Physiological and Ionic Expressions of Different Hybrids of Maize (Zea Mays L.) under Different Salinity Levels

A pot culture experiment was conducted to estimate the behavior of maize under saline soil conditions in the wire house of Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad. There were three treatments with different levels of salinity and equal amount of Ammonium Nitrate (NH4NO3) in all pots including control was applied to three genotypes of Hybrid Maize i.e. 32F10 (Poineer), King Cross (Auriga), and NK-8441 (Syngenta), in five replications. Treatments were applied according to Completely Randomized Design (CRD). Plant and soil samples were collected for determination of different physiological and ionic expressions. Genotype NK-8441 comparatively performed better at all salinity levels. All studied physiological parameters viz. relative water contents, membrane stability index, total chlorophyll contents, CO2 assimilation, stomatal conductance, transpiration rate and photosynthetic rate showed a declining trend at higher salinity level, remarkably. However a very small decline was observed in NK-8441 compared with other genotypes. Due to salinity stress plant uptake of K, NO3, and N from soil was reduced and Na uptake was increased significantly as the salinity level was increased. NK-8441 showed its worth by up taking more K, NO3 and N and discouraging Na uptake.


Introduction
Maize (Zea mays L.) is a vital cereal crop for human as well as for animal utilization and is grown under different conditions in various parts of the world. In Pakistan, maize is the 3 rd most valuable cereal after wheat and rice. Its grain has high nutrient value and oil is exploited for cooking purposes while green feed is quite rich in protein [11]. Area under maize crop is 1085 000 ha and production of maize for the year 2012-13 is 4631 000 tones [16]. In view of its increasing importance betterment in agronomical aspects of maize has received big attention in Pakistan especially in saline soil conditions.
Salinity is one of the major constraints of agriculture in the arid and semiarid region. The salt-affected area of Pakistan is 6.67 million hectares [24]. Pakistan falls under arid and semiarid regions where evapotranspiration greatly exceeds precipitation and salts tend to accumulate in upper soil surface. The use of inadequate drainage, saline water and low soil permeability condition contributes to soil salinity. There are various detrimental effects of salt stress in crop plants, responsible for severe decreases in the growth and yield of plants. Osmotic stress (drought problem), ion imbalance, particularly with Ca +2 , K + and the direct toxic effects of ions on the metabolic process are the most important and widely studied physiological impairments caused by salt stress [29,42].
Salt stress, like many abiotic stress factors, also induces oxidative damage to plant cells catalyzed by reactive oxygen species [8]. Excessive buildup of Na + and Cldamages root membrane and causes ionic imbalances that may discourage potassium uptake by plants [17] Na + and Clconcentration in different plant parts, tissues, cells and cell organelles increases by increasing salinity [25] N fertilization not only promotes plant growth it may also lessen the effects of salt on plants [13]. Plants take up nitrogen mainly as NO -3 and NH + 4 form. When both are present in solution then NH + 4 is preferred. Under salinity nitrate consumption is slowed down and salinity reduces NO -3 absorption with the possible consequence of N insufficient in the plant [31].
It is difficult to study plant response to fertilizers under saline conditions due to high concentration of salts and nutritional imbalances. Studies showed that application of fertilizers in saline soils might result in increased, decreased or unchanged plant salt tolerance. In other words, plant response to fertilizers depends on severity of salt stress in the root zone [28] The present study was planned to flourish the knowledge about growth, yield components and to judge the salt tolerance behavior of maize genotypes and role of nitrogenous fertilizer in enhancing the salt tolerance.

