S. lycopersicum is mesophytic; members of the species are not significantly drought or salt resistant. So while there is variability for drought and/or osmotic stress resistance within S. lycopersicum, this variability is limited (Foolad and Lin 1999; Srinivasa Rao et al. 2000; Fellner and Sawhney 2001); the best sources of resistance for cultivated tomato are from other species in the genus.
S. pennellii and S. chilense are indigenous to arid and semi-arid environments in South America (Rick 1973) (Table 1). S. pennellii is adapted to the coastal cliffs of Peru. This location has very low precipitation, but lots of early morning dew. S. chilense is adapted to the Atacama desert in northern Chile (Maldonado et al. 2003). This is the most arid temperate desert on the planet. S. chilense plants are often found in dry arroyos, with no other vegetation in the area (Rick 1973).
S. pennellii and S. chilense produce small green fruit, and they both have an indeterminate growth habit. S. pennellii has small thick rounded leaves, light green in color. This species also has a very small root to shoot ratio. S. chilense has thin finely divided leaves and a well-developed root system. These morphological differences are well matched to the environments in which they grow. S. pennellii appears adapted to arid environments by virtue of high water-use efficiency (WUE) (Martin and Thorstenson 1988) and an ability of its leaves to take up dew (Rick 1973). S. pennellii increased WUE under water deficit conditions, 3.42 g/kg at 25% field capacity compared with
2.71 g/kg at 100% field capacity, while the WUE for S. lycopersicum did not change in plants grown at these two field capacities 2.22 g/kg (O'Connell et al. 2007). Three major QTL or genetic loci were linked to the water-use phenotype in S. pennellii (Martin et al. 1989).
A second feature of the drought response of S. pennellii is stomatal response (Kebede et al. 1994). Stom-ata close rapidly upon water deficit stress; the detached leaf wilt rate reflects this adaptive response. In cultivated tomato, water is lost the most quickly, 6.3% leaf fresh wt/h, in S. chilense the rate is slower at 4.2% and in S. pennellii the rate is the slowest, 1.2% leaf fresh wt/h (O'Connell et al. 2007). The leaves of S. pennellii perceive the reduction in water availability once the leaf is cut off the plant and immediately close all of the stoma, reducing evaporative water loss. The leaves of L. pennellii have 20 times the amount of epicuticular lipid as cultivated tomato (Fobes et al. 1985). The cuticle of S. pennellii was much thicker than cultivated tomato, 5.5 |im vs. 1.5 |im. This trait also appears to be controlled by multiple genes as most of the F2 individuals had intermediate cuticle dimensions, ranging between the two parental values (Treviño 1997; O'Connell et al. 2007). Together, the stomatal response and the waxy leaves allow S. pennellii to conserve leaf water despite a drying soil.
S. chilense has a very well-developed root system (Sánchez Peña 1999), presumably to explore deep soil layers in arroyos following seasonal rains. Chen and Tabaeizadeh (1992b) observed root growth increase during drought treatments, while leaf area and seedling growth rates were reduced in S. chilense. O'Connell et al. (2007) observed enhanced root development in mature S. chilense plants under non-stress conditions. S. chilense had a longer primary root, and twice the number of secondary roots as cultivated tomato. No differences were observed in distance of the root tip to the youngest secondary root or distance from the root base to oldest secondary root.
Measurements of plant water status during drought episodes also indicated that these two species are more drought resistant than cultivated tomato (Kahn et al. 1993; Sánchez Peña et al. 1995; O'Connell etal. 2007). The time until leaf water potentials dropped to the levels reported at wilt was different for each species. As expected, cultivated tomato usually wilted within 2 to 3 days after water was withheld; equivalent sized S. pennellii plants wilted within 4 to 6 days while S. chilense plants often wilted only after 15 days without water. Inspection of the leaf, osmotic and turgor potentials of these plants during a drought cycle demonstrate that S. chilense increased osmotic potential to much higher values than those observed for cultivated tomato or S. pennellii, -2.37 MPa vs. -1.62 or -1.20 MPa. This increase in osmotic potential maintains the leaf turgor potential, such that the leaves do not appear to wilt as soil water decreases; rather the leaves curl and become slightly brittle (Sánchez Peña 1999).
Other species may provide sources of drought resistance or tolerance; S. pimpinellifolium, has been investigated as a source of drought resistance during seed germination (Foolad et al. 2003). This species is more commonly investigated as a source for salt stress resistance (see Sect. 1.13.3). In this study, polyethylene glycol was used to generate the water deficit conditions during seed germination. Four QTLs were identified to be associated with seed germination drought tolerance, of which two were contributed by the S. pimpinellifolium parent and two by the cultivated tomato parent.
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