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Braz. J. Plant Physiol. vol.22 no.1 Campo dos Goytacazes 2010
Impact of saline water stress on nutrient uptake and growth of cowpea
Prakash R. PatelI, Sushil S. KajalII, Vinay R. PatelI, Vimal J. PatelIII* and Sunil M. KhristiII
IDepartment of Biosciences, Sardar Patel University, Vallabh Vidyanagar, Gujarat - 388120, INDIA.
IIAshok & Rita Patel Institute of Integrated Study & Research in Biotechnology and Allied Sciences, New Vallabh Vidyanagar, Gujarat 388121, INDIA
IIIKrishi Vigyan Kendra (ICAR), Mangalbharti, Dist. Vadodara, Gujarat-391125
Soil salinity is a major limitation to crops production in many areas of the world. The present study reports the impact of salt stress on seeds germination, plant growth parameters and leaf ions accumulation in three cowpea [Vigna unguiculata (L.) Walp] Indian cultivars: Akshay102, Gomti vu-89 and Pusa Falguni. The electrical conductivity (EC) of the soil was 0.75 dS m−1 and the NaCl treatments increased it to 2, 4, 6, 8 and 10 dS m−1. Germination percentage was recorded 10 days after sowing, while shoot and root dry weights were measured in 45 days old plants. Leaf ion concentrations for Na+, K+, Ca2+, Cl− and proline content were determined. The results showed that the germination percentage of seeds was highly affected by salinity in all cultivars in all salinity levels from 2 to 10 dS m−1. On the other hand, height and weight of all cultivars were less affected by the salt stress. Only at 10 dS m−1 EC, significant reduction in plant height and root length could be found for all three cultivars. Salinity induced a significant increase in Na+, Cl− and proline concentrations, while reduced the accumulation of K+ and Ca2+ in leaves of all the cultivars. Moreover, the tolerance difference between the cultivars was better observed at the lowest levels of salt stress, as reveled in the measurements of K+/Na+ ratio and proline content. In conclusion, this study characterizes Akshay102 as the most tolerant cultivar and establishes the measurements of germination capacity, K+/Na+ ratio and proline accumulation as an important features to be explored in programs for selection and/or development of tolerant cultivars which make possible the utilization of waste saline water as well as the cultivation of vast areas of the tropical world affected by salinity.
Key words: Salt stress, germination, proline, potassium/sodium ratio
Salinity is a wide spread environmental stress for crop plants in arid and coastal regions. The salinity of the soil and irrigated water is a problem that restricts yield on almost 40 million hectares of irrigated land, which is approximately one-third of the irrigated land on earth (Norlyn and Epstein, 1984). Due to increasing salt salinity large areas of arable land are being removed from crop production every year (Chapman, 1975, Epstein et al., 1980). Use of saline irrigation water and application of fertilizer are the main factors responsible for increasing soil salinity (Epstein et al., 1980).
Seed germination, seedling emergence, and their survival are particularly sensitive to substrate salinity (Mariko et al., 1992 and Baldwin et al., 1996). Changes in morphological and developmental stress as well as physiological and biochemical processes are some of the mechanisms adapted by plant towards stress (Zhu, 2001). Generally the growth of plant is reduced by salinity but may vary from species to species in their tolerance (Munns and Termaat, 1986). High levels of soil salinity can significantly inhibit seed germination and seedling growth, due to the combined effects of high osmotic potential and specific ion toxicity (Grieve and Suarez, 1997). Intolerance to salinity may result in physiological and biochemical disorders which prevent or delay germination or cause abnormal seedlings (Rehman et al., 1996; Ungar, 1996). Hence, introducing of salt tolerant plants is one of the ways to utilize the waste saline water and lands (Baccio et al., 2004).
Although extensive work has been carried out on the effects of salinity on cowpea (Vigna unguiculata L.) using NaCl as a source of salinity (Plaut et al., 1989; Jamal et al., 2006; Patel and Ramana Rao, 2007; Chanyou et al., 2007; Hussein et al., 2007; and Tawfik, 2008), tolerance against salinity at the germination stage is important in the establishment of cowpea in saline soils. Looking to the above fact, an investigation was undertaken to determine the effect of salinity levels on seed germination and plant growth under natural conditions as well as nutrient composition of plant leaf tissues of different cultivars.
