Agriculture Reference
In-Depth Information
accomplished in Texas by introgression of resistance to the root destroying fungus
Phoma terrestris
, and selection in dry regions for large, vigorous root systems.
Cultivated tomato is probably the most important vegetable crop on a worldwide
basis, and is often grown in arid regions to avoid serious foliar diseases. Breeding
for drought tolerance in tomato (
Solanum lycopersicum
) has had limited success,
possibly due to the extreme sensitivity of the large fruit to water stress. Investiga-
tions into leaf physiology have revealed differences in cell structure between geno-
types with greater drought tolerance and more susceptible ones. Thicker leaves,
longer palisade mesophyll cells and fewer, larger stomata were found in the more
drought tolerant genotypes (Kulkarni and Deshpande
2006
). They described a sim-
ple selection protocol for these leaf traits to breed tomato cultivars with enhanced
drought tolerance. Another investigation of drought responses among different to-
mato species found no differences in stomatal conductance and leaf water potential,
despite diversity in their natural habitats (Easlon and Richards
2009
). Lack of shoot
and leaf response variation, among the five species, for these traits suggests that
other physiological attributes may contribute to drought tolerance. As in melon and
onion, root physiology traits may contribute to differences in water uptake from na-
tive soil under stress conditions.
Breeding for Salt Stress Tolerance
Salt stress is frequently associated with drought conditions, poor water quality and
high pH soils in arid regions. Much progress has been made in breeding agronomic
crops for tolerance to salt in soils and irrigation water. More recently, efforts to
exploit salt tolerance traits in major vegetable crops have produced mixed results.
Inherent salt tolerance is evident in some vegetable species such as asparagus, beets
and melons. Efforts to screen germplasm for salt tolerance have yielded positive
results for some major crops such as tomato, pepper, and cabbage, but not for onion,
carrot and radish (Shannon and Grieve
1999
).
Salt sensitivity varies among species depending on the environmental condi-
tions, source of the salts and irrigation method. As many vegetable crops are being
grown with drip or other limited irrigation systems, salt damage will likely increase.
Flood irrigation helps leach detrimental salts from the soils, but is not sustainable
in most regions where population growth is straining available water supplies. Gen-
erally, tolerance to sodium salts is more important for many vegetable crops, as
calcium and potassium are important nutrients with high threshold levels before
negative impacts occur. Screening germplasm and exploiting salt tolerance traits to
develop novel cultivars has been successful in certain vegetable species. Variation
for salt tolerance in lettuce revealed significant differences among germplasm ac-
cessions (Shannon and McCreight
1984
), and Romaine types showed greater salin-
ity tolerance than iceberg cultivars (Pasternak et al.
1986
).
Salt tolerance in tomato has been investigated for at least 60 years. Wild spe-
cies of tomato, including
Solanum pennellii. S. cheesmanii,
and
S. pimpinellifolium
,
