REGULATION OF ASSIMILATE PARTITIONING IN FLOWERING PLANTS OF THE MONOPODIAL ORCHID ARANDA-NOORAH-ALSAGOFF

Radiocarbon dosing of leaves and source-sink manipulation treatments were used to clarify how assimilate partitioning is regulated in flowering plants of a monopodial Aranda orchid. The inflorescence was the dominant sink for all test leaves that were selected to determine patterns of C-14-assimilate partitioning. Defoliation treatments had only minor effects on the distribution of C-14-assimilate between the inflorescence and vegetative apical shoot. Excision of the inflorescence resulted in greater proportions of C-14-assimilate being sent to all other sinks. Loss of the vegetative apical shoot had a quite different effect in that greater proportions of C-14-assimilate were exported only to the inflorescence. Rates of C-14 assimilation of remaining leaves were often raised in defoliated plants although this effect was not consistent for a single, residual leaf unless the vegetative apical shoot was also excised. Our results highlight the complexity of source-sink relationships in indeterminate plant types showing simultaneous vegetative and reproductive growth. We suggest that inflorescence growth in monopodial orchids such as Aranda is primarily source-limited although significant sink limitations for assimilate gain by the inflorescence exist because of a modulating effect of the vegetative apical shoot on inflorescence sink strength and the ability of source leaves to respond positively to increased sink demand.