EFFECT OF POTASSIUM NUTRITION ON SOLUTE ACCUMULATION, ION COMPOSITION AND YIELD OF MAIZE HYBRIDS GROWN UNDER SALINE CONDITIONS

This article was downloaded by: [UQ Library] On: 18 April 2013, At: 05:51 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Plant Nutrition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lpla20 EFFECT OF POTASSIUM NUTRITION ON SOLUTE ACCUMULATION, ION COMPOSITION AND YIELD OF MAIZE HYBRIDS GROWN UNDER SALINE CONDITIONS MUHAMMAD AKRAM a a University College of Agriculture and Environmental Sciences, The Islamia University of Bahawalpur, Bahawalpur, Pakistan Accepted author version posted online: 11 Oct 2012.Version of record first published: 05 Dec 2012. To cite this article: MUHAMMAD AKRAM (2013): EFFECT OF POTASSIUM NUTRITION ON SOLUTE ACCUMULATION, ION COMPOSITION AND YIELD OF MAIZE HYBRIDS GROWN UNDER SALINE CONDITIONS, Journal of Plant Nutrition, 36:1, 143-163 To link to this article: http://dx.doi.org/10.1080/01904167.2012.737885 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Journal of Plant Nutrition, 36:143–163, 2013 Copyright C© Taylor & Francis Group, LLC ISSN: 0190-4167 print / 1532-4087 online DOI: 10.1080/01904167.2012.737885 EFFECT OF POTASSIUM NUTRITION ON SOLUTE ACCUMULATION, ION COMPOSITION AND YIELD OF MAIZE HYBRIDS GROWN UNDER SALINE CONDITIONS Muhammad Akram University College of Agriculture and Environmental Sciences, The Islamia University of Bahawalpur, Bahawalpur, Pakistan � An experiment was conducted to study the response of two maize hybrids to external potassium (K) application under saline conditions. The data showed that there was an increase in the organic solute contents and sodium ion under salinity stress, though potassium, calcium, nitrogen and phosphorus were decreased. There was a non-significant effect of K application on glycinebetaine and total soluble sugar, however; the proline, protein and total free amino acids were increased with the application of external K. The enzymatic activity like nitrate reductase and nitrite reductase activity were severely reduced under salinity stress and improved by K application. The maize hybrids differed significantly for all the parameters discussed in the study except sugar, phosphorus and number of grain rows per cob. The increase in yield parameters was more pronounced under control than under saline conditions. The enhanced yield and yield components of these maize hybrids might be due to the quick response to external K application, resulting in high contents of leaf potassium, calcium, nitrogen and phosphorus. The results indicated that the maize hybrid ‘Pioneer32B33’ might perform better than ‘Dekalb979’ under saline conditions when sufficient potassium is applied in the rooting medium. Keywords: Zea mays, organic solute accmulation, ion contents, enzyme activity, plant nutrition, potassium, salinity tolerance, yield INTRODUCTION At the present time about 20% of the world’s cultivated land and ap- proximately half of all irrigated land is affected by salinity (Zhu, 2001). Therefore, salinity is one of the most important abiotic stress factors limiting plant growth and productivity (Khan and Panda, 2008), particularly in arid and semi-arid environments has faced an increase in soil salinity. Salinity Received 20 June 2010; accepted 19 January 2012. Address correspondence to Muhammad Akram, University College of Agriculture and Envi- ronmental Sciences, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan. E-mail: [email protected] 143 D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 144 M. Akram affects almost every aspect of the physiology and biochemistry of plants and significantly reduces yield. High exogenous salt concentrations affect water deficit, cause ion imbalance of the cellular ions resulting in ion toxicity and osmotic stress (Khan and Panda, 2008). A plant’s ability to acclimate to salt stress includes alterations at the leaf level, associated with morphological, physiological and biochemical charac- teristics whereby many plants adjust to high salinity and the consequent low soil water availability (Ashraf, 2004). One of the major effects of salt stress in plants is induced nutritional disorders; these disorders may result from the effect of salinity on nutrient availability, competitive uptake and transport or partitioning within the plant (Munns and Tester, 2008). Under saline conditions, halophytic plants tend to take up and accumulate sodium (Na+) in their vacuoles and use it as an osmoticum (Glenn and Brown, 1999); however nonhalophytic monocotyledons tend to exclude Na+ and uptake of more potassium (K+), which seems to be crucial for salt tolerance (Grattan and Grieve, 1999). Osmotic adjustment or accumulation of solutes by cells is a process by which water potential of a cell can be decreased without an accompanying decrease in cell turgor. It is a net