REFINEMENT OF INNOVATIVE WATERMELON GRAFTING METHODS WITH APPROPRIATE CHOICE OF DEVELOPMENTAL STAGE, ROOTSTOCK TYPE, AND ROOT TREATMENT TO INCREASE GRAFTING SUCCESS (2010)

ABSTRACT Watermelon grafting methods used in Europe and Asia vary, but are based on efficiency, skill and needs. China mainly practices the whole insertion grafting method, whereas, Europe and Japan employ the one cotyledon (splice/slant–cut) grafting method. These methods are not suitable for grafting production in the U.S. due to the labor intensive and high labor cost necessary to successfully produce grafted transplants. This thesis introduced a modified grafting technique called the “Cotyledon Devoid Method” and in three experiments determined; 1) the rootstock leaf number stage (RLNS) at which the greatest grafting success is achieved; 2) the relationship between total soluble carbohydrates in rootstock hypocotyl seedlings and grafting success; and 3) the effects of root excision performed after grafting but prior to healing on grafting success and hypocotyl carbohydrate depletion. Grafting was performed on ten plants in five replications using four different rootstocks: Lagenaria siceraria ‘Emphasis’, Citrullus lanatus var. citroides ‘Ojakkyo’, Cucurbita moschata x Cucurbita maxima ‘Strong Tosa’, and Citrullus lanatus var. lanatus ‘Tri–X 313’. All scion material was Citrullus lanatus var. lanatus ‘Tri–X 313’. Rootstocks and scion material were developed in synchrony to the appearance of the first (9–15 days), second (13–18 days), and third (19–24 days) leaf number stage. Aerial measurements were taken on both the rootstocks and scion material before grafting. Both rootstock cotyledons were removed at time of grafting to eliminate any potential rootstock regeneration. Furthermore, roots were excised from the hypocotyl of one set of grafted seedlings to reduce the need to maintain an active root system during healing which allowed the hypocotyl energy reserves to be conserved to initially heal the graft union and then generate new roots (Excision treatment). Grafts were randomly placed inside a healing chamber for 7 days and evaluated 14 days later for grafting success. The second and third experiments were designed to analyze total soluble carbohydrates accumulated in the rootstock plant tissues before and after grafting at each of the three RLNS with and without roots present. Plants were carefully dissected on the day of grafting and 7 days after grafting to measure individual plant organs including root, hypocotyl, cotyledon, and leaf or scion hypocotyl, scion cotyledon, and scion leaf area. All individual plant organs measurements consisted of ten plants per samples replicated five times. Carbohydrates were extracted using the methanol–chloroform–water method. The carbohydrate concentrations were determined using the phenol sulfuric acid assay and read by the micro plate spectrophotometer. Measured samples for carbohydrate analysis consisted of a subsample taken from ten plants ground samples replicated five times. Each ten–plant sub sample was determined by the mean of two read replications on the micro plate with the coefficient of variation values generally less than 10. Grafting success increased with each increase in RLNS. Aerial dimensions taken before grafting revealed that the rootstock hypocotyl diameter, length, and area increased from the first to the third RLNS and were related to grafting success. Total carbohydrate measurements taken from each rootstock hypocotyl organ before grafting increased from the first to the third RLNS suggesting a relationship between grafting success and hypocotyl carbohydrates. The overall carbohydrate concentration remained the same among RLNS, but the increase in dry weight from the first to the third RLNS accounted for the vast increase in total carbohydrates per hypocotyl and thereby increased grafting success. Rootstock hypocotyl total carbohydrates greatly decreased when roots were left intact versus excised, indicating root excision can be employed to conserve hypocotyl carbohydrate to encourage healing which is also essential for mechanical grafting. Excising the rootstock root prior to healing but after grafting did not decrease grafting success at the second or third RLNS on three of the rootstocks tested. The “Cotyledon Devoid Method” provides a successful option that may have potential to reduce grafting cost by successfully removing rootstock regeneration; however, precise seed germination and seedling development guidelines must be followed in order to achieve acceptable grafting success.