Recovery of chlorophyll fluorescence and CO2 exchange in lichen soil crusts after rehydration

Lichen soil crusts (LSCs) are an important stage during the development of biological soil crusts (BSCs). The photosynthetic characteristics of this stage are determined by both of the symbiotic and free-living cyanobacteria and algae in the crusts. In this study, the recovery rate, extent, pattern of chlorophyll fluorescence and CO2 exchange in LSCs were detected after rehydration under different water contents (5%, 20% and 40%). The results showed that original fluorescence (Fo) of each group increased dramatically after rehydration and then reached a plateau to steady state; however both maximum quantum efficiency of PS II photochemistry (Fv/Fm) and the ratio of variable fluorescence at different recovery time to the maximal value during the recovery period (Fv/FvMax) showed a similar and corresponding “three steps” recovery pattern. At the end of experiment, Fv/Fm of 20% water content group recovered to the highest level (0.45), although no obvious difference in the Fv/Fm was found among the three water content groups within the first 4 h. Additionally, after the rehydration CO2 release from LSCs (under the light) gradually decreased, and the organic carbon accumulation first occurred in 40% water content group, in which a higher CO2 assimilation rate was also observed. The obvious inconsistency between chlorophyll fluorescence detection and CO2 exchange analysis is considered to be the result of the special structure and photosynthetic characteristics of LSCs.

► Photosynthetic activity of lichen crusts was affected by water content.
► After rehydration Fo first increased then reached a plateau to steady state.
► Three-step pattern was found in the recovery of crust photosynthetic activity.
► Results detected by chlorophyll fluorescence and CO2 analysis were inconsistent.