increase in solute content per cell that is independent of the volume changes that result from loss of water (Taiz and Zeiger, 2002). Osmotic adjustment is recognized as an important adaptive mechanism for poor water availability as it helps maintain growth in many plants (Morgan, 1995). It involves the regulation of the intracellar levels of organic compounds, many of which are compartmentalized principally in the cytoplasm, whereas inorganic ions are sequestered in the vacuole (Matoh et al., 1987). Glycinebetaine, proline and other organic solutes accumulated under stress conditions in plants (Mansour, 2000). Under saline conditions, the osmotic adjustment, which occurs through the accumulation of inor- ganic compounds [mainly Na+ and chloride (Cl−)] in plants, is less energy and carbon demanding than adjustment by organic solutes (Greenway and Munns, 1983). Salinization also affects the different steps of nitrogen (N) metabolism, namely ion uptake, N assimilation, and amino acid and protein synthesis. Nitrate reductase is considered to be a limiting factor for growth, development and protein production in plants, because its activity changes directly and affect plant growth (Zornoza and Gonzales, 1998). This first enzyme of nitrate assimilation is well known to be influenced by external conditions, such as salinity or osmotic stress (Botella et al., 1993). Salinity reduces the ability of plants to utilize water and osmotic effects are due to salt-induced decrease in the soil water potential. Salinity results in a reduc- tion of K+ and calcium (Ca2+) content and an increased level of Na+, Cl−, and sulfate (SO42-), which forms its ionic effects (Mansour et al., 2005). Potassium supplement reduced the concentration of Na+ and increased the concentration of K+ in leaves (Kostas and Georgios, 2006). Characters like yield and yield components have been the most commonly used criteria D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 145 for identifying salinity tolerance (Gamma et al., 2007). Potassium is often considered to be a nutrient of primary importance for cereals crops. It is a major osmoticum, which contributes to osmotic adjustment, stomatal movement and restriction of Na+ uptake under salinity (Mengel and Kirkby, 2001). Potassium content in plant tissues progressively decreases with an increase in salinity, owing to higher absorption of Na+ (Grattan and Grieve, 1994). Maintenance of adequate levels of potassium is important for plant survival in saline conditions (Chow et al., 1990). Potassium also plays a major part in the enzyme system essential for protein synthesis, glycolytic enzymes and photosynthesis; an osmoticum mediating cell expansion and turgor driven movements and competitor of Na+ under salt stress (Hu and Schmidhalter, 2005). Application of potash fertilizer to soil mitigates the ad- verse effects of salinity through its role in osmoregulation, protein synthesis and homeostasis (Sanjakkara et al., 2001) and improved the plant growth and yield and thus salt tolerance in corn (Grattan and Grieve, 1999; Bar-Tal et al., 2004). Maize (Zea mays L.) is one of the most important cereal crop grown in Pakistan. Maize grain is used for both human consumption and poultry feed. It has a great utility in agro-industry. Based on area and production, maize is the third most important cereal crop after wheat and rice in world (Tollenaar and Dwyer, 1999). The yield of maize in Pakistan is very low as compared to other maize producing countries. One of the most important effective factors is non-application of optimal amounts of potassium fertilizer per hectare and maize hybrids differ in their response to different levels of potassium fertilizer under saline conditions. Being an important “Kharif” crop, maize is grown on an area about one million hectares with a total yield of about 4 million tones and an average yield of 3610 kg ha−1 (Government of Pakistan, 2008). The yield of maize is below its potential due to moderate to high soil salinity/sodicity, high pH and scarcity of good quality irrigation water. The unfavorable conditions as well as inadequate and imbalance use of plant nutrients in these and other similar soils cause a considerable decline in the yield of maize and other cereals (Niane, 1987). No doubt soil salinity alters the uptake of nutrients by plants but the use of potash fertilizers alleviates to some extent the detri- mental effects of moderate salinity and help to improve the economic yield of crops. Therefore, in addition to other agronomic practices, successful crop production on moderately salt affected soils demands judicious use of plant nutrients, particularly potassium. Maize crop is moderately sensitive to salinity and considered as salt sensitive of cereals with yield reduction due to salinity varying from 10% at an electrical conductivity (ECe) of 1.7 dS m−1 