The aim of the present investigation was to determine physiological and biochemical attributes of tomato genotypes under salinity stress. The salinity (3 dsm-1, 6 dsm-1 and 9 dsm-1) was applied 28 DAS to green house grown plants.
Growth parameters of tomato genotypes Plant height, Fresh, Dry weight and water content of shoot and root
The present investigation indicated that salinity decreased growth in all tomato genotypes. The % decrease in plant height increased significantly from 6dsm-1 salinity and become maximum at 9 dSm-1. Plant height of tomato is inversely proportional to salt concentration. Tomato genotypes Avinash, NTH-242 and Gol possessed higher plant height in concomitant with higher shoot and root fresh weight at highest salt concentration of Ec 9dSm-1. Genotypes Pant bahar, Lyp No.1 and Nagina showed maximum decrease at higher salt concentration. The commercial genotypes Roma and Checo maintained higher plant height but exhibited lower shoot and root fresh weight which was due to production of less lateral growth at highest salt concentration.
During salt stress osmotic stress and ion toxicity are the problems and this causes cells to lose turgor. Turgor pressure stretch the cell walls and cell growth depends on it but lack of turgor implies danger for cell survival and due to the same reason reduction in plant height was observed in tomato as well as other crops. The root and shoot growth decreases rapidly in salt sensitive plant species and this does not appear to depend on salt concentration on the growing tissues, it is rather a response to the osmolarity of the exogenous solution (Serrano et al., 1999., Rengel, 1992, Aslam et al. 1993a, Lin and Kao 2001, Shereen et al., 2001, Arshi et al., 2002., Munns 2002).
These results are in accordance with previous findings that genotypes response differentially to salinity stress (Cruz and Cuartero (1990). Salinity cause stunted growth in glycophytes which results in reduced shoot and root fresh weights (Parida and Das, 2005; Hajer et al., 2006). The decrease in shoot and root fresh weights might be due to decreased uptake and accumulation of nutrients in the plant body (Dadkhah and Grrifiths 2006). The adverse effects of salinity on nitrogen fixation of cotton plants have been reported which resulted in lower fresh weights of shoots and roots (Pessarakli and Tucker (1988).
The plant height in Avinash and NTH-242 showed significant and negative correlation with Na uptake in shoot and root, and proline and ABA content whereas positive correlation existed with potassium uptake in root and shoot and root fresh weight. Whereas, in genotype Checo, the plant height had significant and positive correlation with shoot and root fresh weight and dry weight while plant height in same genotype had significant and negative correlation with proline and ABA content in shoot and root. The decrease in shoot and root fresh weight in Roma had significant negative correlation with Na uptake in root whereas positive correlation with shoot and root dry matter accumulation and RWC (Appendix).
Under salinity stress, plants accumulate excessive amounts of Na+ at the cost of K+ and Ca2+ (Rengle, 1992) and can lead to nutrient deficiencies (Francois and Donovan, 1991). The tolerant genotypes generally establish by lowering the toxic concentration of Na+ influx or its sequestration into the vacuole and/or its extrusion (Binzeil et al. 1988) which might have contributed to minimize the adverse effects of salinity on genotype Gol, Avinash and NTH-242 which performed better under stress than other tomato genotypes.
The results indicated that lowest % decrease in shoot dry weight was observed in genotypes BARI > Avinash > Checo > NTH242 > Gol but have different ranking pattern for root dry weight among above listed genotypes which can be rated as follow: Checo > BARI > Gol > NTH242 > Avinash. Whereas, in case of water content, minimum decreased in shoot and root water content was observed in BARI, NTH242, Avinash and Gol. The genotype Checo showed tremendous decrease in shoots and root water content at 9dSm-1 as compared to 6dSm-1.
Inhibition of growth by salinity is a common phenomenon in crop plants which might be due to reduction in cell division, cell enlargement and cell wall expansion (Greenway 1973). The decrease in dry weight of glycophytes in response to salinity has been reported in many plants like rice (Alam et al. 2004), tomato (Satti and Al-Yahyai, 1995) and pepper (Kaya et al., 2001) etc. The decrease in dry weight of tomato genotypes in response to salinity stress might be due to several factors like adverse effects of salinity on photosynthesis, reduction in turgor pressure of expanding tissues and salinity response of root to down regulate shoot growth through a long distance signal (Alam et al. 2004). The genotype BARI possessed higher root dry weight under salinity stress with concomitant decrease in shoot dry weight.
The decrease in relative water content under salinity stress is of common occurrence in plants and is closely related to plant water status, reflecting the metabolic activity in plant tissues (Flower and Ludlow 1986). The decrease in RWC of tomato genotypes under salinity could be attributed to the root systems which loses their capability to reduce water loss through transpiration by reduction of the absorbing surface (Gadallah 2000). Chartzoulakis and Loupassaki (1997) found significant decrease in RWC in leaves of egg plant (Solanum melongena L.) under salt stress.
