Flower formation as studied by grafting

Graft combinations were made between flowering (donor) and non-flowering (receptor) plants, and grown in a photoperiod unfavourable for flowering of the receptor.

Donor leaves of the short-day (SD) plant Perilla crispa caused a maximum flowering response after exposure to 30 SD. By the use of radioactive sucrose it was shown that a functional phloem connection between donor and receptor was necessary for transmission of the floral stimulus. Induced Perilla leaves transferred to LD had not lost their capacity to induce flower formation after 3 months. But leaves taken from flowering receptor branches did not function as donors. Thus, photoperiodic induction of Perilla was permanently conserved in the SD-treated leaves. This was in contrast to Xanthium (SD plant) in which indirectly induced shoots could themselves transfer the floral stimulus to receptors.

Transmission of the floral stimulus was also demonstrated between: Kalanchoë (SD plant) and Sedum (LD plant), and between Maryland Mammoth tobacco (SD plant) and Nicotiana sylvestris (LD plant). In either case, the LD plant as well as the SD plant could function as donor for the other partner. A day-neutral tobacco variety did not cause flower formation in Maryland Mammoth tobacco, but when grafted as interstock between N. sylvestris and Maryland Mammoth tobacco it permitted transfer of the flowering stimulus. It was concluded that in graft-compatible species the floral stimuli of SD and LD plants seemed to be identical.