Materials and Methods
A pot experiment was conducted in the wire house, Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad to estimate the behavior of maize (Zea mays L.) genotypes in saline soil conditions. Three genotypes of maize (Zea mays L.) 32F10 (Poineer), King Cross (Auriga), and NK-8441 (Syngenta) were tested against three levels of salinity viz. (1) Control (1.2 dS m -1 ) (2) EC e 4 dS m -1 (3) EC e 8 dS m -1 . These saline treatments were selected keeping in view the tolerance and yield potential of the crop. Initially soil samples were collected from research area of University of Agriculture Faisalabad, after then soil was sieved and analyzed to develop salinity with addition of NaCl salt. Salinity was developed artificially in the pots using calculated amount of NaCl salt [24]. Properties of the soil and tap water used for irrigating the pots are as under in Table 1.
The experiment was replicated five times with completely randomized design (CRD). Leaf samples were taken after two months for determining membrane stability index (MSI) and relative water contents (RWC). Chlorophyll contents of each plant were determined with chlorophyll meter. Different parameters like CO 2 assimilation, stomatal conductance, transpiration rate and photosynthetic rate were recorded by Infra Red Gas analyzer (IRGA). Leaf samples were taken again for extraction of leaf sap. Sodium (Na + ) and Potassium (K + ) was calculated from the leaf sap. Plants were harvested after three months and separated into shoots and roots. Soil samples after harvesting the crop were taken and analyzed for sodium (Na + ) and potassium (K + ) by Flame Photometer, nitrate (NO 3 ) by Spectrophotometer and total nitrogen by Kjaldahl method. The collected data were statistically analyzed by using the Fisher's analysis of variance technique and differences among treatment means were compared by using least significant difference (LSD) test at 5 % probability level with the help of statistical package MSTAT-C version 1.3 [38].