MATERIAL AND METHODS
Three cowpea cultivars, Akshay102, Gomti vu-89 and Pusa Falguni were grown outdoors in earthen pots containing 8 kg of soil. Soil used for the experiment was collected from the ploughed field of a nearby agriculture farm. The salinity of the soil used in the present study gave an electrical conductivity (EC of 0.75 dS m−1). The texture of the soil was clay loam (clay 49.3%, silt 30.5% and sand 21.2%). Different salinity levels of 2, 4, 6, 8 and 10 dS m−1 were prepared by mixing NaCl salt along with the soil until the soil sample measured the desired levels.
Ten seeds of each cultivar were planted in each pot. Thinning was carried out 10 days after planting, leaving four plants in each pot. The pots used during the present study had no holes at the bottom. A randomized complete block design was used with three replications. The experiment was carried out during the months of February to April 2007 and the daytime temperatures during the course of the experiment ranged from 33 - 45°C.
Germination percentages were recorded 10 days after seed sowing. Shoots and roots were collected after 45 days of seed sowing and separated after cleaning the roots of soil. Fresh weights were recorded immediately after harvest and were later placed in hot air oven at 70°C for 24 h in order to determine the dry weight. Leaves were used for determination of proline content (Bates et al., 1973) and mineral composition. Ash of plant samples was prepared by heating the samples to 550°C in a muffle furnace for 5 h and was later dissolved in dilute HCl with a few drops of Nitric acid and was used to determine Na+, K+ and Ca2+ using flame photometer, while Cl- contents of the leaves were determined by titration with AgNO3 (Richards, 1954). The data obtained from the various analyses and measurements were statistically analyzed using DMRT (Duncans multiple range test) (Bliss, 1967).
RESULTS AND DISCUSSION
A comparison of the responses of the different cultivars indicated significant decrease in germination percentage with increasing salinity level. Increase in salinity from 2 to 10 dS m−1 drastically reduced the germination percentages from 84 to 15, 82 to 10 and 64 to 6 for cowpea cv. Akshay-102, Gomti vu-89 and Pusa Falguni, respectively (Figure 1). After 45 days of sowing, plants at 10 dS m−1 of Akshay-102 cultivar were found to be taller than the plants of the other two cultivars. With the increase in salinity level (2 to 10 dS m-1) reduction in plant height was observed by 38, 44 and 50% for Akshay102, Gomti vu-89 and Pusa Falguni, respectively (Figure 2). The results reported here are in good agreement with those reported by Bernardo et al., (2006), who observed significant reduction in seed germination, plant height and shoot dry weight with increasing salinity-induced stress in cowpea cultivars. This might be due to NaCl affects the permeability of the plasma membrane and increases influx of external ions and efflux of cytosolic solutes in plant cells (Allen et al., 1995). Secondly NaCl causes hardening of the cell wall (Nabil and Coudret, 1995) and a decrease in water conductance of the plasma membrane causing reduction in plant height (Cramer, 1992).
Reduction in shoot dry weight was significantly affected with increase in salinity levels. Shoot dry weight of Akshay102, Gomti vu-89 and Pusa Falguni was reduced by 33, 39 and 43 % (Figure 3), while reduction of 37, 44 and 53 % in root dry weight was observed (Figure 4). The present results confirm that root growth was highly affected by salinity levels than shoot growth. Decrease in shoot biomass indicates an inverse relationship between salinity and biomass production (Gururaja Rao et al., 2005).
