Carbohydrate Accumulation In Cucurbit Rootstock Hypocotyls Correlates with Grafting Success

Watermelon grafting is an important part of watermelon production in areas where land rotation is not possible, to avoid soil borne diseases and/or to avoid chemical fumigation. Graft survival depends on maintaining at least one rootstock cotyledon during the healing period following grafting. One purpose of the cotyledon is to supply necessary resources for the developing seedling by fixing carbon at twice the rate of a normal leaf. If the cotyledon is removed during grafting, the rootstock hypocotyl begins to die. The rootstock hypocotyl continues to senesces until death which suggests that insufficient reserves were available for the hypocotyl before the newly grafted vegetative tissue could supply needed resources.  The objective of this study was to determine whether carbohydrate accumulation in hypocotyls of rootstock and scion material directly correlate with grafting success. Three different rootstocks were used: Lagenaria siceraria cv. Emphasis, Citrulus lanatus var. citroides cv. Ojakkyo, and Cucurbita mochata x Cucurbita maxima cv. Strongtosa. Scion material was the same for all rootstocks (Citrulus lanatus var. lanatus cv. Tri X 313). Rootstock and scion material were allowed to develop to the appearance of the 1^st, 2^nd and 3^rd leaf consecutively at which time grafting was done at each developmental stage. That same time, plant samples were taken of the rootstock hypocotyls and scion material for carbohydrate analysis in a destructive manner. After seven days plants were evaluated for grafting success and plants were again analyzed for total carbohydrates in the same procedure as before.  Carbohydrate extraction and analysis were performed using a methanol:chloroform:water and phenol-sulfuric acid assay.  Grafting success increased with additional rootstock and scion development. Total carbohydrates were positively correlated with plant survivability. Carbohydrates in the scion material and rootstock hypocotyl increased significantly with each developmental stage. Therefore, grafting should be performed at the appearance of the 2^nd or 3^rd developmental stage to maximize grafting efficiency and carbohydrate load.