Results and Discussions
Salinity adversely affected the physiological and ionic characteristics of maize, when exposed to salinity level of 4 dS m -1 and 8 dS m -1 .
Effect on salinity on physiological and ionic parameters of Maize: Maize genotype NK-8441 showed better physio-chemical attributes at all salinity levels. Salinity effect on physiological parameters is discussed below:-Relative Water Contents (RWC): The data presented in the table 2 showed that at all salinity levels control (1.2 dS m -1 ), 4 dS m -1 and 8 dS m -1 EC e the maximum and the minimum relative water contents were calculated in NK-8441 and King Cross, respectively. Relative water contents decreased significantly with increased in salinity. At all EC e levels, the maximum and the minimum relative water contents recorded in the order NK-8441 > 32F10 > King Cross. The results were supported by [19] drought and salinity affected the dry weight content in a different way, resulting from the reduction in the relative water content. [10] also concluded that salinity reduces leaf relative water contents in maize (Zea mays L.).    2 showed that at all salinity levels control (1.2 dS m -1 ), 4 dS m -1 and 8 dS m -1 EC e maximum and the minimum stomatal conductance recorded in the order NK-8441 > 32F10 > King Cross. [22] during early seedling growth of maize and sunflower salinity reduces leaf water potential and stomatal conductance. Photosynthesis is an accepted fact that determines the final plant productivity. Salinity decreased the net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration and transpiration rate, but increased the leaf temperature of rice [37] CO 2 Assimilation (Ci): The data presented in the table 2 showed that CO 2 assimilation decreased significantly with increase in salinity at all salinity levels. NK-8441 was better in CO 2 assimilation than 32F10 and King cross. Salinity decreased the net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration and transpiration rate, but increased the leaf temperature of rice [37]. Similar results were found by [33].
Transpiration Rate (E): The data (table 2) revealed that transpiration rate decreased significantly with increased salinity level. Transpiration rate recorded in the order NK-8441 > 32F10 > King Cross at all salinity levels of control (1.2 dS m -1 ), 4 dS m -1 and 8 dS m -1 EC e . Salinity decreased the net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration and transpiration rate, but increased the leaf temperature of rice [37].
Photosynthetic Rate (A): The data (table 2) cleared that photosynthetic rate decreased significantly with increase in salinity and maximum Photosynthetic rate was found in NK-8441 while, the minimum in King Cross at control (1.2 dS m -1 ) EC e level. At 4 dS m -1 EC e level, the maximum and the minimum Photosynthetic rate was recorded in NK-8441 and King Cross, respectively. At 8 dS m -1 EC e level, NK-8441 was better in photosynthetic rate than 32F10 and King cross. Photosynthesis is an accepted fact that determines the final plant productivity. Salinity decreased the net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration and transpiration rate, but increased the leaf temperature of rice [37].
Effect on salinity on ionic expressions of Maize: Salinity has very clear affects on ionic composition of maize and maize grown soil, especially when exposed to salinity index of 8 dS m -1 . Maize genotype NK-8441 showed better chemical attributes at all salinity levels. Salinity effect on ionic composition of plant and soil is discussed below:-Leaf Sodium Level (Na + ): The data presented in the table 3 showed that Na + concentration in leaf sap was increased significantly with increased EC e levels. At all levels, the maximum Na + concentration was recorded for King Cross. The minimum Na + concentration was noticed for NK-8441 at all salinity levels. A positive correlation exists between Na + exclusion and relative salt tolerance in many crops including barley [34], rice [7] beans and sunflower [ 4,6,20]). Efficient Na + exclusion is a good selection criterion for salt tolerance in maize and other glycophytes [4,9,30]. The concentration of N, NO -3 , P and K was reduced in the salinity treatments, but the concentration of Na + and Clwas increased. [12].
Leaf Potassium Level (K + ): The data (table 3) revealed that K + concentration in leaf sap was decreased considerably with increased EC e levels. The minimum K+ was found in King Cross at all the EC e levels, whereas the maximum K + concentration was found in NK-8441 which varied significantly at all EC e levels. Almost similar results had been observed by [27]. Salt stress induced Na + and Claccumulation it decrease K + and Ca 2+ level in shoots and roots, therefore salt decrease relative growth rate of shoots and roots. Decreased K + concentration with increased salinity was also reported earlier by [3,30] in maize.
Soil Sodium Level (Na + ): The data (table 3) showed that Na + concentration in soil was decreased significantly with increased EC e levels. At all levels, the minimum Na + concentration was recorded for King Cross. The maximum Na + concentration at moderate NaCl induced salinity was observed for NK-8441, while at 8 dS m -1 EC e level, the maximum was observed for NK-8441 and minimum for King Cross. Na + uptake by plant under saline conditions was more so as a result Na + concentration increases in soil as the salinity level increases. [12] the concentration of N, NO 3 , P and K + was reduced in the salinity treatments, but the concentration of Na + and Clwas increased.
Soil Potassium Level (K + ): The data presented in the table 3 shows that K + concentration in soil was increased significantly with increased EC e levels. The maximum K + was found in King Cross at all the EC e levels, whereas the minimum K + concentration was found in NK-8441 which varied significantly at all EC e levels. K + up take from soil under salinity is suppressed by Na + uptake, resulting in high concentration of K + in saline soils. There is a debate that K + influx could be used as an index to salinity tolerance [36]. Increased K + concentration may be attributed to K + selectivity for absorption.
Soil Nitrate (NO 3 -): The data (table 3) revealed that analyzed soil NO -3 concentration at all salinity levels of control (1.2 dS m -1 ), 4 dS m -1 and 8 dS m -1 was minimum for cultivar NK-8441and maximum for cultivar King Cross. [12] the concentration of N, NO 3 , P and K + was reduced in the salinity treatments, but the concentration of Na + and Clwas increased. [28] Salinity in a straight line can affect nutrient uptake such as Na + , reducing K + uptake or by Clreducing NO 3 uptake. Soil Total Nitrogen (N): The data presented (table 3) cleared that N concentration was increased significantly with increased EC e levels. At all levels, the maximum N concentration was recorded for King Cross. The minimum N concentration at moderate NaCl induced salinity was observed for NK-8441, while at 8 dS m -1 EC e level, the minimum N concentration was observed for NK-8441 and maximum for King Cross. [2] total dry matter production and root and shoot N contents decreased with increasing salinity in the rooting medium. [18] soil salinity affected crop yield, crop total nitrogen uptake and the nitrogen contribution of the soil. [31] The NH 4 + : NO 3 -(25:75) application resulted in increasing the total N contents in plants of control and saline treatments, but did not cause a large decrease in plant Na + contents under salinity.

Conclusions
After investigating the all above mentioned physiological and ionic attributes, results were found significant. Among the three commercial hybrids of maize NK-8441(Syngenta) was found comparatively best variety due to its better mechanism against salinity stress.
On an overall basis NK-8441 (Syngenta) and 32F10 (Poineer) were proved to be the best at intermediate salinity level. However, at 8 dS m -1 salinity level NK-8441 proved to be relatively salt tolerant of all cultivars. It also had higher K + as well as lower concentration of Na + concentration in its leaf sap. It means that it had a relatively better mechanism of Na + exclusion. At the same time it had better system of K + maintenance in its leaves.