Pattern of photoassimilate partitioning in pseudobulbous and rhizomatous terrestrial orchids

Abstract

Photoassimilate partitioning patterns in two terrestrial orchids, Spathoglottis unguiculata and Bromheadia finlaysoniana, were mapped using a 14CO2 dosing technique. A highly integrated source-sink photoassimilate partitioning pattern was observed in both orchids. In S. unguiculata, a pseudobulbous terrestrial orchid, all test leaves supplied similar percentages of 14C-assimilates to all plant parts on a single shoot. In both orchids, the inflorescence during the reproductive stage and the axillary bud during the vegetative stage had the highest sink activity and percentage distribution of 14C-assimilates. A high percentage of 14C-assimilates was imported by the pseudobulb of S. unguiculata. The pseudobulb accumulated a higher percentage of 14C-assimilates (44%) during the vegetative stage than during the flowering stage (21-30%). In B. finlaysoniana, a rhizomatous terrestrial orchid, a high percentage of 14C-photoassimilates was observed in the stem internodes at all three developmental growth stages (the vegetative stage (stage 1), the flowering stage (stage 2) and the fruiting stage (stage 4)). Sink activities of the rhizome were higher at stages 1 and 4 than at stage 2. A similar pattern of photoassimilate partitioning was observed for B. finlaysoniana grown naturally in its habitat. A polar movement of 14C-assimilates towards the major sink (inflorescence) was observed in the current shoot of field plants with a competing sink (axillary bud).