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Rice Genomics and Genetics 2012, Vol.3, No.7, 39
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41
produced by proton electrochemical gradient in the
system. As a result, plant, under salt-stress situation,
will transform low-affinity K
+
absorption system to
high-affinity system thereby enhancing the plant’s
absorption of K
+
and less accumulation of Na
+
. In this
way, plant will survive longer under salt-stress
condition with an improvement of salt-tolerance (Qi et
al., 2007; Bing et al., 2008; Cheng et al., 2008).
1.3.2 Regulation of small organic molecular
Studies found that in high salt conditions, most plants
began to accumulate some small molecular weight
metabolites in cytoplasm such as proline, lycine, sugar
alcohols and others to maintain high osmotic pressure
within cells, making it possible for plants to absorb
water under high-salt conditions with an increased
stress resistance. As organic osmolytes, they must
have some characteristics as follows: small molecular
weight; soluble in water; without net charges at
physiological pH; maintaining by membrane; little
affection towards the structure of enzyme; quickly
produced and accumulated enough contents to initiate
osmotic regulation. In order to eliminate the
imbalance caused by salt-stress, usually, plant
accumulates two different types of substances to lower
intracellular osmotic potential level and improve its
salt tolerance. One kind of substance is small
molecular organic compounds such as betaine,
mannitol, sorbitol, proline and polyamines while the
other is proteins including rice exogenous gene
BADH,
CMO
, etc. (Shen et al., 2001; Guo et al., 1997).
2 Excavation of rice salt-tolerance germplasm
2.1 Identification methods of salt damage
Under the salt stress, rice plants usually present a
variety of symptom response, such as hairy leaves,
dehydration, no tiller, death, decrease of production,
etc. This adaptability existed among different species,
which presents various sensitivity to salt stress, is the
foundation of salt-tolerance in rice germplasm resources.
There are three methods about identification of salt
tolerance. (1) Salt resistance capacity of rice, which
means that survival skills under the tough salinity-
alkalinity stress, it is evaluated by biomass, vigor, etc.
It can determine the potential and factual value in use
under appointed tough environment, and it can also
provide countermeasure of eco-cybernetics and basis
of make rational use of rice resources. (2) Rice
agricultural salt resistance capacity, which means rice
agriculture productivity under salt stress, or rice yield
under salt stress. It reflects the sensitivity of different
varieties to salt stress. It can also determine the
performance and adaptability of different varieties
under appointed tough environment. (3) The response
of isolated rice cell under salt stress, which is that the
cell suspension cultures and callus of rice are cultivate
under salt stress, and then compare the relative
biomass or livability to determine salt-tolerance (Guo
et al., 2003; Qi et al., 2005).
2.2 Rice salt-tolerant germplasm
The elite salt-tolerant germplasm is the carrier of
genetic analysis of the salt tolerance and the breed of
variety with resistance to salt stress. In the early 1930s,
the breeders had launched to cultivate the salt
tolerance rice variety. For example, the high salt
tolerance native variety Pokkali which was cultivated
in Sri Lanka; the salt tolerance commercial variety
Kala Rata l-24 and hura Rata 4-10 were cultivated in
India, the salt tolerance variety BRI, BR 203-26-2 and
Sail were from Bengal. Since 1970, the International
Rice Research Institute (IRRI) identified 10 salt
tolerant varieties including Pokkali, Getu and Nona
Bokra from 9 000 rice varieties and pedigree, which
provided the technical reserve for the breed of the salt
tolerance variety.
In China, the salt tolerance variety 80-85 was selected
by the Academy of Agricultural Sciences in Jiangsu
Province working with IRRI. The Chinese Academy
of Agricultural Sciences selected 103 salt tolerance
varieties including 27
indica
, 76
japonica
(the salt
tolerance of some varieties were higher than Pokkali)
from the 2808 introduced varieties, among which the
81-210, Nonglin72 and American rice achieved large
area in coastal area of Jiangsu Province. Wu et al
(1989, Jiangsu Agricultural Sciences, (l): 4-5) found
some salt-tolerant germplasm such as Jiucaiqing,
Laohuangdao, Huangjingnuo and Hongmangxiang-
jingnuo from the native
japonica
in Taihu Lake Basin.
China National Rice Research Institute has also
screened out some salt tolerance varieties such as
Mang Rice3, Hair Rice, Big Mang Rice and Sorghum
rice (Table 1).