Rootstock Affected In- and Poststorage Performance of Grafted Watermelon Seedlings at Low Temperature

Low-temperature storage is a technique to hold seedlings for a short period of time to adjust the production schedule of young seedlings. Labor-intensive grafting propagation can potentially benefit from the effective use of this technique to minimize peak labor inputs. Watermelon (Citrullus lanatus) seedlings are generally chilling sensitive and therefore difficult to store at low temperatures. However, the rootstocks used for watermelon grafting, interspecific squash (Cucurbita maxima x Cucurbita moschata) and bottle gourd (Lagenaria siceraria) are known to be chilling tolerant. To examine the influence of rootstocks on storability of watermelon seedlings, young seedlings of 'Tri-X-313' seedless watermelon grafted onto 'Strong Tosa' interspecific squash, 'Emphasis' bottle gourd, and 'Tri-X-313' watermelon as rootstock were placed for 2 or 4 weeks under 12 degrees C air temperature and 12 mu mol center dot m(-2)center dot s (-1) photosynthetic photon flux (PPF). Nongrafted watermelon seedlings were also treated in these same conditions. In addition, nonstored (grafted and nongrafted) seedlings were prepared for comparison. Regardless of seedling type (nongrafted or grafted with different rootstocks), all seedlings stored for 2 weeks had lower dry weight, comparable or greater number of leaves and stem length, when compared with their respective nonstored control groups after 2 weeks in the greenhouse. Seedlings stored for 4 weeks had lower number of leaves and stem length after 2 weeks in the greenhouse, except for those grafted onto the interspecific squash rootstock. Nongrafted and grafted watermelon seedlings with the same watermelon cultivar as rootstock showed significantly lower leaf net photosynthetic rates after 2 weeks in the greenhouse after the 2-week storage than those of nonstored control groups. In contrast, when grafted onto interspecific squash and bottle gourd rootstocks, seedlings showed comparable net photosynthetic rate to the control group. For all seedling types, 20% to 35% of seedlings died during 4-week storage or poststorage in the greenhouse, whereas all seedlings survived for the 2-week storage, except when grafted onto watermelon as rootstock. Therefore, chilling-tolerant rootstocks 'Strong Tosa' interspecific squash and ' Emphasis' bottle gourd improved storability of grafted 'Tri-X-313' watermelon seedlings but could not extend the storability beyond 2 weeks.