Storing Seedlings at Low Temperature as a Key Technology to Introduce Vegetable Grafting In North America

Low temperature storage of seedlings is a technique to suppress their growth while preserving their transplant quality, in an attempt to accommodate the concomitant rapid growth of transplant production industries.  Much research has been done for environmental requirements (air temperature, light, and gas) to store vegetable seedlings (e.g., Kubota, 2003). Our research objective is to use this technology to help North American propagators to establish a large capacity of labor intensive vegetable grafting.  While grafted seedlings have been used in hydroponic greenhouse production for increasing yields over years, introduction of the technology to conventional open-field cropping system is still limited.  A barrier preventing wider use of grafting is the small undeveloped propagation capacity relative to potential demand.  We believe that incorporation of short term storage of vegetable seedlings enables distributing necessary labor over time to produce large number of seedlings (>100,000 plants per shipment), while manual grafting processes only 1000 - 2000 grafts per day per worker. However, environmental requirements for grafted seedlings, which often consist of two species with different temperature responses, have not been studied. Some rootstocks for cucurbits are known to add the chilling tolerance to the scions, which may be advantageous in storing grafted seedlings when scion is chilling sensitive and therefore difficult to store. Over two years of repeated experiments, we stored grafted and non-grafted muskmelon seedlings (scion Cucumis melo cv. ‘Olympic Gold’; rootstock Cucurbita maxima × Cucurbita moschata cv.‘Tetsukabuto’)  at 9, 12 or 15^oC under ~100% relative humidity and 12 μmol m^-2 s^-1 (400-700 nm) continuous light up to 4 weeks.  The results showed that grafted ‘Olympic Gold’ seedlings can be stored at 12 ^oC for 4 weeks without affecting marketable visual quality, photosynthetic capacity, post-storage growth or development. Non-grafted seedlings were susceptible to low temperatures of 9 and 12 ^oC, and lowered the visual quality to unmarketable level after 4-week storage.  Storing grafted seedlings at 15 ^oC accumulated dry mass and induced a greater magnitude of elongation during the storage. Use of rootstock tolerate to chilling extended the storability of cantaloupe-type muskmelon when grafted to such a rootstock. However, due to the vigor of rootstock at low temperature, ‘Olympic Gold’ scion of grafted seedlings elongated more than did the non-grafted seedlings.  Although further optimization of storage conditions is needed, low temperature storage was shown as a promising technology to help propagators to produce the larger number of grafts required for open-field transplanting.