Vegetable transplant production in high-density plug trays can induce excessive stem elongation as a result of shade avoidance responses (Marr and Jirak, 1990; Smith, 1994). The resulting spindly transplants are generally considered unsuitable for shipping and transplanting, because they are susceptible to damage during these operations (Garner and Björkman, 1996; Shaw, 1993) and to wind damage in the field (Garner and Björkman, 1999; Latimer and Mitchell, 1988). Consequently, their field establishment can be slow and non-uniform, potentially delaying early harvest and limiting marketable yield.
Height control is important for producing compact and high-quality vegetable transplants. Although several gibberellin inhibitors such as daminozide, paclobutrazol, and uniconazole are commercially used to produce compact plants in ornamentals and flowers (Gibson and Whipker, 2001; Whipker et al., 2000), they tend to have long-term growth inhibitory effects (Cantliffe, 1993; Latimer, 1991) and only uniconazole is currently registered for vegetable crops. Furthermore, the approved vegetables are limited mostly to solanaceous crops, including eggplant (Solanum melongena L.), pepper (Capsicum annuum L.), and tomato (Solanum lycopersicum L.). Alternatively, stem elongation can be reduced by mechanical stimulation such as brushing the upper canopy, shaking, and vibration by wind or forced aeration (Baden and Latimer, 1992; Björkman, 1999; Garner and Björkman, 1997). These mechanical conditioning methods inhibit stem elongation by stimulating ethylene production, which in turn inhibits cell elongation and promotes stem thickening (Hiraki and Ota, 1975; Zarembinski and Theologis, 1994). However, their commercial application is limited by high costs of automation and labor (Latimer, 1998).
Abscisic acid can act as a physiological inhibitor of stem elongation in some vegetable transplants, including pepper, eggplant, tomato, and cucumber (Cucumis sativus L.) (Biai et al., 2011; Latimer and Mitchell, 1988; Yamazaki et al., 1995). In contrast to gibberellin inhibitors, ABA can be rapidly inactivated in plant tissues by oxidation or conjugation (Davies and Jones, 1991), suggesting that it may be more suitable for vegetable transplants because of its transient growth-inhibitory effects. The potential of ABA as a height control agent has been evaluated mainly in bell pepper seedlings. For example, Leskovar and Cantliffe (1992) reported that the concentration effect of ABA on stem elongation was quadratic with height suppression occurring above 10 μm. Biai et al. (2011) suggested that the effectiveness of height control by ABA is age-dependent and that ABA application should be initiated at the cotyledon stage. However, this recommendation is based solely on plant height, although other growth components are also known to be affected by ABA (Taiz and Zeiger, 2010). Moreover, high-dose applications of ABA have negative side effects such as leaf chlorosis and abscission (Agehara and Leskovar, 2012; Kim and van Iersel, 2011; Waterland et al., 2010). Therefore, the overall growth modification must be considered to further optimize ABA application methods for height control.
Seedless (triploid) watermelon is generally the most expensive vegetable to produce transplants, mainly because of the high cost of seeds, low seedling vigor (Grange et al., 2003), and extra care required for transplant production (Vavrina, 2002). Nonetheless, this highly valuable crop has been neither approved for the use of uniconazole, the only growth regulator currently available for height control of vegetable transplants, nor tested for ABA responses. The first objective of this study is, therefore, to examine the age-dependent sensitivity of various growth variables to ABA in diploid and triploid watermelon seedlings under greenhouse conditions. This information will be useful to determine the optimal application timing for the most effective height control. The second objective is to evaluate if the advantages of height control in ABA-treated transplants would be translated in improved field performance.
