Pathway of phloem loading in the C3 tropical orchid hybrid Oncidium Goldiana

Abstract

The apoplast of mature leaves of the tropical orchid Oncidium Goldiana was perfused with 0.5 mM p-chloromercuribenzenesulphonic acid (PCMBS) via the transpiration stream in order to test the mode of phloem loading. The efficacy of introducing PCMBS by perfusion was shown by saffranin O dye movement in the veins and leaf apoplast in control experiments. Photoassimilate export as the result of phloem loading was measured by collection of 14CO2-derived photoassimilates from the basal cut-ends of intact leaves. Phloem loading and translocation of photoassimilates was inhibited by 89% in leaves perfused with PCMBS for 1 h. The effect of PCMBS on leaf photosynthesis was minimal. The amount of radiocarbon fixed by PCMBS- treated leaves averaged 89% of control leaves perfused with distilled water. A negative correlation between the total amount of photoassimilate exuded and the calculated concentration of PCMBS in the leaf apoplast was also observed. The results indicate that phloem loading in Oncidium Goldiana occurs via the apoplastic pathway.