The response of maize (Zea mays .L,) to exogenously applied sulfur under saline conditions

Abstract

Salt tolerant and salt sensitive cultivars of maize were screened for improvement in salinity tolerance by exogenously applied sulfur as K2SO4. Seeds of eight cultivars of maize were subjected to 0, 25, 50 and 75 mM NaCl. Six levels (0, 20, 40, 60, 80, 100 mM) of sulfur were prepared in Hoglands nutrient solution by using potassium sulphate and applied in sand culture at sowing time. Results showed that salt stress reduced all germination attributed such as; germination percentage, emergence index, mean emergence time, coefficient of uniformity of emergence, vigour index, coefficient of velocity of germination, germination energy, germination speed, mean daily germination, germination value and length, fresh, dry weights of plumule and radicle and ionic accumulation. Salt stress also caused a delay to achieve 50% germination in all selected maize cultivars. The lower levels of sulfur (20, 40, 60 and 80 mM) enhanced salinity tolerance of all studied maize cultivars. Though the highest level of sulfur (100 mM) also improved salinity tolerance to some extent, it was comparatively less effective as compared to lower levels. All germination and early growth data was fed to NTSyS PC software. The tree was generated based on distance matrixes of all attributes with a higher distance reflecting higher similarity between groups. All genotypes were clustered a tolerant group (Agatti 2003, MMRI, Pearl basic, Sadaf) and a sensitive group (Sahiwal 2003, Hybrid 1898, Pak Afgoi 2003, Yousaf wala hybrid). Among the tolerant group, the highest distance was calculated for Agatti 2003 while among salt sensitive group, the least distance was observed for Pak Afgoi 2003 and Hybrid 1898. All other genotypes had intermediate tolerance levels. Among the salt tolerant group, Agatti 2003 had more germination, growth and ionic accumulation at all levels of salinity as compared to other maize cultivars therefore selected as a tolerant variety for further studies. Among the sensitive group, PakAfgoi 2003 was selected as salt sensitive one because of lowest seed germination, growth and ionic accumulation attributes under salinity. From the findings of screening experiment, 25mM and 75mM concentration of NaCl and 40mM and 80mM of sulfur was selected for further study. In the second part, seeds of selected maize hybrids (Agatti 2003 and Pak Afgoi 2003) were subjected to salinity (25, 75 mM) and sulfur (40, 80 mM) as soil amendment. Sulfur (40, 80 mM) was also applied as foliar spray. The plants were harvested after 45 days of seed germination. Various growth, physiological and biochemical parameters were studied. The results of the second experiment revealed that sulfur application improved all growth and biochemical parameters studied. The concentration of different ions (K+, Ca2+, NO3-, PO42-, SO42-) were reduced by salt treatment in leaf, shoot and root of both varieties of maize. The concentration of phenolics, lycopene and carotenoids were increased by increasing sulfur that effectively scavenged MDA and H2O2 contents in Agatti 2003 and helped plants to tolerate adverse effects of salt stress. Excessive production of secondary metabolites, such as, alkaloids and flavonoids also contributed in enhancing salt tolerance in Agatti 2003. The total soluble sugars, total free amino acids and total soluble proteins increased to a greater extent in Agatti 2003 than Pak Afgoi 2003. In comparison sensitive cultivar Pak Afgoi 2003 had high Na+ ions, H2O2 and MDA concentration. It also exhibited lower concentration of osmoprotectants, vitamins, antioxidants, biomolecules that lowered its capability to tolerate salinity. Sulfur at 40 mM level proved to be very effective for improving all growth and biochemical parameters. While higher levels (80 mM) of sulfur were not proved much effective for growth enhancement of maize cultivars both in salinized and non-salinized conditions. The tagged plants from second experiment were harvested at full mature stage. The data was recorded for yield and yield components and forage value of maize shoot. Results showed that salt stress reduced all studied yield parameters including cob length, cob per plant, total number of cobs, total number of grains, weight of 50 grain, harvest index, yield per plant, ionic contents, protein, carbohydrate, starch, vitamin, ash, fiber and ionic contents (Na+, K+, Ca2+, NO3-, PO42-, SO42-) in leaves and grains. This decrease was accompanied with an increase in sodium contents in both maize varieties. Sulfur at 40 mM level effectively increased all studied yield and biochemical parameters. In conclusion, although both varieties responded differentially to the sulfur application, Agatti 2003 showed more tolerance to salt stress by application of sulfur as compared to Pak Afgoi 2003. Additionally, the lower level of sulfur (40 mM) was much more effective in enhancing salt tolerance potential of tested genotypes as compared to its higher level i.e. 80 mM S applied as K2SO4.