Regulation of assimilate partitioning in flowering plants of the monopodial orchid Aranda Noorah Alsagoff

summary 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 14 C‐assimilate partitioning. Defoliation treatments had only minor effects on the distribution of 14 C‐assimilate between the inflorescence and vegetative apical shoot. Excision of the inflorescence resulted in greater proportions of l4 C‐assimilate being sent to all other sinks. Loss of the vegetative apical shoot had a quite different effect in that greater proportions of 14 C‐assimilate were exported only to the inflorescence. Rates of 14 C 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.