The time-course of CO(2) assimilation rate and stomatal conductance to step changes in photosynthetic photon flux density (PPFD) was observed in Chrysanthemum x morifolium Ramat. ;Fiesta'. When PPFD was increased from 200 to 600 micromoles per square meter per second, the rate of photosynthetic CO(2) assimilation showed an initial rapid increase over the first minute followed by a slower increase over the next 12 to 38 minutes, with a faster response in low-light-grown plants. Leaves exposed to small step increases (100 micromoles per square meter per second) reached the new steady-state assimilation rate within a minute. Both stomatal and biochemical limitations played a role during photosynthetic induction, but carboxylation limitations seemed to predominate during the first 5 to 10 minutes. Stomatal control during the slow phase of induction was less important in low-light compared to high-light-grown plants. In response to step decreases in PPFD, photosynthetic rate decreased rapidly and a depression in CO(2) assimilation prior to steady-state was observed. This CO(2) assimilation ;dip' was considerably larger for the large step (400 micromoles per square meter per second) than for the small step. The rapid photosynthetic response seems to be controlled by biochemical processes. High- and low-light-grown plants did not differ in their photosynthetic response to PPFD step decreases.