The K+ contents in the leaves of Akshay-102 was higher than that in Gomti vu-89 and Pusa Falguni. With increase in the salinity level from 2 to 10 dS m−1 resulted decreases in K+ concentration by 18, 22 and 27% for Akshay-102, Gomti vu-89 and Pusa Falguni. Concentrations of K+ at the higher salinity level were 25.33, 24.18, and 21.35 mg g-1 for Akshay-102, Gomti vu-89 and Pusa Falguni, respectively (Table 1). At the 10 dS m−1 level of soil salinity, the concentration of Na+ in the leaves of Akshay-102, Gomti vu-89 and Pusa Falguni increased 4.3, 3.6 and 3.5 fold, respectively (Table 1). The ratio of K+/Na+ was influenced significantly by soil salinity level. High ratios of K+/Na+ were found in Akshay-102 leaves than in Gomti vu-89 and Pusa Falguni leaves at all salinity levels (Table 1). Increase levels of soil salinity resulted in decreasing K+/Na+ ratios. The K+ content in plant tissues represents the main cation in plant cells, and is an important component of the cell osmotic potential (Reggiani et al., 1995).
Ca2+ contents in leaf decreased with increased salinity levels (2 to 10 dS m-1) by 22, 26 and 29% for Akshay-102, Gomti vu-89 and Pusa Falguni, respectively (Table 2). Essa (2002) reported that the main response of the plant to salt stress is a change in Ca2+ homeostasis and attributed that the salt tolerance of plants is their ability to avoid Na+ toxicity and to maintain Ca2+ and K+ concentrations. Also Na+ is said to maintain turgor but it is unable to substitute for specific functions of Ca2+ and K+. According to Ioneva (1988), increase in Na+ contents, decrease in K+ contents and K+/Na+ ratios in plant leaves can be attributed to the effect of competition between Na+ and K+ ions on the absorptive sites of the plant roots.
There was a substantial difference in Cl- content and rate of accumulation between cultivars with increasing salinity. Increasing the salinity level from 2 to 10 dS m−1 increased the Cl- concentrations in the leaf to the tune of 3.4, 3.9, and 4.1 fold for Akshay-102, Gomti vu-89 and Pusa Falguni, respectively (Table 2). It has been reported that salt-tolerant species maintain high concentrations of Ca2+ and K+ and low concentrations of Na+ and Cl-. Sodium is not considered an essential element for plants and plants accumulate Na+ at the expense of Ca2+ and K+ in saline conditions (Kuiper, 1984). According to Greenway and Munns (1980), the reduction in K+ concentration could inhibit growth by reducing the capacity for osmotic adjustment and turgor maintenance or by adversely affecting metabolic functions.
Increasing level of salinity showed higher proline content in the leaf. With increasing salinity levels Akshay -102 cultivar measured 3.12 to 6.45 mg g-1 proline followed by Pusa Falguni measuring 2.72 to 6.23 mg g-1 proline and least amount was measured in Gomti vu-89 with 2.77 to 5.96 mg g-1 proline (Table 2). Higher proline was noticed at higher salinity levels coupled with higher Na+ constitute the osmoregulating substances, which favours water uptake from saline medium, thereby enabling the plants to maintain its physiological activity. Higher amounts of proline noticed in cowpea helps them in turgor regulation and thus the physiological activity. Moreover, proline is also known to act as major reservoir of energy and nitrogen for utilization upon exposure to salinity (Tawfik, 2008).
Moreover, the tolerance differences between the cultivars are better reveled at the lowest levels of salt stress, as noticed in the measurement of K+/Na+ and proline content. In general, this study suggested that Akshay102 is the most tolerant cultivar and this tolerance is mainly due to the genetic variability among the seeds of each cultivar that may account for this phenomenon. The preset study confirms that at all salinity levels the variation in germination, plant growth, dry matter accumulation, ionic content and proline levels could be better explored in determining the tolerance capacity of the cowpea cultivars. In view of its better ion uptake and higher salt tolerance the Akshay-102 cultivar can be used as a salt tolerant cultivar.
Thus, the salt tolerance of all three Indian cvs studied are quite similar, despite of the higher performance exhibited by the cv Akshay102. Comparisons of the tolerance features demonstrated by these cvs in relation to the characterization of other cvs described by Bernardo et al., (2006) highlights the relevance of germination capacity, K+/Na+ ratio and proline accumulation as important features to be explored in programs to select and/or develop tolerant cultivars, to make possible the utilization of waste saline water as well as the cultivation of vast areas of the tropical world affected by salinity.
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Received: 27 November 2009; Accepted: 16 March 2010
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