Photosynthesis and chlorophyll fluorescence response to low sink demand of tubers and roots in Dahlia pinnata source leaves

Photosynthetic rate (PN) and chlorophyll (Chl) fluorescence induction of source leaves in response to a low sink demand created by girdling the branch (GB) between the root-tuber-system and the leaves were studied in Dahlia pinnata L. cv. Rigolet during the stage of rapid tuber growth in the greenhouse. GB resulted in significantly lower values of PN, stomatal conductance (gs), and transpiration rate (E), but in higher leaf temperature (Tl) compared with those of controls. With exception of maximum quantum yield of photosystem 2 (PS 2) photochemistry (Fv/Fm) and maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PS 2 (Fv/F0), no significant differences were observed in Chl fluorescence parameters between girdled and control leaves on days 1 and 2 after GB, indicating no apparent damage in the photosynthetic apparatus. However, longer girdling duration resulted in higher non-photochemical Chl fluorescence quenching (NPQ), but lower Fv/F0, actual efficiency of energy conversion in PS 2 under steady-state conditions (ΦPS2), and photochemical quenching coefficient (qP) in comparison with controls from 10:00 to 16:00 or 15:00 on days 4 and 5, respectively, indicating reversible injury in the photosynthetic apparatus.