to 50% at ECe 7 dS m−1 (Mass and Hoffman, 1997). Because of great sensitivity of this crop, improvement for salt tolerance would be of consid- erable value. Effective and accelerated improvement would be required in order to improve the productivity. Therefore, the present study emphasized D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 146 M. Akram on different aspects of salt tolerance of maize crop and its response to the applied potassium. MATERIALS AND METHODS The experiment was conducted under natural conditions in the wire house of Plant Stress Physiology Laboratory of Nuclear Institute for Agricul- ture and Biology (NIAB), Faisalabad, Pakistan. Maize (Zea mays L.) hybrids ‘Pioneer32B33’ (salt tolerant) and ‘Dekalb979’ (salt sensitive) was grown in the pots of 24.5 cm diameter and 28 cm deep. The pots were filled with 12 kg of sandy loam soil having a bulk density of 1 500 kg m−3, ECe 1.09 dS m−1, pH 7.70, sodium absorption ration (SAR) 10.9. The field ca- pacity of the soil was 16.35% and the saturation percentage was 35%. After germination, four plants were maintained in each pot and recommended doses of fertilizers were applied at appropriate time. Three salinity [sodium chloride (NaCl)] levels, S0 (Control, i.e., 1.09 dS m−1) S1 (5 dS m−1) and S2 (10 dS m−1) was developed by adding calculated amount of NaCl in the soil. Four potassium levels i.e. K0, K1, K2 and K3 (without application, 75, 125 and 175 kg K ha−1) were applied in the present study. Salinity level, i.e., 5 dS m−1, was developed before sowing the seed by mixing the calculated amount of salt (NaCl) with the soil and latter on 5 dS m−1 was applied 25 days after sowing. Nitrogen was applied to the plants in splits, i.e., 1/3 at sowing, 1/3 at knee height and 1/3 at tasseling stage. The nitrogen and phosphorus were applied at recommended rate, i.e., 225 kg ha−1, 125 kg ha−1, respectively. The whole phosphorus was applied at sowing time. The nitrogen, phospho- rus and potassium were used in the form of ammonium nitrate, single super phosphate (SSP) and sulfate of potash (SOP). The experiment was laid out in a three factors completely randomized design (CRD). There were three replicates for each treatment. Plants irrigated with normal water as and when required. Observations were recorded on fully expanded top third leaves at tasseling stage. Meteorological data for the growth period of crop is depicted in Figure 1. Proline (µ mol g−1 F.W) from the fully expanded leaf from top was es- timated according to the method of the Bates et al. (1973). Glycinebetaine (µ mol g−1 F.W) was determined following the Grieve and Grattan (1983) method. The nitrate reductase activity (NRA; µ mol NO2 g−1 F.W hr−1) was determined according to the method of Sym (1984). Nitrite reductase (NiR; µ mol NO2 g−1 F.W hr−1) was determined according to Ramarao et al. (1983). Protein (mg g−1 F.W) was determined as described by Lowry et al. (1951). Total soluble sugars (mg g−1 F.W) were determined according to the method of Razi et al. (1985). Total free amino acids (mg g−1 F.W) were de- termined according to the method of Moor and Stein (1948). To determine mineral elements in plant tissues, the dried ground shootmaterial (0.5 g) was D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 147 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 1-1 5 J uly 06 15 -31 Ju ly 0 6 1-1 5 A ug ust 06 15 - 31 Au gu st 06 1-1 5 S ep tem be r 15 - 30 Se pte m be r 1-1 5 O cto be r 15 - 30 Oc tob er 1-1 5 N ov em be r 15 -30 No ve m be r Te m p. 0 C (M ax . & M in . ) 0 10 20 30 40 50 60 70 80 Growing season Ra in fa ll (m m ), R .H . (% ) Max.Temp Min.Temp R.F (mm) R.H (%) FIGURE 1 Metrological data for the growth period of the crop. digested with sulfuric acid and hydrogen peroxide according to the method of Wolf (1982). Digestion Dried ground plant material (0.5 g) was taken in digestion tubes and 2.5 mL of concentrated sulfuric acid (H2SO4) were added to each tube and incubated overnight at room temperature. Then 1 mL of hydrogen peroxide (H2O2) (35%) was poured down the sides of the digestion tube, ported the tubes in a digestion block and heated at 300◦C until fumes were produced. They were continued to heat for 30 minutes. The digestion tubes were removed from the block and cooled. Then 1mLofH2O2 was added and placed the tubes back into the digestion block. The above step was repeated until the cooled digested material was colorless. The volume of the extract was maintained to 50 mL in volumetric flasks. The extract was filtered and used for the determination of Na+, K+ and Ca2+. Determination of Na+, K+, and Ca2+ Sodium, K+ and Ca2+ cations in the digests were determined with a flame photometer (Jenway, PFP-7; Jenway, Stone, UK). Nitrogen was estimated by micro-Kjeldhal’s method (Bremner, 1965). Phosphorus (P) was determined on a spectrophotometer (Jackson, 