PHYSIOLOGIA PLANTARUM 117: 58–63. 2003 Copyright C Physiologia Plantarum 2003 Printed in Denmark – all rights reserved ISSN 0031-9317 Growth of tomato and an ABA-deficient mutant (sitiens) under saline conditions Pirjo Mäkelä a , Rana Munns b , Timothy D. Colmer c and Pirjo Peltonen-Sainio d a Department of Applied Biology, Crop Production, PO Box 27, 00014 University of Helsinki, Finland b CSIRO Plant Industry, GPO Box 1600, Canberra ACT 2601, Australia c Faculty of Natural & Agricultural Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia d MTT Agrifood Research Finland, Plant Production Research, 31600 Jokioinen, Finland *Corresponding author, e-mail: pirjo.makela/helsinki.fi Received 15 July 2002 To determine whether ABA accumulation inhibits or promotes shoot growth under stress, an ABA-deficient mutant tomato, sitiens, and its wild-type, the cultivar Rheinlands Rhum, were exposed to moderate salinity stress. Plants were grown at 75 mM NaCl for 2 weeks under conditions of moderate or high relative humidity (70% and 95% RH, respectively). At 70% RH, shoot DW and relative growth rate were reduced more in sitiens than in the cultivar, but the major difference between genotypes was in the degree of injury suffered by older leaves. Most leaves of sitiens died after 2 weeks, but those of the Introduction Stress-induced abscisic acid (ABA) accumulation plays a role in the acclimation of plants to soil water deficits by regulating stomatal aperture (Cummins et al. 1971) and the expression of genes involved in protecting cells against dehydration (Chandler and Robertson 1994). However, ABA may also inhibit growth (Quarrie 1991, Trewavas and Jones 1991). Evidence for its inhibitory effect on growth comes from the strong correlations between internal ABA con- centrations and growth rate of plants under stress. The extent of the decreases in leaf expansion rates in water- stressed plants correlates with increases in ABA levels in shoots (Creelman et al. 1990, Zhang and Davies 1990, Creelman and Mullet 1991, Montero et al. 1998). Further, detailed time courses following the onset or re- moval of stress have shown that changes in leaf growth rates coincide with changes in ABA concentrations in the expanding tissues of soybean hypocotyls (Bensen et al. 1988, 1990) and maize leaves (Cramer and Quarrie 2002). Further evidence for an inhibitory effect on growth comes from experiments with applied ABA. Applied Physiol. Plant. 117, 2003 58 cultivar remained alive. When plants were grown at 95% RH, to maximize the leaf water status of both genotypes, there was no significant effect of salt on shoot DW of either genotype. However, there was still considerable leaf death in sitiens whereas no visible injury appeared in the cultivar. Cl – accumu- lated to higher levels in leaf tissues than Na π , but to similar concentrations in both genotypes, and so could not explain the injury in the sitiens leaves. The results indicate that ABA maintains rather than inhibits new growth under stress, and has a major effect on preservation of older leaves. ABA reduces growth in detached shoots (Munns 1992, Dodd and Davies 1996) and intact plants (Bensen et al. 1988, Creelman et al. 1990, He and Cramer 1996, Ben Haj Salah and Tardieu 1997, Cramer et al. 1998). De- tailed time courses have shown that the rate of change in leaf growth rates of plants fed ABA are tightly linked with changes in ABA concentrations measured in the expanding tissues (He and Cramer 1996, Cramer et al. 1998). All these observations suggest that increased ABA in stressed plants is causing the reduction in shoot growth. Studies with ABA-deficient mutants have yielded in- consistent results. It is clear that stress-induced ABA helps to maintain root growth (Saab et al. 1990, Sharp et al. 1994, Spollen et al. 2000). In roots, the increased ABA under water stress is essential for maintaining growth; in ABA-deficient mutants, the root elongation rate is reduced more than in the wild-type (Saab et al. 1990). This beneficial effect of ABA is linked with sup- pression of ethylene synthesis (Spollen et al. 2000, Sharp and LeNoble 2002). In leaves, the picture is not so clear. Shoot elongation at low soil water potential in the early