Seasonal Variation in Vegetative and Reproductive Development in Zinnia elegans Jacq.

Abstract

Cultivars of Zinnia elegans Jacq. were grown under long day (LD) photoperiods in the greenhouse in spring (Sp), summer (Su), fall (F), and winter (W) over 2 years to determine days from seed sowing to flowering (DTF), flower diameter (FD), height (H), node number below first inflorescence (NN), and the role of irradiance and temperature on DTF. All cultivars flowered earliest in Su, and latest in W. Cactus-flowered (cactus) cultivars (‘Carved Ivory’, ‘Rosy Future’, ‘Torch’) flowered later and were greater in H, FD, and NN compared to pompon cultivars (‘Cherry Ruffles’, ‘Pink Ruffles’, ‘Scarlet Ruffles’, ‘Yellow Ruffles’). The principal factor affecting DTF and NN was the season, whereas H and FD variation were due mainly to cultivar differences. Year X season interactions were significant for DTF, H, FD, and NN, but seasonal trends each year were similar. Mean daily temperature (MDT) from transplanting until flowering (DTFT) was lowest in W, highest in Su, and showed a linear correlation with DTFT for cactus (r = −0.89) and pompon (r = −0.92) cultivars. Mean daily photosynthetic photon flux (PPF) for DTFT was highest in Sp, followed by Su, F, and W due to greenhouse shading during Su and part of F; the relation between PPF and DTFT was described by a 2nd degree polynomial for cactus (r = −0.96) and pompon (r = −0.98) cultivars. DTFT was more a function of MDT than PPF for Sp, Su, and F crops, whereas late flowering of the W crop was due to lower MDT and PPF. Results demonstrate that Z. elegans can be accurately scheduled as a greenhouse crop on a year-round basis.