass gain rate was 129 mg·d-1 at 20 °C compared to 252 mg·d-1 at 32 °C under a DLI of 17.5 mol·m-2·d-1. We suggest torenia may be a good model to study the basis for inhibition of flowering under high temperatures as flowering, but not dry mass gain, was reduced at 32 °C.