1962). D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 148 M. Akram Measurements Cob length was taken from each plant with the help of scale and then average was taken. It was expressed in centimeter (cm). To determine grains per cob, grains were shelled with hands and number of grains was counted. Then average of these cob’s grains was calculated. The number of grain rows in each cob was carefully counted and averaged them. 100-grain were taken at random from the grain lot of each cob and weighed by means of electric balance. To determine grain yield (g plant−1), cobs from each plant were shelled, sun dried and grain yield of each plant was calculated by weighing on electric balance. Statistical Analysis Three factor completely randomized design (analysis of variance tech- nique) of the data was computed for all attributes by using the MSTAT computer program (MSTAT-C, Michigan State University, East Lansing, MI, USA). The treatment’s means were compared using least significant differ- ence test at 5% probability level (Steel et al., 1997). RESULTS Effect of Potassium Nutrition on Organic Solute Accumulation Interactive effects of salinity andpotassiumonproline accumulation indi- cate that potassium application increased the proline contents under saline conditions (Figure 2b). At the highest salinity level proline accumulation was maximum in K3 (175 kg K2O ha−1), which was statistically on par with K2 (125 kg K2O ha−1), K1 (75 kg K2O ha−1) and minimum was recorded in control treatment, where crop was grown without application of potas- sium. Interaction between maize hybrids and salinity (Figure 2a) show that there was significant increase in proline concentration by the application of potassium in both the hybrids. Salinity stress significantly increased the glycinebetaine concentration and maximum glycinebetaine accumulation was recorded at the highest salinity level (10 dSm−1) in ‘Pioneer32B33’. Glycinebetaine accumulation in- creased with increase in the salinity, while in ‘Dekalb979’ maximum glycine- betaine accumulation was recorded at the highest salinity level which was at par with S1 (5 dS m−1) and minimum was found in control, where no salinity stress was applied (Figure 3). Protein content was increased with increase in salinity levels and sig- nificant differences were noted among various potassium levels (Figure 4). Interactive effect of maize hybrids and different potassium levels show that in ‘Pioneer32B33’ protein content significantly increased in all the potassium D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 149 Salinity levels 0 1 2 3 4 5 6 7 8 9 Pr ol in e (µ m ol g-1 F. W ) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 Control 5 dS m-1 10 dS m-1 0 1 2 3 4 5 6 7 8 9 10 Pioneer 32B33 Dekalb 979 Pr ol in e (µm ol g - 1 F. W ) Control 5 dS m-1 10 dS m-1 (b) (a) FIGURE 2 Interactive effect of A) maize hybrids x salinity and B) salinity x potassium on leaf proline accumulation in maize hybrids ± SE. levels, whereas, in ‘Dekalb979’ maximum protein contents was recorded in K2 (125 kg K2O ha−1) which was followed by K3 (175 kg K2O ha−1) and minimum was in control where no potassium was applied (Figure 4). Effect of different salinity and potassium levels indicate that total soluble sugar significantly increased with increase in the salinity stress but potassium application did not significantly affect sugar accumulation (Figure 5). Figure 6 indicate that maximum total free amino acids accumulation was found in case of treatment where potassium was applied at 175 kg K2O ha−1 but it was statistically at par with K2 (125 kg K2O ha−1) and minimum was observed in control (no application). It indicates that application of D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 150 M. Akram 0 5 10 15 20 25 Pioneer 32B33 Dekalb 979 G ly ci ne be ta in (µ mo l g - 1 F. W ) Control 5 dS m-1 10 dS m-1 FIGURE 3 Interactive effect of maize hybrids and different salinity levels on leaf glycinebetaine accu- mulation in maize hybrids ± SE. potassium significantly increased the total free amino acids in all the salinity treatments. Effect of Potassium Nutrition on Leaf Enzymatic Activity Maize hybrids had highly significant (P ≤ 0.01) difference from each otherwith respect to nitrate reductase activity (NRA) It is reveal fromFigure 7 that in both the hybrids NRA, was increased with increase in the potassium application, but in ‘Dekalb979’ maximum NRA was recorded in K2 (125 kg 0 1 2 3 4 5 6 Pioneer 32B33 Dekalb 979 Pr ot ei n (m g g- 1 F. W ) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 FIGURE 4 Interactive effect of maize hybrids and different potassium levels on leaf protein contents in maize hybrids ± SE. D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 151 0 5 10 15 20 25 Salinity levels To ta l s ol u bl e su ga r (µg m L-1 ) K0: Control K1: 175 kg ha-1 K2: 225 kg ha-1 K3: 