Remotely sensed passive chlorophyll fluorescence emission has a potential to become one of the major global-scale reporter signals on vegetation performance and stress. In contrast to the actively probed parameters such as maximal (F_M′) or minimal (F₀′) emission, the steady-state chlorophyll fluorescence, Chl-F_S, (F_M′ > Chl-F_S > F₀′) has not been adequately studied. Using fluorescence imaging of leaves, we explored the modulation of Chl-F_S by actinic irradiance and by temperature in laboratory, as well as the changes that occurred in three coniferous and broadleaf plant species grown in field. The experiments revealed that Chl-F_S is largely insensitive to the incident irradiance once this is above early morning or late evening levels. The characteristic, pre-noon measured Chl-F_S correlated positively with the CO₂ assimilation rate when measured in field during the year. It was low and stable in the cold winter months and steeply increased with the spring onset. The high values of the characteristic Chl-F_S persisted throughout the vegetation season and rapidly decreased in the fall. The seasonal Chl-F_S transitions coincided with the last spring frosts or the first fall frosts that persisted for several consecutive nights. The transitions were marked by an elevated variability of the Chl-F_S signal. We propose that the signal variability occurring during the transition periods can be used to detect from satellites the beginning and the end of the photosynthetic activity in evergreen canopies of the temperate zone.