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

Radiocarbon dosing of leaves and source-sink manipulation treatments w ere used to clarify how assimilate partitioning is regulated in flower ing plants of a monopodial Aranda orchid. The inflorescence was the do minant sink for all test leaves that were selected to determine patter ns of C-14-assimilate partitioning. Defoliation treatments had only mi nor effects on the distribution of C-14-assimilate between the inflore scence and vegetative apical shoot. Excision of the inflorescence resu lted in greater proportions of C-14-assimilate being sent to all other sinks. Loss of the vegetative apical shoot had a quite different effe ct in that greater proportions of C-14-assimilate were exported only t o the inflorescence. Rates of C-14 assimilation of remaining leaves we re often raised in defoliated plants although this effect was not cons istent 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 vegeta tive 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 o n inflorescence sink strength and the ability of source leaves to resp ond positively to increased sink demand.