Agriculture Reference
In-Depth Information
Extensive research into mechanisms of drought and salinity tolerance has been
conducted by plant physiologists for more than 60 years. Recently, geneticists and
plant breeders have begun to screen for traits and genes conditioning resistance to
these stress factors. Melon and watermelon are very important vegetable crops in
warm climates, often produced in regions with both drought and salinity problems.
Screening watermelon germplasm by breeders in Turkey, Japan and the USA has
revealed genetic variation for drought stress resistance. Wild species of
Citrullus
demonstrated enhanced drought tolerance and produced elevated levels of citrul-
line. The investigators were able to identify a gene which codes for an enzyme of
the deacetylase/carboxypeptidase family, involved in producing free citrulline (Ka-
wasaki et al.
2000
). Seedling drought stress screening revealed 25
Citrullus lanatus
accessions from Africa with high tolerance amongst 1066 germplasm and breeding
lines assayed (Zhang et al.
2011
). Many of these accessions were from Zimbabwe,
with an equal split between domestic (var lanatus) and wild (var citroides) types.
These have potential for use as breeding parents to create both cultivars and root-
stock lines with enhanced drought tolerance. The author has investigated root vigor
and morphology in crosses between cultivated ('Crimson Sweet' and 'Dixie Lee')
and wild (var. citroides) watermelon. Extreme heterosis for root length, area, diam-
eter and lateral numbers was observed. Subsequent field trials revealed enhanced
tolerance to both drought and vine decline due to
Monosporascus
root rot (Crosby
2000
). These lines are currently the focus of rootstock trials in Texas. In Turkey, 85
watermelon accessions were screened for drought tolerance in field experiments
with deficit irrigation. Over a third demonstrated drought tolerance based on 9 trait
measurements, with one accession rating 99 on a scale of 100 (Karipcin et al.
2008
).
These drought tolerant genotypes are serving as the basis for development of new
watermelon cultivars adapted to deficit irrigation and periodic drought conditions.
The shift to smaller fruit size in commercial watermelon markets will complement
enhanced drought tolerance traits due to reduced water demand at fruit maturity.
Onion is another major vegetable crop, produced and consumed on a year round
basis. Resistance to drought in onion has been documented since ancient times. This
crop evolved in semi-arid regions of central Asia. However, breeding has tradition-
ally focused on increasing bulb size, and thus water content of the crop, under ir-
rigated production systems. Survival of onions under deficit or dryland production
systems has been demonstrated, but quality and yield are typically sacrificed. The
shallow root system of onions suggests that the mechanism for drought tolerance
may relate to leaf structure and transpiration more than root absorption capacity
(Levy et al.
1981
). Specific candidate genes involved in drought tolerance were
isolated from leaf tissue of onions based on their homologies to
Arabidopsis
genes
(Kutty et al.
2012
). Protein candidates based on the sequences included Aquaporin
and Calcium-dependant protein kinase. Thus, internal flow of water in onion leaf
tissues and modulation of ABA activity may reduce water loss or limit oxidative
stress under drought conditions. Selection of genotypes with enhanced expression
of such genes could lead to more drought tolerant onion cultivars. Another approach
would be to select onion genotypes with enhanced root area and vigor. This has been
