Salt stress is one of the major constrains that limits crop productivity worldwide
By Ashwani Kumar
| September 16th 2009 06:24 PM | Print
Salt stress is one of the major constrains that limits crop productivity worldwide ( Boyer, 1982, Queseda and Ponce 2000 ) . Agricultural productivity in many aird and semi-arid regions of the world is threatened by the occurrence of salt affected soils. Estimates show that these soils occupy at least 20 percent of the irrigated land with an annual global income loss of about US $ 12 billion ( Ghassemi et al 1995). An important category of salt affected soils, sodic and saline-sodic soils show structural problems created by certain physical processes ( slacking, swelling and dispersion of clay ) and specific conditions ( surface crusting and hardsetting) ( Quadir and Schubert, 2002) Such problems may affect water and air movements, plant available water holding capacity, root penetration, seedling emergence, runoff and erosion as well as tillage and sowing operations in sodic and saline sodic soils ( Oster and Jayawardane, 1998). In addition, osmotic and specific ion effects together with imbalances in plant available nutrients in such soils affect plant growth.
Arid and semi-arid climates which consist of 60 percent of the salt affected area (Tanji 1990)
covers early 955 x 106 ha ( abolcs, 1994, Quadir and Schubert, 2002). Such regions have sodic soils which are characterized by excess of Na + to levels that can adversely affect soil structure and crop growth. Deterioration in sodic soils occurs through changes in the proportion of soil solution and exchangeable ions and soil reaction (pH) that lead to osmotic and specific ion effects together with imbalances in plant nutrition, which may range from deficiencies of several nutrients to high levels of Na+ ( Mengel and Kirkby, 2001).
In most saline enviroments NaCl is the predominant salt species which inhibits cell division and expansion. Work on the components involved in relative tolerance has identified several proteins that determine end points of physiological responses ( Hasegawa, et al 2000). Studies with transgenic plant supports the notion that altered gene expression can lead to improvements in tolerance ( van Camp et al 1996, Apse et al 1999) .
In addition components of abiotic stress signal recongnition and transduction pathways and in part the transcription activators now have become known ( Hasegawa et al 2000, Mizoguchi et al 2000 , Zhu 2001 ). These components initiate and control the expression of downstream biochemical reactions by, for example the action of Calcium sensor on a protein kinase that affects the acitivity of an Na+/ H+ antiporter ( Halfter et al 2000, Mizoguchi et al 2000, Zhu, 2001).
Maize : Maize is a salt sensitive crop. The response of Maize to salinity varies depending on the stage of development ( Fortmeier and Schubert, 1995) . Vegetative growth appears to be most sensitive to salinity, while plants are much less affected at later stages ( Cramer, 1994)
Early proteins associated with response to salinity:
Increased protein synthesis and protein turnover in salt stressed rice was recorded at early time points, followed by the induction of known stress responsive transcripts within hours and the induction of transcript for defence related functions later.
" Instantaneous response " cluster ( 15 min) contained only a few transcripts. Rice GRP isoforms GRP-I and GRP-II showed 60 percent amino acid sequence identitiy to maize Zm GRP4 in overlapping regions. ZmGRP4 has been localised in root cap region, but ist function is unknown ( Matsuyama, et al 1999). One theory is that this class of transcripts might function as an immediate defense to stress for e.g. by strengthening cell walls.
" Eearly response " cluster ( 1hr ) is examplified by sequence hologous with a Calcium dependent protein kinase (CDPK). This transcript shows 100 percent identitiy to rice CDPK 7 to Arabidopsis AT CDPK 1 ( 71 percent identitiy ). These CDPK transcripts are reported as salt stress and low temp induced transcripts ( Urao et al 1994, Berverich and Kusano, 1997)
Upregulated sequences at 3 and 6 hr of salt stress constitute "early recovery". Several of these transcripts have been identified by two dimensional electrophoresis as salt stress and growth regulator induced proteins ( Abscisic acid and jasmonic acid) Moons et al 1995, Moons at 1997) . One of the transcript is rice 40 kD protein. Osr 40 c 1, the induction of which is related to endogenous or exogenous applied abscisic acid. Osr 40 c 1 is suggested to perform a structural role in preventing water loss and preserving the rigidity of cell walls ( Moons et al, 1997). Some of the transcripts 167 and 1190 both of which are upregulated are identical to rice S-adenosylmethionine decarboxylase ( SAMDC Z ; AJ 251 899 ) . Transcript 1342 is identical to rice Beta glucosidase ( U 28047 ). Function of upregulated SAMDC could be polyamine synthesis in which enzyme catalyzes a rate limiting step. Polyamines play a very important role in plant development, are known to increase under abiotic stress. In rice seedling polyamines increases have been reported as early as 6 hr after salt stress. ( Basu and Maitra, 1988). Again within 3 to 6 hr time frame, upregulation of protease inhibitors ( trypsin inhibitor and subtilisin-chymotrypsin inhibitor ) was observed. These were inducible by stress such as high aluminium, fungal infection and wounding ( Richards et al 1998 ) . With the same kinetics B glucosidases were found that catalyse the hydrolysis of 1,3 Beta D glucosidic linkage in 1,3, Beta D glucans.
Firstly they are involved in alteration of specific B linked polysaccharides during cell expansion in development ( Laeh et al 1995). Second they are involved in pathogen defence reactions by cyanogenesis wherein enzyme catalyses the hydrolysis of glucosides after pathogen attack( Hugh et al 1992). Third Beta glucosidases could release active cytokinins, gibberellins and auxin from biologically inactive hormone glucoside conjugates. .
A major region is that traits that realize resistance are inherited in a multigenic way. Thus, although resistant plants do exist ( eg wild plants) the combination of resistance with high yielding potential still awaits accomplishment: However Schubert (1999) reported physiology based approach to improve salt resistance of maize. According to this concept salt resistance in maize is subdivided into two attributes, namely drought resistance and Na+ exclusion. The later has been studied in detail and has led to an efficiently Na + excluding inbred line. It is suggested that combination of Na+ exclustion and drought resistance will significantly improve salt resistance of Maize.
Sven Schubert ( 1999)
Anpassung von Mais ( Zea mays L. ) an Bodensalinitat strategien und konzepte Stoffumsatz im wurzelnahen Raum. 9. Borkheider Seminar zur Okophysiologie des Wurzelraums. Hrsg: W. Merbach, L. Wittenhayer und J. Augustin: B G: Teubner Stutgart. Leipizg 1999, S 74-79.
196. Shekhawat, V.P.S., Kumar, A., and K.H. Neumann. (2006). Effect of NaCl salinity on growth and ion accumulation in some chenopodiaceous halophytes. Communication in Soil Science and Plant analysis 13-14 (37), 1933 – 1946