Direct and interactive effects of daylength and temperature preconditioning of above- and below-ground parts of Chrysanthemum morifolium Ramat. 'Astrid' are examined in relation to net photosynthesis. Most previous work on thermal preconditioning has concentrated on the effects of a given thermal regime on entire plants, without attempts to separate the effects of temperature acting on shoots vs. below-ground organs (roots and rhizomes).Vegetatively propagated cuttings were placed under 2 different photoperiods and 4 different thermal regimes for a total of 8 treatments. Subsequent to 40-day preconditioning regimes, the temperature-dependence of net photosynthesis was determined between 8 and 40 degrees C under a photon flux density of about 540 ฮผEยทmยทs. The photosynthetic rate data were treated by multiple analysis of variance to examine direct and interactive factor effects.Each of the preconditioning factors (daylength, shoot preconditioning temperature, root preconditioning temperature) individually affected the measured photosynthesis rates, and the effects were very highly significant (p<0.001). In the two-way interactive effects, both daylength X shoot environment, and shoot environment X root environment were very highly significant (p<0.001) and daylength X root environment was significant (p<0.05). Comparison of sums of squares indicated the greatest effect was due to shoot preconditioning environment, while the least important was daylength. Photosynthesis temperature at the time of gas exchange measurements did not interact with the preconditioning temperatures or with daylength, with the implication that preconditioning affected the elevation (absolute rates) of the photosynthesis curves but not the shapes of the curves.The data are further discussed in relation to models of resource allocation and theory of natural selection for optimization.