In genotype Avinash, the SDW and RDW showed significant and negative correlation with Na+ uptake in root and shoot, proline and ABA content whereas positive correlation was existed with SWC, RFW, RDW, RWC and K+ in shoot and root.
In tomato genotype Gol, shoot water content and root water content had significant and negative correlation with Na+ content in shoot, ABA content in shoot and root while had positive significant correlation with K+ content in root. The shoot water content was positively correlated with root water content and vice versa.
Ionic contents in tomato genotypes
The genotypes showed differential response for accumulation of ions in shoot and root. The Na+ content of shoot was increased in all genotypes with the increase in salt concentration from 3dSm-1 to 9dSm-1. The genotypes Avinash, Gol, NTH242, Nagina and Pakit have relatively low Na+ accumulation in shoot in contrast their root has greater accumulation of Na+ which indicated translocation of Na+ to shoot was less in these genotypes. Although Roma has higher Na+ content in root but shoot accumulation was comparable to other genotypes. The basal level of Na+ content of shoot was lower in tolerant genotypes than sensitive genotypes.
The uptake of K+ content was inversely proportional to Na+ content, in shoot K+ accumulation is greater than in root as indicated by Na/K ratio. There was found marked decrease in the K+ content of tomato genotypes when subjected to higher salt concentration. Maximum decrease in K+ content of shoot at higher salt level was recorded in genotype Riogrande while minimum decrease in K+ content was observed in Roma. The maximum decrease in root K+ was recorded in Pakit and Pant Bahar while minimum root K+ was found in Avinash and NTH-242 at maximum salt concentrations. At higher level of salinity, maximum value of Na/K ratio was observed in tomato genotype 88572 for both root and shoot. While lowest Na/K ratio was observed in Nagina and NTH-242 in shoot and Checo and NTH-242 in root.
Perhaps accumulation of Na+ in root diminishes plant growth by causing Na+ toxicity that affects the cell permeability which adversely effects cell elongation/cell division (Khan et al., 2000). Salt stress is responsible for creating both ionic as well osmotic stress in plants and such ionic stresses could be apparent at several levels (Tester and Davenport 2003). The tomato genotypes showed differential response to salt stress for Na+ accumulation. It has been found previously that salinity tolerant barley and wheat cultivars accumulate lower concentration of Na+ in shoot than sensitive ones (Kook et al. 2009., Colmer et al., 2005). The tolerant species during the present investigation indicated greater accumulation of Na+ content in root but lower transportation to shoot having less Na+ content in shoot as compared to other genotypes. This may be due to removal of Na+ by the exclusion systems operating in the upper part of the root, the stem, petiole or leaf sheath (Munns, 2002; Tester & Davenport, 2003).
Talbott and Zeiger, (1996) have earlier reported that K+ concentration decreased in plant tissues under NaCl stress. The decrease in K+ concentration was also observed in salt tolerant plants at higher salt concentration (Asghari et al., 2001).
The levels of availability of K+ to plants determine their tolerance capacity to salt stress. Potassium act as balancing charge and the plant must maintain higher levels of K+ to counter balance the excess salt. Due to similar structure of sodium and potassium, there is a competition between both of them for plant uptake under salinity stress (Maathius and Amtmann, 1999). The genotypes NTH-242 and Roma possessed higher K+ concentration both in shoot and root than other genotypes indicating their better adaptation to salinity as some previous studies showed that salt tolerant plants maintain a high level of potassium under salinity (Volkmar et al., 1998). The concentration of K+ in root was less than that of shoot in all most all the tomato genotypes, which indicated that K is mobile element and its maximum requirement is in leaves to control different metabolic activities (Qureshi, 2004).
Na/K ratio is considered as indicator of salinity tolerance in plants (Saqib et al., 2004; Saqib et al., 2005; Munns, 2005) because it has been found that low Na/K ratio in cytosole is essential for normal cell metabolism (Chinnusamy, et al., 2005). The tomato genotype NTH-242 possessed lower Na/K ratio in both shoot and root, indicating its tolerance nature towards salinity. The high K/Na ratio can be used as selection criteria for the salt tolerant plants (Alhagdow et al., 1999). In tolerant cultivars of rice, the Na/K ratio was lower as compared to sensitive cultivars under salinity stress (Cha-um, 2009).
In all most all the genotypes like Aviansh, Pant Bahar, NTH242 and Roma etc the Na+ content of shoot and root were negatively correlated with plant height, shoot fresh weight, dry weight, root fresh and dry weight, water content and K+ content in shoot and root. It had significant and positive correlation with proline and ABA in both root and shoot. Whereas, K+ content in shoot and root had significant and positive correlation with plant height and biomass. Negative correlation of K+ was observed with proline and ABA content of root and shoot (Appendix).
