The physiological behavior of PSII measured by chlorophyll a fluorescence explains stress responses; wonders if it can differentiate plants from different populations. For this purpose, acclimated young plants of two C. pachystachya populations were cultivated from seeds. Chlorophyll-a fluorescence was measured after fertilization and [CO(2)](e). In the first 48 h after fertilization there was a reduction in the maximum quantum yield of PSII, while the means obtained under [CO(2)](e) were significantly higher than in other treatments (0.8 and 0.81). The variable PI best expressed the different conditions tested. Compared to their respective controls, the reduction of DIo/CS was 35.89 % in population (P) and 41.89 % in population (I), while the polyphasic fluorescence kinetics differed between treatments, but not necessarily between populations, except for post-fertilization at I-P steps. The analysis of kinetics between Fo and Fj (Wt) showed no K band during the O-J phase. The interferences found in PSII reinforces the idea of reversible damage to PSII. This effect is directly related to the reduced electron transport rate and increased non-photochemical dissipation and may be similar to those observed under field conditions after planting; adjustment time depends, among other factors, on the genetic potential of the species.