275 kg ha-1 Control 5 dS m -1 10 dS m-1 FIGURE 5 Interactive effect of different salinity and potassium levels on leaf total soluble sugar in maize hybrids ± SE. K2O ha−1) which was followed by K3 (175 kg K2O ha−1) and minimum was found in control (no application). Interactive effect of salinity and potassium levels (Figure 8) show that in control treatmentmaximumNRAwas recorded in K2 (125 kg K2O ha−1) treatment which was followed by K3 (175 kg K2O ha−1) and minimum was recorded in K0 (no application), whereas, in both the salinity treatments application of potassium significantly increased the NRA. Salinity levels 0 5 10 15 20 25 30 35 To ta l f re e am in o ac id s (m g g-1 F. W ) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 Control 5 dS m-1 10 dS m-1 FIGURE 6 Interactive effect of different salinity and potassium levels on leaf total free amino acids in maize hybrids ± SE. D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 152 M. Akram 0 2 4 6 8 10 12 14 16 Pioneer 32B33 Dekalb 979 N R A (u m ol NO 2 g- 1 F. W hr - 1 ) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 FIGURE 7 Interactive effect of maize hybrids and different potassium levels on leaf nitrate reductase activity (NRA) in maize hybrids ± SE. Maize hybrids differed significantly (p≤ 0.01) for nitrite reductase (NiR) activity (Figure 9). Salinity stress severely decreased the activity of NiR and application of potassium improve it; however there was no significant dif- ference in NiR activity at K2 (125 kg K2O ha−1) and K3 (175 kg K2O ha−1) levels. Interactive effect of different salinity and potassium levels indicate 0 2 4 6 8 10 12 14 16 Salinity levels N R A (µ m o l N O 2 g- 1 F. W hr - 1 ) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 Control 5 dS m-1 10 dS m-1 FIGURE 8 Interactive effect of different salinity and potassium levels on leaf nitrate reductase activity (NRA) in maize hybrids ± SE. D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 153 0 5 10 15 20 25 30 35 40 45 50 Pioneer 32B33 Dekalb979 Pioneer 32B33 Dekalb979 Pioneer 32B33 Dekalb979 Salinity levels NR A (µ m ol NO 2 g- 1 F. W hr - 1 ) N0: Control N1: 175 kg ha-1 N2: 225 kg ha-1 N3: 275 kg ha-1 Control 5 dS m-1 10 dS m-1 FIGURE 9 Interactive effect of hybrids x salinity x potassium on leaf nitrite reductase activity (NRA) in maize hybrids ± SE. salinity stress decreased while potassium application improved the activity of NiR (Figure 9). Effect of Potassium Nutrition on Ion Accumulation and Yield Parameters The analyzed data (Table 1) indicate that hybrid ‘Dekalb979’ accumu- lated more sodium as compared to ‘Pioneer32B33’. Sodium concentration was increased with increasing salinity levels and the increase with each incre- ment of salinity was statistically significant. Application of potassium signifi- cantly decreased the sodium concentration in plants. Significant difference was noted in the accumulation of potassium in both the hybrids. Potassium in the plants decreased significantly with increasing salinity levels (Table 1). Inverse relationship between salinity and potassium in maize crop plants was estimated. However, Potassium application significantly increased the potassium concentration in the maize plants. Salinity stress significantly affected themaize calcium contents andmaize hybrids also differed significantly (p≤ 0.01) from each other for leaf calcium contents (Table 1). The effect of salinity treatments on calcium contents was more pronounced at high salinity level (10 dS m−1). Potassium application improved the uptake of calcium. Maximum uptake was observed in case of treatment where potassium was applied at 175 kg ha−1 but it was statistically at par with that where potassium was applied at 125 kg ha−1 and minimum D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 T A B L E 1 E ff ec to fd if fe re n ts al in it y an d po ta ss iu m le ve ls on so di um ,p ot as si um ,c al ci um ,n it ro ge n ,p h os ph or us ,c ob le n gt h an d n um be r of gr ai n ro w s pe r co b in m ai ze h yb ri ds So di um Po ta ss iu m C al ci um N it ro ge n Ph os ph or us C ob le n gt h N um be r of (m g g− 1 D .W ) (m g g− 1 D .W ) (m g g− 1 D .W ) (m g g− 1 D .W ) (m g g− 1 D .W ) (c m ) gr ai n ro w s co b− 1 T re at m en t M ea n M ai ze h yb ri ds (H ) ‘P io n ee r 32 B 33 ’ 1. 56 a 19 .2 2 a 12 .6 1 a 15 .2 8 4. 22 a 13 .7 3 a 13 .1 1 ‘D ek al b 97 9’ 1. 87 b 17 .0 5 b 10 .8 7 b 14 .7 6 3. 98 b 12 .4 1 b 13 .0 5 L SD at 5% 0. 12 0. 55 0. 53 N S N S 0. 30 N S Sa lin it y le ve ls (S ) (d S m −1 ) S 0 :C on tr ol (1 .0 9) 1. 17 c 20 .1 5 a 12 .9 9 a 16 .4 2 a 4. 34 a 13 .6 4 a 14 .0 8 a S 1 :5 1. 52 b 18 .2 5 b 11 .6 9 b 15 .2 2 ab 4. 19 a 13 .1 1 b 13 .0 8 b S 2 :1 0 2. 47 a 16 .0 1 c 10 .5 6 c 13 .4 3 b 3. 78 b 12 .4 7 c 12 .0 8 c L SD at 5% 0. 17 0. 81 0. 78 1. 83 0. 33 0. 45 0. 61 Po ta ss iu m le ve ls (K ) (k g h a- 1) K 0 :0 2. 