Date of Award


Document Type


Degree Name

Master of Science (MS)


Plant and Environmental Science

Committee Member

James E Faust

Committee Member

Guido Schnabel

Committee Member

Patrick Gerard


During greenhouse production of poinsettia, flower initiation can be delayed by exposure to supra-optimal temperatures; this phenomenon is termed “heat delay.” Poinsettias that are delayed by high temperatures may mature too late to be shipped in time for the Christmas market. This can lead to a significant loss of revenue on a crop that is considered to be marginally profitable. The increasing global temperatures brought on by climate change are expected to amplify the magnitude of heat delay in susceptible regions and spread this problem to new areas previously unaffected by heat delay. The goal of this thesis was to evaluate the interaction of temperature and night length (NL) on flower initiation as well as differentiate the effect of NL on flower initiation and flower development. In Chapter 1, the heat-sensitive cultivar ‘Prestige Red’ and the heat-tolerant cultivar ‘Orion Red’ were subjected to sixty treatments consisting of three day temperatures, four night temperature, and five NLs for the first 17 d of the experiment; following these 17 d, all plants were consolidated to one greenhouse with an inductive environment. The results from this chapter demonstrate that the poinsettia flowering response to temperature depends on NL; however, in both cultivars, the rate of flowering increased in a sigmoidal pattern as NL increased. High day temperatures affected flowering during the first 17 d when flower initiation took place at a 12-h NL for both cultivars, while high night temperatures uniquely delayed flowering of the heat-sensitive cultivar at NL from 12-14 h. In Chapter 2, the effect of NL on flower development in ‘Prestige Red’ was evaluated by providing NLs from 11 to 14 h following 10 or 17 d under 14-h NLs. Leaf number was not affected by the NL treatments suggesting that flower initiation occurred during the 10 SD prior to the start of the NL treatments; thus, the NL treatments only affected flower development. The results from this chapter demonstrate that minimal differences in flower development occur at NL > 12 h; however, an 11-h NL resulted in significantly fewer plants reaching anthesis, fewer stem bracts, and lower stem bract ratings at all node positions compared to the 12-, 13-, or 14-h NLs. The results from this project demonstrate that a 14-h NL results in rapid flower initiation, an optimal rate of flower development, and optimal bract color development. Furthermore, providing a 14-h NL will not entirely alleviate heat delay in heat-sensitive cultivars, but flowering will occur faster at a 14-h NL than if the plants were receiving natural NL during flower initiation. For this reason, black clothing poinsettias is an effective method for reducing the magnitude of heat delay when high temperatures cannot be avoided. As NL decreases below 14 h, the rate of flower initiation is increasingly delayed, which causes slower overall crop response time. During flower development, NLs between 12 and 14 h have a minimal effect on crop response time, but a 12-h NL will negatively affect color development relative to 13- or 14 h NLs. Night lengthsdevelopment.



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