Vegetable seedlings often suffer transient water stress after transplanting. This so-called transplant shock is caused by the imbalance between water uptake and transpiration. In newly transplanted seedlings, water uptake is reduced because of root injury during transplanting (Kramer, 1983) and disturbed root–soil contact (Burdett, 1990). In contrast to roots, shoots are relatively undamaged, maintaining high transpiration capacity. Moreover, upon transplanting, plants are exposed to direct sunlight, wind, and temperature extremes, which increase crop evapotranspiration. Successful field establishment depends on how quickly plants can recover water uptake capacity to support transpiration demand for normal growth.
Water stress increases accumulation of ABA in leaves (Davies and Jones, 1991). It is well documented that ABA acts as a stress signal, which triggers adaptive changes in physiology and morphology of plants (Taiz and Zeiger, 2002). For example, ABA synthesized in roots or mesophyll is transported to guard cells where it promotes stomatal closure by inducing net efflux of potassium ions and thus reducing turgor pressure (Fan et al., 2004; Li et al., 2006; Schroeder et al., 2001). It is also known that ABA is involved in inhibition of leaf growth (Van Volkenburgh, 1999). Several studies reported that restricted leaf expansion was correlated with ABA increases in xylem sap (Ismail et al., 2002; Salah and Tardieu, 1997) or leaves (Alves and Setter, 2000; He and Cramer, 1996; Van Volkenburgh and Davies, 1983). In a study using ABA-deficit mutants, Bacon et al. (1998) demonstrated that ABA is required to mediate pH-regulated cell expansion in dehydrated barley (Hordeum vulgare L.). Whereas stomatal closure has an immediate effect in reducing transpirational water loss, restricted leaf expansion minimizes plant water use by limiting increases in transpirational area.
In addition to these functions in leaves, ABA plays an important regulatory role in root systems. Root growth is usually less inhibited than shoot growth under water deficit conditions (Creelman et al., 1990; Sharp et al., 2004; van der Weele et al., 2000; Watts et al., 1981). In maize (Zea mays L.) seedlings, Saab et al. (1990) proposed that endogenous ABA, which accumulates in root tips at low water potential, is required for the maintenance of primary root elongation. Their approach was to inhibit ABA accumulation using fluridone, an inhibitor of the carotenoid (ABA precursor) biosynthesis pathway, or using a mutant with deficient carotenoid synthesis. Inhibition of ABA accumulation by either method resulted in severe reductions in root elongation at low water potential. This finding was further confirmed in a subsequent study that showed full recovery of root elongation when ABA in the elongation zone was restored to normal levels with exogenous ABA (Sharp, 1994).
The overall effect of ABA can be summarized as an increase in root-to-shoot ratio, which, along with the regulation of stomatal closure, helps plants cope with water stress (Taiz and Zeiger, 2002). Thus, ABA application may reduce transplant shock in vegetable transplants. Berkowitz and Rabin (1988) found that bell pepper (Capsicum annuum L.) seedlings dipped entirely in 1 mm ABA solution had higher stomatal resistance and leaf water potential than untreated seedlings after transplanting. When irrigation was withheld for 15 h after transplanting to impose water stress, the improved water status by ABA resulted in increased field survival and yield. Similar results have been reported by Goreta et al. (2007). In their study, bell pepper seedlings were sprayed with ABA at 2000 mg·L−1 (7.6 mm) and subjected to two cycles of 4-d water withholding in a greenhouse. They suggested that reductions in stomatal conductance (gS) by ABA enabled the maintenance of leaf water potential and prevented increases in electrolyte leakage and leaf abscission. On the other hand, Latimer (1992) reported that root-drench application of ABA at 660 mg·L−1 (2.5 mm) did not affect either transplant growth or field establishment of tomato (Solanum lycopersicum L.) seedlings under optimal irrigation. In maize seedlings, foliar application of 100 μm ABA increased root-to-shoot ratio but stimulated leaf chlorophyll degradation under water deficit conditions (Hejnák and Kykalová, 2009).
The beneficial effects of exogenously applied ABA are not consistently evident in previous greenhouse and field studies. Most of these studies used a single concentration or narrow concentration range of ABA, which may not represent the optimal rate for the tested crop to promote desired responses. In fact, the magnitude of drought-induced increases in endogenous ABA varies among crop species, indicating a crop specific sensitivity to ABA (Davies and Jones, 1991). Furthermore, high-dose applications of ABA tend to have negative side effects such as leaf chlorosis and abscission (Kim and van Iersel, 2011; Waterland et al., 2010b). Therefore, exogenous ABA must be tested over a wide range of concentrations to accurately evaluate its potential as a stress control agent. The objective of this study was to characterize concentration effects of exogenous ABA on alleviating water stress and stimulating post-stress growth of muskmelon seedlings.
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