04 a 14 .2 9 d 10 .1 8 c 12 .0 7 c 3. 26 c 11 .3 9 d 11 .5 6 d K 1 :7 5 1. 81 b 17 .8 0 c 11 .0 0 c 14 .2 7 b 3. 88 b 12 .8 2 c 12 .5 6 c K 2 :1 25 1. 57 c 19 .6 4 b 12 .2 9 b 16 .3 0 ab 4. 61 a 13 .7 2 b 13 .6 7 b K 3 :1 75 1. 45 c 20 .8 2 a 13 .5 1 a 17 .4 5 a 4. 66 a 14 .3 7 a 14 .5 6 a L SD at 5% 0. 20 0. 94 0. 90 2. 11 0. 38 0. 52 0. 70 M ea n s sh ar in g th e sa m e le tt er s in a co lu m n do n ot di ff er si gn ifi ca n tl y at p 0. 05 . ∗ , ∗∗ = Si gn ifi ca n ta tP ≤ 0. 05 an d P ≤ 0. 01 le ve ls ,r es pe ct iv el y. N S = N on -s ig n ifi ca n t, F. W = fr es h w ei gh t, D .W = dr y w ei gh t. 154 D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 155 was observed in control (no application). Nitrogen contents decreased with increasing the salinity levels; however, application of potassium significantly increased the uptake of nitrogen inmaize plants (Table 1).Maximumuptake was observed in K3 (175 kg K2O ha−1), which was closely followed by K2 (125 kg K2O ha−1) and minimum was noted in control where no potassium was applied. Non-significant difference was observed in both the hybrids for phos- phorus accumulation (Table 1). Salinity level means indicate that there was significant decrease in phosphorus uptake at the highest salinity treatment (10 dSm−1); however, by the application of potassium significantly increased in the phosphorus uptake was observed. Maximum uptake was observed in K3 (175 kg K2O ha−1) which was at par with K2 (125 kg K2O ha−1), while minimumwas recorded in control (no application). Analyzed data (Table 1) show that maize hybrids differed significantly from each other with respect to cob length. Salinity significantly decreased the cob length, whereas, appli- cation of potassium ameliorated the effect of salinity and enhanced the cob length. It is revealed from the results that potassium application showed signifi- cant improvement in number of grains per cob especially in ‘Pioneer32B33’. In ‘Dekalb979’, maximum number of grains was recorded in K3 (175 kg K2O ha−1), which was statistically at par with that of K2 (125 kg ha−1) and mini- mum was recorded in control, where no potassium was applied (Figure 10). The hybrids differed non-significantly from each other for number of grain rows per cob. However, there was significant decrease in number of grain 0 50 100 150 200 250 300 350 400 Pioneer 32B33 Dekalb 979 N um be r o f g ra in s co b-1 K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 FIGURE 10 Interactive effect of maize hybrids and different potassium levels on number of grains per cob in maize hybrids ± SE. D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 156 M. Akram 0 2 4 6 8 10 12 14 16 18 Pioneer 32B33 Dekalb 979 10 0- gr ai n w ei gh t ( g) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 FIGURE 11 Interactive effect of maize hybrids and different potassium levels on 100-grain weight in maize hybrids ± SE. rows per cob with increasing salinity stress but application of potassium in- creased the number of grain rows per cob (Table 1). Salinity level caused a decrease, whereas, potassium application signifi- cantly increased 100-grain weight. Potassium application show that potas- sium increased 100-grain weight at all the levels in both the hybrids (Figure 11). Grain yield is the end result of many complex morphological and physiological processes occurring during the growth and development of crop. Maize hybrids differed significantly from each other for this param- eter. It is also clear from the data that the salinity stress significantly reduced the grain yield, whereas, application of potassium reduced the injurious effect of salinity on grain yield (Figure 12). DISCUSSION Soil salinity reduced absorption of nitrogen and caused imbalance of mineral nutrients that resulted in reduction of plant growth and productiv- ity (Mer et al., 2000). Negative effect was observed in growth and yield of maize plants due to increase in the salt concentration in the soil solution (Rodriguez et al., 2007). Salt stress led to a significant reduction in yield and yield attributes of maize hybrids (Table 1). These results are also supported with the findings obtained by Hafeez et al. (1988) who reported that num- ber of grains per pod in mungbean was decreased when plants were grown under saline conditions. In the present study, application of potassium signif- icantly improved the yield components and also the previous investigations indicated that potassium plays an important role in the osmotic adjustment D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 157 0 10 20 30 40 50 60 70 Pioneer 32B33 Dekalb 979 G ra in y ie ld p la nt - 1 (g) K0: Control K1: 75 kg ha-1 K2: 125 kg ha-1 K3: 175 kg ha-1 FIGURE 12 Interactive effect of maize hybrids and different potassium levels on grain yield per plant in maize hybrids ± SE. of plants under stress and yield