The efficiency of magnesium (Mg) on rice growth, biomass partitioning and chlorophyll contents in alkaline soil condition

Magnesium (Mg) is recognized as an essential macronutrient because of its involvement in several morphological and physiological processes in numerous plant species. While, its deficiency in the soil lead to cause the severe reduction in plant growth and yield. The purpose of the current study was to evaluate the efficiency of Mg at (40, 80 and 160 kg/ ha) for rice growth, yield, biomass production and chlorophyll contents as well as Mg availability in in calcareous soil. Results demonstrated that the addition of Mg showed the prominent increase in rice plants vegetative growth, shoot and root fresh and dry biomass as well as enhance the availability of NPK in calcareous soil. Similarly, the significant increase in chlorophyll contents was also observed with the increasing rate of Mg fertilization. In conclusion, addition of Mg fertilizer in calcareous soils is considered an effective strategy for the rice growth.


Introduction
Rice (Oryza Sativa) is recognized as one of the world's most important staple food crops; it has a major share to fulfil the food demand of the world population. According to the FAO 2016 statistics, globally rice production was more than 749 million tons, superseded by maize that accounts for 959 million tons. While, In Pakistan, Punjab and Sindh provinces have greater share in rice production that accounts 88% of the total country's basic food needs. Rice is cultivated on about 2.5 million hectares' area which has major share after cotton and wheat production. Recently, the cultivated land is shrinking due to the huge rise in urbanization, industrial revolution, environmental and agro climatic conditions, which pose a serious threat to is the huge world population food requirement [1]. It is interesting to address some important counter measures to increase crops yields including, well organized water conservation projects, ameliorate soil quality, innovative techniques that should be implemented to manage plant protection and fertilizers applications. The use of fertilizers is an important strategy to meet the large population food demands all around the world. However, the nutrient use efficiency was very low in Pakistani soils, especially Nitrogen (N) and Phosphorus (P)/ due to calcareous nature relative to European countries [2]. Therefore, the use of secondary nutrient such as Mg as a mixture and alone nutrient is considered the beneficial approach for rice growth and yield. Magnesium (Mg) is recognized an essential nutrient for various living organisms including, plant species, animals and as well as human beings [17] and thereby its deficiency may cause the reduction in sustainable agricultural production and negative impacts on other organisms Recently, more attention has been paid on the primary macronutrients N, P and K related fertilizers than secondary nutrients Ca, Mg and S to attain maximum crops yield [4, 22]. While, there is also little information is available regarding the significant importance of Mg for agricultural production, contribution in plant structure and physiological attributes as well as its deficiency in farmlands and crops [1, 4]. However, it is essential to recognize the behavior of crop yield to Mg-fertilization under various soils types, cropping pattern, and fertilizer management strategies in large-scale field experiments. There has been no effort made to examine the effects of Mg as a fertilizer on rice growth and development. In this study, a meta-analysis was conducted to (1) examine the overall impacts of Mg applications on rice yield and subsequent agronomic efficiencies; (2) To understand the effects of Mg fertilization on the photosynthetic rate of rice plants in alkaline condition.

Experimental particulars
The impact of Mg on rice crop growth and yield was investigated by conducting a field trial at the Research site of College of agriculture, Bahauddin Zakariya University, Multan. The research site was organized with Randomized Complete Block Design (RCBD). The research site was divided and managed into small plots with 5m length and 2m width. Super Basmati rice was selected as a test plant, seeds were sown in the month of July, 2018. Rice nursery was transplanted manually. The experimental units were amended with MgSO4 at the following doses: (T1), Control; (T2), 40 Kg Mg/ ha; (T3), 80 Kg Mg/ha; (T4), 160 Kg Mg/ ha along with the basic application of NPK fertilizers. Super Basmati Punjab II rice variety was selected as a test crop. Recommended dose of NPK (75:76:60 kg/ ha) fertilizer was added from the sources of Urea, MOP and DAP, respectively at the time of seedling transplantation. The physico-chemical properties were presented in the (Table 1).

Growth and yield attributes
Plant height in (cm) was measured by taking plants above ground biomass to the tip of the panicle at the time maturity and harvesting stage using meter rod. Similarly, other crop growth and yield attributes were measured including, number of tillers (m -2 ), shoot diameter (cm), thousand grain fresh weight (g), thousand grain dry weight (g), total grain yield (kg/ha), fresh shoot weight (g) and dry shoot weight (g).

