DEVELOPING RELATIONSHIPS BETWEEN ENVIRONMENTAL VARIABLES AND STEM ELONGATION IN CHRYSANTHEMUM

The main objective of this research was to model the relationships bet ween the environmental variables and stem elongation in chrysanthemum with the end-goal of producing a model appropriate for use in the dyna mic control of a greenhouse environment. The plants used were Dendrant hema grandiflora cv. 'Spice'. The model developed uses Richards' growt h equation (Richards, 1969) cw its base. Adaptations were made to Rich ards' growth equation to explicitly include the effects of day and nig ht temperature, daily PPF (photosynthetic photon flux), end-of-day red to far-red ratio, and position of the internode on the stem on intern ode elongation. The model fit the observed final length data reasonabl y well (R-2 = 0.89). Sensitivity analyses indicated that increasing da y temperature had a positive effect on internode length while increasi ng night temperature had a negative effect, with night temperature hav ing a considerably larger effect than the effect of day temperature, T he analyses suggests that both high and low end-of-day red to far-red ratios will produce increased lengths and that increasing daily PPF wi ll produce decreased lengths. The analyses also suggests that internod es which develop later on the plant will generally have larger lengths as reflected by the measured data.