Wild species and landraces of Cucurbita are important resources for plant breeders as they often contain novel genes for economically important traits. Utilizing such germplasm presents challenges to breeders in temperate climates because wild species and landraces from tropical and subtropical regions may flower late in development or only under short days. Although previous researchers have induced flowering in recalcitrant flowering cucurbit cultigens by grafting them to early flowering rootstocks, details of methodology and floral development are scant, and there has been no confirmation of fruit set and seed production from these efforts. We sought to develop a grafting method that would reliably induce flowering and fruit set in a short-day flowering landrace of C. moschata and the short-day flowering species, C. ficifolia. Initial experiments revealed leaf removal from the scion is necessary, presumably to prevent synthesis of an inhibitor molecule. Grafts performed at the one and two leaf stage of rootstocks failed to induce flowering of scions. Scion insertion into rootstock by a cleft graft at or above the 4th node together with leaf removal from the main stem of the scion for 20 nodes was effective in causing floral induction. Vigorous shoot development in a lateral branch of the rootstock between nodes 0-4 also proved to be necessary as a source of photosynthate for the scion. In a greenhouse study, once floral induction occurred, flowering continued even with renewed leaf growth along the scion main stem; however, flowers aborted prior to fruit set. Without leaf removal, flower buds were initiated at early nodes on scions, but aborted early in development. In field studies conducted in 2018, flowering was again induced with grafting and leaf removal for 20 nodes, and in the C. moschata accession, fruit set and growth was obtained. Self-grafted plants with and without leaf removal, un-grafted plants with and without leaf removal, and grafted plants without leaf removal all failed to flower. This method should be a useful tool for plant breeders and curators of Cucurbita germplasm.