may increase significantly due to potassium application to soils (Davis, 1994; Finck, 1998). One of the most common stress responses in plants is over production of different types of compatible solutes (Serraj and Sinclair, 2002). Compatible solutes are low molecular weight, highly soluble compounds that are usu- ally non-toxic at high cellular concentrations. Generally, they protect plants from stress through different courses, including contribution to cellular osmotic adjustment, protection of membrane integrity and stabilization of enzymes (Bohnert and Jensen, 1996). Furthermore, because some of these solutes also protect cellular components from dehydration injury, they are commonly referred to as osmoprotectants. These solutes include proline, sucrose and quaternary ammonium compounds such as glycinebetaine, and proline (Rhodes and Hanson, 1993). Two major roles have been suggested for proline accumulation during hyperosmotic stress. One is its action as an osmotic regulator. An alternative suggested role is connected with the cellular water structure, which protects against reduction of the hydration of cytoplasmic constituents induced by high salt stress (Nakamura, 1979). The results of the present work show that the high accumulation of proline in maize cells induced salt tolerance which indicated its active role for pro- line in the intercellular processes connected with salt tolerance (Figure 2). Moreover, the salt stress can induce an increment in the de novo synthesis of some particular amino acid, especially proline (Lutts et al., 1999). Salinity is associatedwith reducedNRA in both leaves and roots ofDurum wheat (Carillo et al., 2005), and in the leaves of tomato (Debouba et al., 2006) and in tomato and wheat depression of NRA is due to reduced nitrate D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 158 M. Akram (NO3−) concentration, which itself is linked toNO3− influx andplays a direct role on the production of NR protein and its activation. Cl− is believed to cause a reduction in root xylem loading of NO3− as a significant decrease in concentration occurred only in the leaves of wheat, similar to tomato. A concomitant decrease in NiR activity occurs with reduced NRA since NR induces NiR production, as seen in salt-stressed tomato leaves (Debouba et al., 2006). In the present study, the accumulation of total free amino acids signif- icantly increased under salinity stress and application of potassium further improved their accumulation (Figure 6). Amino acid proline is known to occur widely in higher plants and normally accumulates in large quantities in response to environmental stresses (Kavi Kishore et al., 2005). In response to drought or salinity stress, proline accumulation normally occurs in the cytosol, where it contributes sustainability to the cytoplasmic os- motic adjustment in plants (Ketchum et al., 1991). Accumulation of proline under stress in many plant species has been correlated ion stress tolerance and its concentration has been shown to be generally higher in stress tol- erant than in salt sensitive plants (Petrusa and Winicov, 1997). Potassium enhances several enzyme functions as a result of which metabolic activities of the plants are maintained even under adverse condition. Present study on maize hybrids showed that soil amendment with K was successful in increas- ing nitrate reductase activity under normal and saline conditions (Figures 7 and 8), the results of Idrees et al. (2004) also indicated that application of potassium increased nitrate reductase activities, nutrient uptake and growth of sugarcane under saline conditions. Abd-El Baki et al. (2000) reported that the nitrate reductase (NR) activity and NR-mRNA were both reduced by salt stress in maize seedlings. Inhibition in plant growth due to salinity is the result of osmotic and ionic effects and different plant species have developed different mecha- nisms to combat these effects (Munns, 2002). The osmotic adjustment i.e., reduction of cellular osmotic potential by solute accumulation has been con- sidered an important mechanisms to salt and drought tolerance in plants. The reduction in osmotic potential in salt stressed plants can be a result of inorganic ion and compatible organic solute (proline, glycinebetaine, sugar and amino acids) accumulations (Hasegawa et al., 2000). Osmotic adjust- ment in plants subjected to salt stress can occur as a result of accumulation of inorganic ion (K+) and compatible solutes i.e. proline and glycinebetaine, which contribute in maintaining water uptake and cell turgor and control- ling physiological processes, such as stomatal opening and photosynthesis (Serraj and Sinclair, 2002). In most crop species, salt tolerance is found to be associated with the ac- cumulation of only low amounts of both Na+ and Cl− in the shoots (Ashraf, 1994). In contrast, K+ and Ca2+ accumulation in salt-stressed plants was con- siderably reduced. However, adequate amounts of potassium are required D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 Potassium Nutrition Affects Maize Salt Tolerance 159 for integrity and functioning of cell membranes, and other cellular processes (Davenport et al., 1997). It is well established that salinity tolerance was typ- ically characterized to enhanced Na+ exclusion and increased absorption of K+ ion in the shoots (Gorham et al., 1990). Thus maintenance and acquisi- tion of N, K+, and Ca2+ are important determinants of salt tolerance. In the present study, accumulations ofNa+ andCl− in the shoots of both the hybrids were significantly increased due to salt stress, while K+ and Ca2+ accumula- tion was decreased. But application of potassium reduced the accumulation of Na+ and Cl−. Overall, hybrid ‘Pioneer32B33’ was better in discriminating Na+ over K+ and Ca2+. From the above discussion, it can be perceived that improvement in growth and yield in maize hybrids under saline conditions with potassium applicationmay have been due to discriminating Na+ against K+ and Ca2+ in salt stressed plants, it needs to be elucidated. Plants use low and high affinity transporters, present on biological membranes, for uptake of K+ from the growth medium (Blumwald et al., 2000). Salinity increased Na+ uptake in maize plants. The sodium uptake showed a decreasing tendency with the application of potassium. Munns et al. (2006) reviewed several reports showing that reduced Na+ transport capacity of varieties from the roots to the shoots greatly contributes to high tolerance to NaCl toxicity. Potassium is essential for osmoregulation, pro- tein synthesis and activation of enzymes, maintaining cell turgor and stim- ulating photosynthesis (Buschmann et al., 2000). In saline environments where Na+ predominates over potassium, the plants paramount nutritional requirement is potassium in adequate amount. Application of higher lev- els of potassium showed an increase in potassium accumulation. Increased treatment of NaCl induces increase in Na+ and Cl− and decrease in Ca2+ and K+ levels in a number of plants (Khan, 2001). The inhibition of K+ uptake by NaCl has been observed previously in glycophyte plants (Tarakcioglu and Inal, 2002). There is competition between Na+ and K+ ions for binding site in enzyme and substrate action during uptake of ions. Fertilizer application under saline cultivations is now being tried to al- leviate or neutralize growth inhibition due to salinity and to increase the productivity of the saline soils. Fertilization with nutrients like potassium or nitrogen increases the ability of plants to take up sufficient potassium, calcium, phosphorus and nitrogen to avoid the accumulation of too much sodium and chloride, respectively. Helal and Mengel (1979) reported that salt stress on barley caused by NaCl could be reduced by an increase of potas- sium supply. Therefore, it is assumed that the application of optimum level of potassium preferably in the form of K2SO4 will improve plant tolerance to salinity. Calcium is very important macro nutrient during salt stress e.g., in pre- servingmembrane integrity and signaling in osmoregulation (Rengel, 1992). The uptake of Ca2+ from the soil solution may decrease because of ion inter- action, precipitation and increase in ionic strength that reduce the activity D ow nl oa de d by [U Q Li bra ry] at 05 :51 18 A pri l 2 01 3 160 M. Akram of Ca2+ (Garg and Gupta, 1997). The decrease of K+ and Ca2+ in plants has been well reported under sodium chloride salinity (Cramer et al., 1991). The detrimental effect of salt stress on crop plants is responsible for severe decreases in the yield components of plants. Osmotic stress (drought problem), ion imbalances, particularly with Ca2+ and 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 (Munns et al., 2006). Salinity reduces nitrogen accumulation in plants (Garg et al., 1993). This is due to the fact that an increase in chloride uptake and accumulation is mostly accompanied by a decrease in shoot nitrate concentration (Garg and Gupta, 1997). In the present study, uptake of phosphorus decreased by salinity stress, however; increasing application of potassium increased the phosphorus accumulation (Table 1). These results are in accordance with Abd-Ella and Shalaby (1993) who reported that potassium addition diminished the salinity effect and improved the status of ion relationships in cotton under saline conditions. CONCLUSIONS It can be concluded from the present study and literature quoted above that the application of potassium fertilizer mitigates the adverse effects of the salinity stress condition imposed on maize plants and resulted in better response in terms of nutrient acquisition and yield parameters. 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