The Importance of Photoassimilate Contribution from the Current Shoot and Connected Back Shoots to Inflorescence Size in the Thin-Leaved Sympodial Orchid Oncidium Goldiana

Abstract

Inflorescence size of the current shoot of the thin-leaved sympodial orchid hybrid Oncidium Goldiana is dependent on the number of connected back shoot(s). Our study showed that the size of the inflorescence increased progressively from plants having no, one, or two connected back shoot(s). The size of the inflorescence was not affected by more than two back shoots connected to the current shoot. Defoliation experiments demonstrated that the leaves of the current shoot are the main source of photoassimilates for the inflorescence on the current shoot itself while the leaves of the connected back shoots are secondary sources of photoassimilates. Significantly lower dry mass of pseudobulbs for the defoliated current shoot indicated some degree of remobilization of storage reserves for inflorescence production. The relative importance of leaves on the current shoot and connected back shoots as a source of photoassimilates was confirmed using 14CO2 feeding experiments. The test leaf on the current shoot contributed significantly more 14C assimilates (83%) to the inflorescence after a transport time of 57 h. All selected test leaves on back shoots exported 61%-72% of 14C assimilates to the inflorescence of the current shoot. The connected shoots of O Goldiana are physiologically interdependent in terms of carbon economy during the flowering stage as test leaves on all shoots exported 14C assimilates to all plant parts within the connected shoots, including the mature leaves. There was sufficient evidence to indicate a polar movement of 14C assimilates toward the major sink on the current shoot with some bidirectional transfer of 14C assimilates among the different shoots.