Agehara, S. & Leskovar, D.I. 2012 Characterizing concentration effects of exogenous abscisic acid on gas exchange, water relations, and growth of muskmelon seedlings during water stress and rehydration J. Amer. Soc. Hort. Sci. 137 400 410
Biai, C.J., Garzon, J.G., Osborne, J.A., Schultheis, J.R., Gehl, R.J. & Gunter, C.C. 2011 Height control in three pepper types treated with drench-applied abscisic acid HortScience 46 1265 1269
Björkman, T. 1999 Dose and timing of brushing to control excessive hypocotyl elongation in cucumber transplants HortTechnology 9 224 226
Davies, W.J. & Jones, H.G. 1991 Abscisic acid: Physiology and biochemistry. BIOS Scientific Publishers, Oxford, UK
Garner, L.C. & Björkman, T. 1996 Mechanical conditioning for controlling excessive elongation in tomato transplants: Sensitivity to dose, frequency, and timing of brushing J. Amer. Soc. Hort. Sci. 121 894 900
Garner, L.C. & Björkman, T. 1997 Using impedance for mechanical conditioning of tomato transplants to control excessive stem elongation HortScience 32 227 229
Garner, L.C. & Björkman, T. 1999 Mechanical conditioning of tomato seedlings improves transplant quality without deleterious effects on field performance HortScience 34 848 851
Gibson, J.L. & Whipker, B.E. 2001 Ornamental cabbage and kale growth responses to daminozide, paclobutrazol, and uniconazole HortTechnology 11 226 230
Grange, S., Leskovar, D.I., Pike, L.M. & Cobb, B.G. 2003 Seedcoat structure and oxygen-enhanced environments affect germination of triploid watermelon J. Amer. Soc. Hort. Sci. 128 253 259
Hiraki, Y. & Ota, Y. 1975 The relationship between growth inhibition and ethylene production by mechanical stimulation in lilium longiflorum Plant Cell Physiol. 16 185 189
Hodges, L. 2007 Growing seedless (triploid) watermelons. Univ. Neb.–Linc. Ext. G1755
Kim, J. & van Iersel, M.W. 2011 Abscisic acid drenches can reduce water use and extend shelf life of Salvia splendens Sci. Hort. 127 420 423
Latimer, J.G. & Mitchell, C.A. 1988 Effects of mechanical stress or abscisic acid on growth, water status and leaf abscisic acid content of eggplant seedlings Sci. Hort. 36 37 46
Leskovar, D.I. & Cantliffe, D.J. 1992 Pepper seedling growth response to drought stress and exogenous abscisic acid J. Amer. Soc. Hort. Sci. 117 389 393
Mishra, A., Khare, S., Trivedi, P.K. & Nath, P. 2008 Effect of ethylene, 1-MCP, ABA and IAA on break strength, cellulase and polygalacturonase activities during cotton leaf abscission S. Afr. J. Bot. 74 282 287
Smith, H. 1994 Sensing the light environment: The functions of the phytochrome family, p. 377–416. In: Kendrick, R.E. and G.H.M. Kronenberg (eds.). Photomorphogenesis in plants. Kluwer Academic, Dordrecht, The Netherlands
Taiz, L. & Zeiger, E. 2010 Plant physiology. 5th Ed. Sinauer Assoc., Sunderland, MA
Taylor, J.E., Tucker, G.A., Lasslett, Y., Smith, C.J.S., Arnold, C.M., Watson, C.F., Schuch, W., Grierson, D. & Roberts, J.A. 1991 Polygalacturonase expression during leaf abscission of normal and transgenic tomato plants Planta 183 133 138
U.S. Department of Agriculture 2006 United States standards for grades of watermelons. 26 Aug. 2013. <http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5050334>
Vavrina, C.S. 2002 An introduction to the production of containerized vegetable transplants. Fla. Coop. Ext Serv. HS849
Waterland, N.L., Finer, J.J. & Jones, M.L. 2010 Benzyladenine and gibberellic acid application prevents abscisic acid-induced leaf chlorosis in pansy and viola HortScience 45 925 933
Whipker, B.E., Dasoju, S.K. & Evans, M.R. 2000 Vegetatively propagated geraniums respond similarly to drench applications of paclobutrazol or uniconazole HortTechnology 10 151 153
Yamazaki, H., Nishijima, T. & Koshioka, M. 1995 Effects of (+)-s-abscisic acid on the quality of stored cucumber and tomato seedlings HortScience 30 80 82