Soil analysis
Soil pH and EC were measured using the 1:2.5 and 1:5 of experimental soil and water ratio, after the harvesting of crop using pH and EC meter [4-6]. Soil organic matter was calculated using the wet oxidation method by taking 1.0 g of studied air dried soil sample [5].
The available phosphorus concentration was also determined by taking 5.0 g of tested soil into plastic sample tubes and add 15 ml of 0.5 M NaHCO3 extractant. The soil sample was shaken and centrifuge at 3000 rpm [5]. Extractable soil Potassium was estimated by extracting the 2.0 g of studied soil with the 10 ml of 1 N C₂H₇NO₂ solution with 1:5 ratios. Then the suspension was shaken for 30 minutes at 200 to 300 rpm. The extractable K contents from studied soil were estimated using the flame. A calibration curve was developed which showed the K levels. Exchangeable Mg from studied soil was measured using the normal ammonium acetate extraction method. Extraction was taken and an aliquot was titrated using EDTA solution and extractable Mg contents were determined using atomic absorption spectrophotometer.

Plant analysis
Harvested plant tissues were collected, dried and ground for chemical analysis. Fresh plants leaves were used for chlorophyll analysis. The dried and ground portion of 1g of rice plant tissues were digested using Diacid (2:1) (HNO3+HCl2) [4, 5]. Similarly, N contents from rice tissues were measured using 0.25 g of rice tissues (root and leaf) and were taken in sample digesting tube with 2 ml of H2SO4 [4, 6]. Then 30% pure Hydrogen peroxide was added and digested plant tissues at 100 0 C for 20 minutes [3]. Phosphorus contents in rice tissues were measured by the following method [20]. We took 0.05 ml of digesting plant sample and took 0.025 ml of grain sample was diluted in 0.950 ml 0.975 distilled water. Finally, 0.2 ml of the reagent 1 was added into tubes and left the suspension for 15 minutes.

Statistical analysis
The subjected parameters were analysed by using their means with the software (statistics 8.1). Analysis of variance (ANOVA) technique was used for the respective statistical analysis (RCBD). The least significant difference (LSD) test was used p ≤ 0.05 to understand the mean difference among the amended doses of Mg.

Effect of Magnesium on rice growth, yield and soil
The results recorded from the present study showed a significant (P≤0.05) ( Table 2 & 3) effectiveness of Mg on rice growth and yield as well as Mg availability in soil. Results showed that the efficiency of Mg along with the basal levels of NPK has the significant improvements in rice growth parameters relative to the experimental unit which were not amended with Mg. The prominent increment in the number of plants per square meter was observed by (18.3) in T4 after the addition of Mg at 160 kg/ ha). Similarly, the maximum number of spikelet plants -1 were calculated in T4 (13), while the minimum was recorded in T1 (9.7). The prominent increment in plant height was recorded by enhancing the levels of magnesium. Likewise, the greater increase in plant height was obtained in T4 (69.7 cm) relative to T1 (54.0 cm). Data regarding the number of thousand grain weight was significantly differed after Mg incorporation. Then statistical observation showed that Mg play an important role to improve the weight of thousand grain. The greater increment in grain weight was recorded Mg was applied at 160 kg/ha by (27.3 g) compared to T1 by (23.4 g) ( Table  1). The significant difference in shoot dry biomass and diameter was recorded in the highest rate of Mg treated soil. The maximum shoot dry biomass and diameter was recorded by 57.3 g and 13.3 cm respectively. The SPAD values regarding chlorophyll contents were altered with the various Mg levels ( Table 4). The greater values were recorded in T4 by (0.23 mg/g) where the application of Mg was applied at 160 kg/ha dose, while the minimum contents were observed in T1 by (0.15 mg/g) where minimum Mg was applied (Table 4).

Role of MgSO4 on organic matter and Mg availability in soil
Soil organic matter was effectively altered after the incorporation of Mg fertilizer (Fig.  1). The greater amount of OM was estimated under T4 by 0.91 %, while the low contents were measured in T1 by 0.68 %. The greater increase in Mg contents were recorded in the soil, where the application of Mg was applied in T4 48.7 mg/g while minimum was observed in T1 by 21.6 mg/g, where no Mg was applied.

Effect of Mg on NPK contents
The accumulation of nitrogen (N), phosphrous (P) and Potassium (K) in rice shoot was significantly altered (P≤0.05) among the treatments (Table 4 & 5). The highest N, P and K contents was calculated in rice shoot in T4 by 1.29, 0.24 and 155 mg kg -1 respectively, while low N, P and K contents were recorded in T1 by 0.74, 0.2 and 129 mg kg -1 respectively. Data about the contents of N, P and K in rice grains was significantly altered (P≤0.05) ( Table 4 & 5). The maximum N, P and K contents were determined in rice grain in T4 0.76, 0.37 and 69 mg kg -1 respectively, while the minimum were measured in T1 0.19, 0.27 and 57.3 mg kg -1 respectively. Similarly, highest concentration of N, P and K was determined in rice leaf in treatment T4 by1.052, 0.33 and 165 mg kg -1 where Mg level was maximum, while minimum Mg dose was determined in control treatment T1 by 0.86, 0.26 and 136 mg kg -1 respectively.