Effect of translocation-hindering procedures on source leaf photosynthesis in cucumber

Three treatments which altered translocation rate were applied to cucumber plants: Girdling of source leaf petiole; removal of all aerial sinks; removal of all source leaves except one. Two different effects were observed, one short-term (during the initial 6 hours), and one long-term (detected after several days).

The short-term effect was observed exclusively in girdled leaves and involved a reduction in ¹⁴CO₂ fixation rate paralleled by an increase in stomatal resistance. The effects were maximal after 3 hours with subsequent recovery. Stomatal closure apparently resulted from the 5 to 10% water deficit temporarily detected in girdled leaves which probably induced the observed temporary increases in abscisic acid content. Kinetin counteracted the effects of girdling.

The long-term effect was detected 3 days after girdling and 3 to 5 days after sink manipulation. An increase or decrease in ¹⁴CO₂ fixation rate was observed when the sink-source ratio was increased or decreased respectively, accompanied by a respective decrease or increase in starch content. Changes in the relative amount of ¹⁴CO₂ incorporated into various photosynthetic products were also observed. Stomatal closure was not involved, and the decrease in CO₂ fixation was not counteracted by kinetin.