Influence of Different Levels and Timing of Supplemental Irradiation on Pot Chrysanthemum Production

Abstract

Supplemental photosynthetically active radiation (PAR; 77, 148 and 231 μmol s -1 m -2 ) was provided to Chrysanthemum morifolium Ramat ‘Paragon’ during 14 days each of rooting (24 hr daily), long days (LD; 24 hr daily) or short days (SD; 9 hr daily) in a greenhouse. Crop growth efficiency (calculated as the ratio of the observed to potential dry weight gain) was highest under the 77 μmol s -1 m -2 treatment during each stage and decreased markedly at higher irradiances. Relative growth rates were increased by all levels of supplemental PAR at each stage. The increases were proportionately smaller than the increase in total PAR in treatments above 77 μmol s -1 m -2 . Flower number, and flower and vegetative dry weight increased in response to 77 μmol s -1 m -2 supplemental PAR, but increased irradiances had no further effect. Extension of the 77 μmol s -1 m -2 treatment beyond rooting to LD significantly improved all traits over single stage treatments. Further extension into SD, however, was not beneficial.