With increase in salinity levels, maximum Ca++ content of shoot was observed in NTH-242 while maximum Ca++ content of root was recorded in Riogrande and 88572. The Na/Ca ratio increased with increase in salt concentration from 3dSm-1 to 9dSm-1. Overall, Checo showed higher Mg++ content in shoot but lower Ca++ content. In root, the genotypes showed marked differences for Mg++ accumulation but it has very little concern with salt concentrations.
These results are in concomitant with those obtained by Khan et al., (2001) in Salicornia rubra. Ca++ is involved in maintaining cell integrity and cell membrane permeability by forming Ca pectate that is essential constituent of cell membrane (Mcainsh and Clayton, 1996). As Ca++ is very important in protein synthesis and carbohydrate transfer, so the inhibition of growth in sensitive genotypes may be due to the significant reduction in Ca++ uptake. In genotype, NTH-242, Ca++ content in shoot had significant positive correlation with plant height, shoot and root fresh weight, shoot and root dry weight, shoot and root water content. The Ca++ also exhibited significant and positive correlation with K content in shoot and root respectively. Whereas had significant negative correlation with proline, ABA and Na+ in root. The tolerant genotypes maintained higher concentration of Ca++ under saline conditions than relatively tolerant and sensitive genotypes in Atriplex griffithii (Tozlue et al., 2000; Khan et al., 2000). The genotype NTH-242 exhibited higher Ca++ content in shoot with concomitant increase in K+ content which indicated its tolerant nature towards salinity stress.
These results are in agreement with those obtained by Khan et al., (2000) who also observed increased levels of NaCl induced decreased Mg++ in Atriplex griffithii. Mg plays a pivotal role in the production of chlorophyll, synthesis of amino acids and cell proteins and resistance against unfavorable factors. The higher Mg content of Checo can be an indication of its better behavior under salinity. In tomato genotypes like 88572, BARI and Nagina, Mg content in shoot had significant negative correlation with proline, ABA content in shoot and ABA content in root.
Proline and endogenous ABA content
It is suggested from our study that Proline increased with increase in salinity in almost all tomato genotypes and maximum proline contents at higher salinity was recorded in Avinash and NTH-242. In case of proline production, it was observed that Avinash was higher producer both at 6 dSm-1 and 9 dSm-1 salinity but Gol had shown less proline accumulated both at 6dSm-1 and 9dSm-1. NTH242 though had proline content lower at 6dSm-1 but had increased proline production statistically at par with Avinash. Roma has similar response to salt as that of Gol. The genotype Gol > NTH242 > Avinash > BARI exhibit shoot ABA and have higher proline production too. Endogenous root ABA was also higher in Pakit, BARI, Checo and Pant Bahar which were not very tolerant. So it seems that it is the shoot ABA which is one of the determinant of salinity tolerance.
Plants need to have special mechanisms for adjusting internal osmotic conditions and changing of osmotic pressure in the root environment under salinity stress by accumulating certain amino acids like proline. The accumulation of proline under environmental stresses has been investigated by many workers as an adaptive trait related to stress tolerance and it is generally assumed that proline is acting as a compatible solute in osmotic adjustment (Larher et al., 1993). The increase in proline is considered to be an index to stress tolerance (Qureshi, 2004). It was found that plants with higher proline concentration show better osmotic adjustment with the adverse conditions (Ashraf et al., 2002).
Most of the plants including halophytes are when subjected to environmental stresses like drought and salt stress; they accumulate low molecular weights of organic substances as proline (Yoshiba et al., 1997) indicating linear increase in proline content with increasing concentration of salt. The accumulation of these solutes may help to maintain the relatively high water content necessary for growth and cellular functions.
ABA is Phyto-hormone which induces different proteins during abiotic stress and helps plant to cope with these stresses (Jin et al., 2000). ABA is an essential mediator which under adverse environmental condition triggers plant responses (Zeevaart, and Creelman 1988, Hartung et al., 1988) and it was reported by many research workers for different crops which include rice, barley, soybean, tomato, cotton, and alfalfa (Henson 1984., Stewart 1985, Benson 1988, Bray 1988, Hartung et al., 1988, Luo et al., 1992). Substantial evidence suggests that increased ABA levels limit water loss by reducing stomatal aperture.
In most of the genotypes, it was observed that proline had significant and positive correlation with Na content in shoot and root and ABA content in shoot and negative correlation with plant height, biomass parameters and K concentration in shoot and root. The tomato genotypes Avinash and NTH-242 possessed higher accumulation of proline under higher salinity levels, which indicated their better osmotic adjustment and their tolerance towards salinity.
Generally in maximum tomato genotypes ABA contents had significant and positive correlation with Proline and Na Content in root and shoot whereas, had significant and negative correlation with plant height, biomass parameters and potassium content in shoot and root. The ABA is produced directly to maintain leaf and root growth and thus aid adaptation to environmental stresses (Cramer 2002, Sharp 2002). The higher accumulation of ABA in root and shoot of Gol might be due to its better adaptability to salinity stress.